GENERAL CHARACTERSTICS
Aquatic, aerial or terrestrial all free living with no fully parasitic forms.
Bilaterally symmetrical and metamerically segmented.
Exoskeleton often present well developed in most vertebrates.
Bodywall triploblastic with 3 germinal layers: ectoderm, mesoderm and endoderm.
Coelomate animals with a true coelom, enterocoelic or schizocoelic in origin.
A skeletal rod, the notochord, present at some stage in life cycle.
Digestive system complete with digestive glands. Blood vascular system closed.
Heart ventral with dorsal and ventral blood vessels.
Hepatic portal system well developed.
Excretory system comprising proto –or meso-or meta-nephric kidneys.
Nerve cord is dorsal and tubular. Anterior end usually enlarged to form brain.
Sexes are separate with rare exception.
ANCESTORYANCESTORY OF CHORDATES
The unique chordate body plan evolved within the deuterostome animals sometimes before the Cambrian (Valentine, Jablonski Erwin 1999; Blair and Hedges 2005). Chordates traditionally include vertebrates, lancelets (cephalochordates) and tunicates, but tunicates do not exhibit a chordate body plan as adults. Hemichordates are sister group to echinoderms and both phyla are an outgroup to the rest of the chordates).
Xenoturbella has been recently included in the deuterostomes as molecular evidence unites them with hemichordates and echinoderms, but their exact position within the deuterostomes is not yet clear. All five chordate characteristics (post anal tail, dorsal nerve cord, notochord, endo style and pharyngeal gill slits) at one time or another been suggested to have homologou structures present in hemichordates, but all these features are lacking in echinoderms and Xenoturbella, the closest relatives to hemichordates, suggesting that they were lost during their evolution.
Integument in Different Classes of Chordates
The fundamental structure of skin in all the vertebrates is the same but there are certain variations in different classes.
1. Protochordata:
In Branchiostoma the skin is simple without keratin. The epidermis is single layered made of tall or columnar cells. These are ciliated in Balanoglossus. Epidermis has numerous unicellular gland cells which secrete a thin cuticle in Branchiostoma. Dermis (corium) is gelatinous in Amphioxus.
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| V.S. of skin of a young Amphioxus |
2. Cyclostomata:
Epidermis is multilayered (stratified) but has no keratin. It has three types of unicellular gland cells- mucous glands secrete mucous, club cells probably scab-forming cells, and granular cells are of unknown function. Below the epidermis is the cutis formed of collagen and elastic fibres. Star-shaped pigment cells are also present in the cutis.
3. Pisces:
The epidermis has several layers of simple and thin cells, but there is no dead stratum corneum. The outermost cells are nucleated and living. The stratum Malpighii replenishes the outer layers of cells which have some keratin. Unicellular goblet or mucous gland cells are found in the epidermis, as in all aquatic animals. The mucus makes the skin slimy reducing friction between body surface and water, protects the skin from bacteria and fungi, and assists in the control of osmosis. Multicellular epidermal glands like poison glands and light producing organs (photophores) may also be found. The epidermis rests on a delicate basement membrane.
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| V.S. of skin of a larval cyclostome |
The dermis contains connective tissue, smooth muscles, blood vessels, nerves, lymph vessels, and collagen fibres. The connective tissue fibres are generally not arranged at right angles, but run parallel to the surface. Scales are embedded in the dermis and projected above the epidermal surface.
These are of five types. The elasmobranchs have placoid scales, Chondrostei and Holostei have ganoid scales, while most Teleostei have cycloid and ctenoid scales lodged in pouches of the dermis. Extinct Crossopterygii had cosmoid scales.
Many bony fishes show more brilliant colours than any other group of animals. The colours of fishes are due to chromatophores and iridocytes.
(a) Chromatophores in the dermis are derived from neural crest cells. They contain pigments which not only produce colours but also cause variations of colours. Chromatophores containing brown or black pigment are known as melanophores and those containing red, yellow, or orange pigment are collectively called lipophores.
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| V.S. of integument of shark |
(b) Iridocytes or guanophores are reflecting cells. They have no pigment but contain crystals of guanin. They lie in the dermis and cause irides-cence. Iridocytes reflect light from guanin crystals to produce white or silvery colours if the iridocytes are below the scales, if the iridocytes are above the scales they cause iridescent hues. By combinations of chromatophores and iridocytes various colours are produced, e.g., blue is produced by reflection from iridocytes, the blue combines with yellow pigment to produce green.
4. Amphibia:
The epidermis is multilayered; the outermost layer is a stratum corneum made of flattened, highly keratinised cells. Such a dead layer appears first in amphibians, and is best formed in those which spend a considerable time on land. The stratum corneum is an adaptation to terrestrial life; it not only protects the body but prevents any excessive loss of moisture.
In ecdysis, the stratum corneum is cast off in fragments or as a whole in some. The dermis is relatively thin, it is made of two layers, an upper loose stratum spongiosum and a lower dense and compact stratum compactum. Connective tissue fibres run both vertically and horizontally.
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| V.S. of skin of bony fish |
There are two kinds of glands, they are multicellular mucous glands and poison glands in the dermis, but they are derivatives of the epidermis. The mucous glands produce mucus which not only forms a slimy protective covering but also helps in respiration. The poison glands found in toads as parotid glands produce a mild but unpleasant poison which is protective, keep the enemies away. In the upper part of the dermis are chromatophores which have black melanophores and yellow lipophores, these produce the colour of the skin. The ability of the skin for changing colour to blend with the environment is well developed. Skin of labyrinthodontia, the stem Amphibia had a armour of dermal seales. Bony dermal scales are found embedded in the skin of some Gymnophiona (Apoda) and a few tropical toads. These scales are absent in modern Amphibia.
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| Chromatophores and irisocytes in skin of fishes |
The skin is sensitive to light in amphibians, especially in cave-dwelling forms. It is an important organ of respiration, and also enables the frog to respire under water for long periods, during hibernation or aestivation it is the only organ of respiration.
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| V.S. of skin of frog |
The skin is loose being attached to muscles only at certain places by connective tissue septa which mark the boundaries of subcutaneous lymph spaces.
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| V.S. of skin of lchthyophis showing dermal scales |
5. Reptilia:
The integument (Fig. 41.17) is thick and dry. It prevents any loss of water. It has almost no glands, this is an adaptation to prevent evaporation of water.
The epidermis has a well-developed stratum corneum which makes the skin dry and prevents any loss of body moisture, thus, well adapted to a terrestrial life.
The epidermis produces horny scales. Scales are shed periodically in fragments or cast in a single slough, as in snakes and some lizards.
The scales often form spines or crests. Below the epidermal scales are dermal bony plates or osteoderms in tortoises, crocodiles, and some lizards (Heloderma). These are retained for life and are not shed off. These may form dermal bones in the skull lying superficially or they may be found in the dermis. The dermis is thick and has an upper loose connective tissue layer and a lower layer tella subjunctiva and separating the two is a horizontal layer of fibrous connective tissue. The upper layer has an abundance of chromatophores in snakes and lizards like fishes and amphibians. Leather of high commercial value is made from the skin of lizards, snakes, and crocodiles.
Many lizards and snakes have elaborate colour patterns, for concealment or as warning colours. There is marked colour change in certain lizards, such as Chameleon, the colour may change with the environment for concealment, or it may change in courtship or threat. In Calotes, the chromatophores have no nerve fibres, they are controlled by hormones of the posterior lobe of the pituitary. In Chameleon, chromatophores are controlled by the autonomic nervous system.
Reptiles essentially lack skin glands. Many lizards have glands near the cloaca. Their openings called femoral or pre-anal pores are generally smaller in female and found only in the male in some species. They are most active in the breeding season. Musk glands in the throat and cloacal opening of crocodilians function during courtship. Generation glands found recently are associated with periodic shedding of the skin.
6. Aves:
The integument is thin, loose, dry and devoid of glands except a uropygial gland at the base of the tail whose secreted oil is used for preening the feathers, especially in aquatic birds. The stratified epidermis is delicate, except on shanks and feet where it is thick and forms epidermal scales. The claws, spurs and horny sheaths of beaks are also the modifications of stratum corneum of epidermis.
Claws and beaks are variously modified in birds according to habitat. The rest of the body has a protective covering of epidermal feathers which are evolved from epidermal scales. Feathers protect and insulate the body, i.e, keep the body warm. The dermis is thin and has interlacing connective tissue fibres, abundant muscle fibres for moving feathers, blood vessels and nerves. The dermis forms an upper vascular and spongy layer and a lower compact layer.
The dermis also contains fat cells. The skin has no chromatophores. Pigment found in melanocytes migrates into feathers and scales. Colour patterns of birds are vivid; they are for concealment, recognition, and sexual stimulation. The colours are mainly produced by reflection and refraction of light from surface of feathers.
7. Mammalia:
The skin is elastic and waterproof and is much thicker than in other vertebrates, especially the dermis is very thick and tough and is used for making leather. The epidermis is thickest in mammals and is differentiated into five layers- stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum and stratum germinativum or Malpighian layer.
The outer
layer of stratum corneum containing keratin, its cells lose their nuclei, but
the cells are not dead as believed before. They secrete several hormones, one
of which represses the mitotic activities of the Malpighian layer. In places of
friction, such as soles and palms, the stratum corneum is very thick. Stratum corneum is variously modified in
various mammals to form epidermal scales, bristles, hairs, claws, nails, hoofs
and horns etc., Below the stratum corneum is a refractive stratum lucidum in
certain regions only. The stratum lucidum is now known as a barrier layer
because the electron microscope has shown that its cells become compact and
closely united to form a region which prevents passage of substances into or
out of the body.
Stratum
lucidum contains a chemical known as eleidin. Keratohyalin and eleidin are
intermediate products in the formation of keratin. Below this is a stratum
granulosum which is having darkly-staining granules of keratohyalin. Below the
stratum granulosum is a stratum spinosum whose cells are held together by spiny
intercellular bridges, each bridge has two arms in close contact, one arm
arising from each cell.
Lastly
there is a stratum germinativum or Malpighian layer which rests on a thin
basement membrane. The Malpighian layer forms new cells continuously which move
towards the surface and become flat and keratinised till the stratum corneum
has flat, cornified cells made only of keratin.
This layer is sloughed off continuously and replaced by new cells. There are no mucous glands in the epidermis of mammals. The keratin from the epidermis at ends of digits forms claws, nails or hoofs.
The
dermis is best developed in mammals. The upper part of the dermis in contact
with the epidermis is the papillary layer which is made of elastic and collagen
fibres with capillaries in between. It is thrown into folds to form rows of
dermal papillae, especially in areas of friction. The greater lower part of the
dermis is a reticular layer having elastic and collagen fibres.
In both
layers there are blood vessels, nerves, smooth muscles, certain glands, tactile
corpuscles, and connective tissue fibres extending in all directions. Below the
dermis the subcutaneous tissue has a layer of fat cells forming adipose tissue
which helps to maintain body heat. In making leather only the dermis is used.
Dermal scales are not found in mammals except armadillos. In the lowest layer of the epidermis are
pigment granules but there are no pigment-bearing chromatophores in mammals. In
man some branching dendritic cells or melanoblasts lie between the epidermis
and dermis, they contain pigment.
The
epidermis forms hairs, sudorific glands, sebaceous glands and mammary glands.
Hairs form an epidermal covering. Shafts of hair project above the skin and
their roots are embedded in hair follicles, into each of which opens a
branching sebaceous gland. Hairs form an insulating layer which prevents a loss
of body heat, thus, hairs keep up the body temperature. Sebaceous glands are
outpushings of the wall of hair follicle and produce an oily substance which
keeps the hair supple and prevents its wetting in water.
It also
lubricates the skin. In the dermis are present coiled sudorific or sweat
glands, which occur all over except lips and glans penis. Mammary glands are
modified sebaceous glands, but in monotremes they are modified sudorific
glands. They are functional only in females for producing milk for the young.
Mucous glands are not found in mammals.
Functions
of the Integument in Vertebrates:
1.
Protection:
i) The integument or skin protects the body from
the entry of foreign bodies and prevent from the mechanical injuries.
ii) The hard dermal and epidermal scales that
protect the skin from surface abrasion and also the soft tissues which lie
beneath it.
iii) Hair, bristles and spines are employed for offensive and defensive purposes.
iv)
The impervious integument helps the body from loss of water.
2. Thermoregulation:
The integument of warm-blooded animals regulates
the body temperature. Feathers of birds, sweat glands and blubber of mammals
help in the regulation of body temperature. Deep covering of the hairs help in
the conser-vation of heat, specially during winter.
3. Storage of food:
In whales, seals and sea cows, a sub- dermal fat
layer forms a thick layer, called blubber, which acts as food storage.
4. Excretion:
The integument of some aquatic verte-brates (e.g.,
aquatic amphibians) serves as an organ for excretion. During ecdysis the waste
material which is stored in the corneal layer of the skin is shed. Sweat of the
sweat glands aids in removing nitrogenous wastes from the body.
5. Respiration:
The moist skin of common eel, mud skip-pers and
swamp eels sub-serve respiration. The skin of amphibians is moist and highly
glan-dular that help air in contact with the skin to be interchanged and thus
performs accessory respiration. In plethodontid salamanders, the lungs are
absent, so rely totally on cutaneous respiration.
6. Secretion:
The skin acts as an organ of secretion. The
different glands are located in the skin those help the vertebrates in
different ways for sur-vival. Fishes possess numerous mucous glands in the skin
that secrete abundant mucous.
The slimy mucus of the fish on the skin reduces
resistance during swimming. The poison glands of fishes, amphibians and snakes
are used for protection and predation. Mammary glands, scent glands, and
sebaceous glands are present in the skin and serve different functions.
7. Locomotion:
Various types of integumentary derivatives
sub-serve different types of locomotion’s. The fins of fishes, web in aquatic
amphibians, terrapins and aquatic birds, scutes in snakes, adhesive pads in
climbing lizards, feathers in birds and patagium in flying lizards help in
different modes of locomotion.
Derivatives of Integument:
Both layers of integument have given rise to various types of derivatives. The epidermis gives rise to integumentary glands, epidermal scales, horns, digital structures, different corneal structures, feathers, and hairs. The dermis forms dermal scales of fishes and of some reptiles, plates or scutes in reptiles, fin rays in fishes and antlers in mammals.
I.
Epidermal Derivatives:
Epidermal derivatives are epidermal glands
(unicellular and multicellular), epidermal scales and scutes, horns, digital
structures (claws, nails and hoofs), feathers and hairs.
1.
Epidermal Glands:
Epidermal glands are formed from the Malpighian
layer of the epidermis. They arise from the epidermis and often penetrate the
dermis. According to their structure they are unicellular or multicellular,
tubular or alveolar and simple or compound (branched) glands. These are lined
by cuboidal or columnar cells.
(a) Unicellular glands are single modified
cells found among other epithelial cells, they are present in amphioxus,
cyclostomes, fishes and larvae of amphibians. Unicellular glands are known as
mucous cells or goblet cells. They secrete a protein mucin which combines with
water to form mucus which lubricates the surface of the body. Other unicellular
glands are granular cells and large beaker cells of cyclostomes and fishes,
they also secrete mucus.
(b) Multicellular glands are of two
types:
i) Tubular glands are multicellular tubes of uniform diameter formed as ingrowths of the Malpighian layer into the dermis, e.g., glands of Moll on the margin of the human eyelids. Tubular glands may become coiled at the base deep in the dermis, e.g., sweat or sudoriferous glands of mammals, or they may divide into many tubules which are then called compound tubular glands, e.g., mammary glands of females and of males in monotremes and primates, etc., and gastric glands in stomach.
ii) Alveolar or saccular glands are
multicellular down growths of the Malpighian layer into the dermis, having a
tubular duct whose terminal parts form a rounded expansion to become
flask-shaped, e.g., mucous and poison glands of amphibians. Alveolar glands may
branch into many lobules which finally open into a common duct, they are then
called compound alveolar glands, e.g., mammary glands of eutherians, and
salivary glands.
Kinds
of Epidermal Glands:
According to function, the epidermal glands of
vertebrates are of the following types:
i.
Mucous Glands:
They may be unicellular or multicellular. The
unicellular glands are mucous gland cells, granular cells and beaker cells of
amphioxus, cyclostomes and fishes. They secrete mucus which keeps the skin
moist and slippery, and also affords protection against bacteria and fungi.
Mucous cells and granular cells lie near the surface, but the beaker cells lie
more deeply and extend from the Malpighian layer to the surface.
Multicelluar mucous glands are alveolar found in some fishes and amphibians. They occur all over the surface of the body and produce mucus for lubricating the skin and in amphibians they keep the skin moist to aid in respiration.
ii. Poison Glands:
Amphibians also have alveolar poison glands which
are larger but less numerous than mucous glands. In toads masses of poison
glands form parotoid glands behind the head. The secretion of poison glands has
a burning taste and is used as a defence. Caecilians have giant poison
glands.
Some tubular glands are found on the feet and
suctorial discs of tree frogs which aid in climbing. Tubular glands are also
found on the swollen glandular thumb parts of male frogs and toads during the
breeding season. They aid in clasping the female during amplexus.
iii.
Luminescent Organs or Photophores:
They are found in longitudinal rows near the
ventral side of the body in those fishes which live in deep sea where no light
penetrates. Each photophore is a group of epidermal cells lying in the
dermis.
Each photophore has a lower layer of luminous cells
below which is a layer of reflecting pigment cells, and the upper layer of
mucous cells forms a lens. The glandular cells produce phosphorescent light
which is transmitted to the outside by other cells. The light helps to attract
the prey of deep sea fishes.
Femoral glands are found in male lizards (e.g.,
Uromastix) below the thighs in a row from the knee to the cloaca. They secrete
a sticky substance which hardens into short spines that are used for holding
the female during copulation.
iv.
Uropygial Glands:
These are the only glands in birds, and they are best developed in aquatic birds. Uropygial glands are branched alveolar glands located on the dorsal side at the base of the tail or uropygium in the form of swelling. They secrete an oil which is odoriferous and attracts the opposite sex. The oil contains pomatum which is picked up with the beak and used for preening and water-proofing the feathers.
v. Sweat Glands:
The largest number and variety of epidermal glands are found in the skin of mammals. They are tubular or alveolar and multicellular. Sudorific or sweat glands are long, coiled tubular glands embedded deep in the dermis. Their upper part forms a duct which opens on the surface by a pore and the lower coiled part lies in the dermis surrounded by a network of blood capillaries.
Epidermal
Glands
Sweat is secreted by sweat glands continuously, may
or may not be visible. Sweat contains a large amount of water having dissolved
salts of sodium, potassium and urea. Sweat removes some metabolic wastes and
regulates the body temperature by evaporation.
Sweat glands are not uniformly distributed. In man
they are more numerous on palms, soles, and arm pits. In cats, rats, and mice
they are confined to the soles of the feet. In rabbit they are around the lips;
in bats on the sides of head; in ruminants on the muzzle and the skin between
the digits and in hippopotamus they are found only on the pinnae.
There are no sweat glands in Tachyglossus, Sirenia,
and Cetacea. In some mammals the secretion of sudorific glands is red-
coloured. In hippopotamus it is red and spreads on the head and back. The male
giant kangaroo Macropus rufus secretes red sweat. Modified sudorific glands
form glands of Moll in the margins of the human eye in connection with
eye-lashes.
Ceruminous glands in the external ear passages of
mammals are modified sweat glands and secrete a waxy substance which combines
with the secretion of sebaceous glands to form earwax which catches dust. Oil
glands form ceruminous glands in the external ears of some gallinaceous birds.
vi.
Sebaceous Glands:
Sebaceous glands are alveolar glands opening in
hair follicles containing hairs. They also independently open at the skin
surface around the genital organs, tip of the nose, and edges of the lips.
Sebaceous glands secrete an oil (sebum) to lubricate the hairs and also cover
the skin with a film of oily coating. The oily secretion of sebaceous glands
contains waxes, fatty acids, and cholesterol, which makes the skin
pliable.
Sebaceous glands are absent in Manis (pangolin),
and Sirenia, and Cetacea which practically have no hairs. Modified sebaceous
glands form Meibomian glands in the eyelids, each has a long straight duct into
which separate alveoli open.
They produce an oily secretion which forms a film over the lacrimal fluid or tears holding them evenly on the surface of the eyeball for keeping the eye moist, in weeping the oily film are broken and tears flow out. Ceruminous glands of external auditory meatus are modified sebaceous glands. Their greasy or waxy secretion, called the cerumen traps the insects and dust particles.
vii. Scent Glands:
Scent glands are modified sebaceous glands or sweat glands. Their secretion is an allurement to the opposite sex. Scent glands are located in the deer family on the head near the eyes. Skunks and carnivores have scent glands around the anus, and pigs and goats have scent glands between their toes.
viii. Preputial Glands:
These are found in many kinds of mammals. In
beaver, large sacs containing a secretion known as castoreum lie beneath the
skin on either side of penis and open by ducts into the prepuce.
ix.
Mammary Glands:
These are characteristic of mammals. They secrete
milk generally in the females for nourishment of the young. In monotremes both
sexes may secrete milk, this condition is called gynaecomastism which is not a
normal condition. Mammary glands of monotremes are compound tubular glands,
while in other mammals they are compound alveolar.
Mammary glands of monotremes have no nipples, they
open into pits on the surface of the skin, and the young ones obtain milk by
licking tufts of hairs. In others the mammary glands open by their ducts
separately into a nipple.
The nipple is a raised outgrowth of the breast,
bearing opening of mammary glands (true teats). The false teat has an elongated
cistern into which the mammary glands open by their ducts. From the cistern a
tube leads to the surface of the nipple.
Mammary glands along with fat form integumentary
swellings called mammae or breasts. The number and location of mammae varies in
different mammals. The number ranges from two in many mammals to 25 in opossum.
Mammae may run along two ventral milk lines from the arm pits to the groin, or
they may be axillary, thoracic, abdominal or inguinal in position.
2.
Epidermal Scales and Scutes:
Epidermal Scales:
After the evolutionary loss of dermal scales of
fishes, amniotes developed an entirely new type of scale derived from the
epidermis. The skin of vertebrates is rarely naked, and it is usually provided with
protective scales, bony plates, feathers or hairs. There are no epidermal
scales in fishes and amphibians. They appear for the first time in
reptiles.
They are cornified derivatives of the Malpighian
layer and are generally shed and replaced. The scleroprotein called the keratin
is continually accumulated in the permanently growing layer of the epidermis
called the stratum germinativum.
These cells are called cornified and they become
dead. Thus, the stratum corneum cells become cornified and form hard horny
structures, such as scales and scutes in reptiles, beaks, claws, horns, nails,
hoofs, feathers and hairs in different vertebrates.
Scales are most noticeable on lizards and snakes.
They are continually being produced by the permanently growing layer of the
stratum germinativum and are generally folded so as to overlap one another.
When they are fully grown they become separated from the stratum germinativum
and appear as nonliving, cornified structures.
The epidermal scales form a protective covering of
the body (a continuous armour). The scales in snakes and lizards are continuous
and undergo ecdysis periodically. Before ecdysis, the new scales that will
replace the old ones are formed. The entire corneal layer of scales is shed as
whole in snakes. The old epidermal covering becomes loosened first in the head
region.
This skin, including even the transparent plates
over the eyes, is turned back and the snake finally crawls out of the old
covering, leaving the old covering inside out. The scales on the ventral side
in most snakes differ from their other scales in being long and transversely
arranged, they aid in locomotion.
Turtles and crocodiles have different kinds of
scales which do not overlap, nor undergo periodic ecdysis, but the scales are
gradually worn off and replaced. Large epidermal scales, such as those on the
shell of turtles and on the head of snakes, are generally called scutes. In
birds the scales are confined to the shanks and feet and some at the base of
the beak.
They generally overlap as in snakes and lizards. In
mammals epidermal scales are found on the tail and paws of rats, mice, beavers,
musk rats and shrews. These scales are not much cornified, nor do they undergo
ecdysis; hairs project from beneath the scales. In scaly ant-eaters large
epidermal scales cover the entire body, except the ventral side, the scales are
reptilian and undergo ecdysis singly.
In armadillos there are large scales which fuse to
form plates on the head, shoulders and hips; in the middle of the body, except
mid-ventrally, the scales fuse to form ring-like bands; these scales do not
undergo ecdysis but are gradually worn off and replaced.
Some epidermal scales of the tail of a rattle-snake
are modified to form a rattle. It consists of a series of old dried scales.
Rattles of rattle snakes represent horny remnants of the skin which adhere to
the base of the tail and are not lost during ecdysis.
During ecdysis the scale at the tip of the tail is
not shed and it forms a ring. Thus, after several ecdysis a series of rings
form a rattle, each new ring being larger than its predecessor. In older rattle
snakes the terminal rattles are often lost. In tortoises and turtles, the
dermal plates forming the carapace and plastron are covered externally with
cornified epidermal scales.
These are not shed regularly like lizards and snakes. A few turtles lack scales and have a leathery skin. The bodies of alligators and their relatives are also covered with epidermal scales which gradually wear off and are replaced.
In turtles, tortoises and modern birds there are no
teeth. Each jaw bone being covered with horny sheath formed of several fused
plates or scales which form a beak. In monotremes there is a soft bill which
differs from the beak of birds is not being covered with modified epidermal
scales.
3.
Horns:
Horns are found in ungulate (even-toed hoofed)
mammals only. True horns of the hollow type are found in pronghorn, cattle,
antelope, sheep and goats consist of an inner core of bone which is an outgrowth
from the frontal bone. It is encased in a keratinised, epidermal covering. True
horns continuously grow throughout life and are not shed.
Following types of horns are recognised:
(a) Rhinoceros
or keratin fibre horn has no skeletal element. It is made by keratinised
cells of the epidermis and consists of matted keratin fibres bound together,
but its fibres are not true hair. It is a permanent epidermal structure and if
broken it grows again. There is one horn in the Indian rhinoceros and two in
the African species.
(b) Pronghorn
is a true horn, consists of a permanent projection of the frontal bone covered
by a hard, horny epidermal sheath. The sheath is forked bearing one to three
prongs made only of horny sheath. The horny sheath is shed annually and is
replaced by another which grows from the skin that surrounds the core. It is
found in Russian antelope Antilocapra.
(c) Giraffe
Horns: They develop from
cartilaginous protrusions which are present at birth. They ossify and fuse at
the top of the skull, where they appear as knobs permanently covered with
living skin and hair. Giraffe possesses three of these knobs, one is median and
anterior to the other two. These horns are short, unbranched and are permanent,
and are present in both sexes.
(d) Antlers
are found in the males of deer family, but they are present in both sexes in
reindeer and caribou. An antler consists of a branching solid outgrowth of the
frontal bone formed of dense connective tissue. It is covered during growth by
hairy, vascular skin called ‘velvet’. The velvet is shed exposing the antler
naked when the antler reaches full growth.
Thus, the antler consists only of dermal bone. The bony antler is also shed annually after the breeding season, and a new antler develops. Antlers are solid mesodermal bone, but they are formed under the influence of the integument. Formation of antlers is controlled by the hormones of testes and anterior lobe of the pituitary.
4.
Digital Structures:
In amniota the distal ends of digits have claws, nails or hoofs formed from the horny layer of the epidermis. They grow parallel to the surface of the skin and are built on the same plan.
i. Claws: Claws made their appearance first in the
reptiles. A claw is made of a hard horny dorsal scale-like plate called unguis
and a relatively soft ventral subunguis, both converge terminally and cover the
terminal part of the last phalanx. Claws of reptiles and birds are similar but
in mammals the subunguis is much reduced and is continuous with a pad at the
end of a digit. In cat family claws are retractile.
ii. Nails: They are found in primates. The dorsal
unguis is large and flat and subunguis is soft and much reduced. Tip of the
digit forms a sensitive and vascular pad over which the nail groove is present.
It is formed by the invagination of epidermis. Growth of the unguis takes place
from the nail root lying below the skin in the nail groove.
iii. Hoofs:
They are found in ungulates. The horny unguis is thick and around the end
of the digit, and encloses the thickened subunguis which touches the ground.
Subunguis surrounds the soft, horny cuneus. Tip of digit, thus, forms a pad
containing a blunt phalanx. Nails and hoofs of mammals are modified claws.
Whalebone plates of toothless whales are also the modification of stratum
corneum.
5.
Feathers:
Feathers are found only in birds and are formed
from the epidermis in which the stratum corneum is highly specialised. Feathers
are light, strong, elastic, waterproof and show many colours due to pigments
and structural arrangement. The pigments are carotenoids and melanins.
Carotenoids are frequently called lipochromes which are soluble in fat solvents
like methanol, ether or carbon disulphide, and insoluble in water.
Animal red is zooerythrin and animal yellow is
zooxanthin are the two groups of carotenoids. Melanins are soluble only in
acids. Eumelanin granules vary from black to dark brown and phaeomelanin
granules may be almost colourless to reddish brown.
Many of the feather colours are the product of both
carotenoids and melanin. They form a protective covering (conserve body heat),
provide buoyancy to the water birds during swimming and at rest, regulate body
temperature and support the body in flight.
There are three kinds of feathers in birds, they
are contour feathers, down feathers, and filoplumes. The development of feather
is like that of scales.
Shedding and replacement of feathers is moulting
which takes place gradually, one moulting usually takes an average time of six
weeks. At the base of each feather follicle a dermal papilla persists from
which new feathers will form, so that there is a continuous replacement of
feathers throughout life. The replacement in some birds is seasonal, while in
others it is gradual throughout the year.
Feathers of birds show varied and often brilliant
colourations. The colours are due to three factors:
(a) Pigment is deposited in the feathers during
development, the colour that is seen is due to absorption of some wavelengths
of light by the pigment, thus, black, red, brown, yellow, and orange colours
are seen. Blue, most greens and violets are not the result of pigment but
depend entirely on the feather structure. Blue feathers in blue birds do not
contain any blue pigment, but they absorb all, but the blue rays of the
spectrum are reflected. White colour is not due to white pigment, but is
caused, due to reflection of light from the feather without absorption of any
wavelengths of light rays;
(b) Structural arrangement or striations of feather
surface are prismatic, these cause iridescence due to reflection of light,
thus, producing iridescent hues, metallic colours, gray, and some shades of
blue;
(c) A combination of pigments and prismatic
striations of the feather produce green in which the yellow pigment combines
with the structural blue. The colours of birds are for concealment,
recognition, and sexual stimulation. The colour of various kinds of birds is
under genetic control but it may be modified by internal and external
factors.
The birds kept in captivity for several years get
their plumage colour changed from red to yellow. In captive birds this change
has been attributed by many to diet. Hormones also play an important role in
the control of feather colour. Estrogenic hormone given to male before moult
result in the assumption of female plumage. Similarly females may acquire male
plumage by giving the testosterone.
Oxidation and abrasion are external factors that
change in colour to a lesser or greater degree in most species of birds.
Carotenoids fades in sunlight.
6.
Hair:
Hair is found only in mammals. It projects at an
acute angle from the skin. Hair covers the entire integument in most (furred
mammals), but in others only traces are left, such as whales have only a few
core hairs on the snout.
But during development the body of the embryos of
all mammals is covered with a coating of fine hair called lanugo which is
usually shed before birth and replaced by a new one. Hair is entirely epidermal
in origin.
The hairs trap air which does not transmit body
heat and, thus, act as insulators for the body. In some animals the colours of
hair are protective. Hair in nostrils and ears prevent entry of dust, eyebrows
and eyelashes protect the eyes, vibrissae are delicate organs of touch (act as
tactile organs), hairs on the tail are used to drive away insects. In some
animals, such as lions and some monkeys the mane distinguishes the male. Hairs
are also modified in spines, scales, horns, etc., in some mammals.
Structure:
Hairs are not modified scales but are new
outgrowths of the epidermis only. A hair has an upper projecting shaft and a
lower root lying in a hair follicle which is a sunken pit in the dermis. The
shaft is made of only dead, keratinised cells. The part of the hair protruding
above the skin is dead.
At the base of the follicle the root is expanded into a bulb and growth of the hair takes place only in the root where the cells of the Malpighian layer divided actively. Below the bulb is a dermal papilla having connective tissue and blood vessels, it nourishes the hair. Beyond the bulb the cells gradually die so that the shaft is made of dead cornified cells.
The hair shaft has an external cuticle of
transparent overlapping cells which have lost their nuclei, inside the cuticle
is a cortex (middle part of hair) containing shrivelled cells and pigments, and
a central core or medulla having air spaces. In the follicle the hair root is
surrounded by two layers of hair sheath cells forming an outer and inner root
sheaths. They do not extend beyond the follicle. A sebaceous gland opens into
the upper part of the hair follicle for oiling the hair.
An arrector pili muscle composed of smooth fibres
extends from the upper part of the dermis to the basal part of the hair
follicle on the side towards which the hair slopes. It pulls the hair base
causing the hair to stand when an animal is confronted with danger. Hair does
not project vertically but at an acute angle from the skin.
Development
of Hair:
A thickening of the epidermis (stratum
germinativum) pushes into the dermis and becomes cup-shaped at its lower end.
The dermis extends into the cup forming a hair papilla which has blood vessels
for supplying nourishment. The epidermal down growth which at first is a solid
cord of cells now splits to form a central shaft of cornified keratinised
cells, and a space around it.
The epidermal cells around the space form the hair
follicle. The lower part of the hair follicle becomes swollen and is known as a
bulb. The cells of the follicle thickened and bud off a sebaceous gland. The
central shaft by addition of new keratinised cells from the root grows in
length and pushes through the solid epidermal cells to emerge outside the
skin.
Thus, the development of hair differs from that of
a feather. It is formed entirely from the solid column of epidermis, while in
the feather there is a mesodermal feather pulp extending into the hollow
quill.
II.
Dermal Derivatives:
The scales arise from the dermis and are, thus,
mesodermal in origin, they are found only in fishes, some reptiles and a few
mammals. Ostracoderm fishes, the earliest known vertebrates, had an armour of
large bony plates. These bony plates became very small in placoderms to give
rise to cosmoid scales which are not found in any living forms today (except in
Latimeria).
1. Cosmoid
scales were also present in primitive Choanichthyes. A cosmoid scale had four
distinct layers, the lowest layer is of isopedine or dentine resembled compact
bone, the next layer was a spongy bone with vascular spaces consisting of pulp
cavities having odontoblasts, the third layer was of hard compact cosmine with
canaliculi and the outermost layer was thin but hard vitrodentine or
enamel.
2. Ganoid
scales are the other type of scales found in earliest primitive bony fishes are
ganoid scales. This implies that two different lines of evolution with regard
to scales appeared very early in the history of fishes. A ganoid scale has a
basal layer of isopedine, above which there may be a reduced cosmine layer or
it may be absent and the uppermost layer is made of a hard, translucent
substance called ganoin. Ganoid scales with a reduced cosmine layer are found
in Polypterus, and with no cosmine in Lepidosteus.
3. The evolution of the ganoid scale by the
loss of its upper layer of ganoin gave rise to a thinner leptoid scale. There
are two types of leptoid scales, namely, (i) cycloid and (ii) ctenoid scales.
In the other line of evolution the cosmoid scale lost its three lower layers,
only the fourth enamel-like dentine layer was retained and somewhat elaborated
to form the placoid scale. Thus, in the present fishes there are four types of
dermal scales- placoid, ganoid, ctenoid and cycloid scales.
Embedded in the dermis of elasmobranchs in oblique rows and projecting from the epidermis are dermal denticles of placoid scales forming an exoskeleton. The placoid scales were evolved by loss of some layers from the cosmoid scales. Placoid scales are found only in elasmobranchs, in sharks they are very small and give the skin a rough texture, but in skates they are large.
Structure:
A placoid scale has a flat bony basal plate bearing
a trident spine which projects above the epidermis and points backwards. Inside
the spine is a pulp cavity containing pulp made of connective tissue, blood
vessels, and a layer of odontoblast cells.
The basal plate is made of calcified dentine, and
the spine has mostly dentine which is covered with a cap of hard modified
dentine called vitrodentine and not enamel as often stated erroneously. The
basal plate and spine are both of mesodermal origin. The cured skin of sharks
containing placoid scales is called shagreen which is used for polishing and
for handle covers.
Placoid scales are the fore-runners of vertebrate
teeth because the two have essentially the same form and structure and a
gradation from placoid scales to teeth is seen in the mouth of shark. Shark
teeth are enlarged placoid scales formed in the skin of jaws. But there are
objections to this supposition.
A more recent view is that both placoid scales and
teeth are modified remnants of the bony dermal plates found in the ancestral
ostracoderms and placoderms, so that teeth and placoid scales are homologous
structures. Moreover, it has been shown that there is no epidermal enamel layer
in placoid scales, but it is a layer of vitrodentine formed from dermal cells.
Enamel is the hardest substance in the body,
whereas vitrodentine is only a hardened outer layer of dentine. The enamel
organ does not secrete enamel, it only plays a role in shaping the spine, so
that the entire placoid scale is mesodermal like the scales of bony fishes,
whereas a tooth has an enamel covering derived from ectoderm.
The claim that a gradation from placoid scales to
teeth is observed in the mouth of a shark is interpreted as a divergence shown
between vitrodentine-covered placoid scales, on the one hand and enamel-covered
teeth on the other.
In bony fishes there are two kinds of leptoid scales- cycloid
and ctenoid scales. They have a thin layer of bony isopedine below which is a
thin layer of connective tissue.
Cycloid
scales are round, thick in the centre and thin towards the
margins. They have a lower layer of fibrous connective tissue and an upper
layer of bone-like isopedine which is elaborated to form dentine. They show
concentric lines of growth which indicate the age of the fish. The scales lie
embedded in the dermis diagonally overlapping each other.
The posterior part of each scale overlapping the anterior part of the scale behind, thus, covering the body with a double layer of scales. The exposed part (posterior) has a smooth edge, while the concealed part may have a wavy margin. Cycloid scales form a dermal exoskeleton in many bony fishes.
Ctenoid
scales probably arose from the more simple cycloid scales. They
have the same shape, structure and concentric lines as the cycloid scales, but
they differ in having small teeth or cteni on their free posterior part. Their
anterior concealed part may have notched or scalloped margin. Ctenoid scales
form a dermal exoskeleton of most bony fishes.
The cycloid or ctenoid scales covering the lateral
line canals are perforated by a vertical tube of the lateral line opening on
the surface. In some flat fishes grooves both cycloid and ctenoid scales are
present. In some catfishes there are no scales, in eels scales are very small
and embedded in the dermis. In seahorses scales form a continuous armour
covering the body. Scales of bony fishes arise only from dermis, they form a
protective exoskeleton but do not hamper movement.
In tetrapoda
dermal scales are known as bony plates or osteoderms. In Apoda (Amphibia) only
vestiges of osteoderms are present. They are found in pockets below the
epidermis and are not visible externally. Two most ancient groups of reptiles,
the turtles and crocodiles, have retained bony dermal plates.
Turtles have continuous osteoderms below the
epidermal plates of the carapace and plastron. These osteoderms form a rigid
dermal skeleton which becomes connected with the endoskeleton. In crocodiles
there are osteoderms below the epidermal plates only on the back and the
throat.
In birds and mammals there is a tendency for
elaborating epidermal structures with an accompanied reduction or loss of
dermal derivatives. In mammals, osteoderms are found in armadillos lying below
the epidermal scales. They are bony plates of spongy texture. Extinct
glyptodons had rigid bony armour of osteoderms. In some whales bony osteoderms
may be present on the back and the dorsal fin.
Girdles and Limbs in Tetrapoda:
Girdles:
The girdles and paired limbs of tetrapoda are
remarkably similar and have homologous parts.
Pectoral girdle has several cartilage bones. They are
a scapula, a suprascaphla, precoracoid, and a coracoid in each half. Between
the scapula and coracoid a glenoid cavity appears for the first time. Two
dermal bones, the clavicle and interclavicle are added in each half. But there
is a gradual reduction or loss of some bones.
Pelvic girdle has three cartilage bones in each half. They are ilium, ischium and pubis. There are no dermal bones like pectoral girdle. All three bones participate in the formation of an acetabulum cavity. A cotyloid or acetabular bone often forms a part of the acetabulum in mammals. Generally a large obturator foramen lies between the ischium and pubis. The two halves of the pelvic girdle often meet midventrally at the pubic or ischiac symphysis. In most tetrapoda the pelvic girdle is attached to the axial skeleton through the ilia articulating with the sacral vertebrae.
Homology:
The bones of the pectoral and pelvic girdles are
homologous, the scapula with the ilium, the precoracoid with the pubis and
cotyloid, and the coracoid with the ischium. The clavicle is a dermal bone and
is not homologous with the pubis which is a cartilage bone.
Limbs:
The limbs of tetrapoda are pentadactyl. They are used
not only for locomotion but also to support the weight of the body above the
ground. Thus, they are provided with joints. Each has three segments, the
stylopodium, zeugopodium and autopodium with joints between the segments. The
stylopodium is the upper arm (or thigh), having a single humerus (or
femur).
The humerus joins the pectoral girdle at the glenoid
cavity, while the femur joins the pelvic girdle at the acetabulum cavity. The
zeugopodium is the forearm (or shank). It contains two parallel bones, the
radius and ulna (or tibia and fibula). The radius and tibia are lateral, while
ulna and fibula are medial. Between the stylopodium and zeugopodium is an elbow
joint (or knee joint). The autopodium
has three divisions, a carpus or wrist (tarsus or ankle), metacarpus or palm
(metatarsus or sole), and digits forming fingers or toes. The carpus or tarsus
in the primitive condition has 10 bones in 3 rows, the first row has a radiale
(or tibiale), an intermedium, and an ulnare (or fibulare), in the second row
are generally two centralia, while the third row has five distal carpals (or
distal tarsals).
The metacarpus (or metatarsus) has five long
metacarpals (or metatarsals). The digits are generally five in number or
pentadactyl, with the first digit on preaxial side and the fifth on the
postaxial side. Each digit has a linear row of phalanges, they are 2, 3, 4, 5,
4 beginning from the first to the last.
There is some evidence that early tetrapoda had seven
digits, one on the preaxial side was prehallex (or prepollex), and the
postaxial digit was a postminimus. But these were probably additional bones of
the wrist or ankle.
Origin
of Pentadactyl Limbs:
I. Gegenbaur’s Theory:
It was erroneously believed by Gegenbaur that Dipnoi
were the ancestors of tetrapoda. The skeleton of their lobed fins, as seen in
Neoceratodus, consists of a jointed axis of numerous bones from which biserial
segmented radials and fin rays arose on both sides for supporting the fin. This
type of fin is called an archipterygium. Gegenbaur’s theory deriving the
tetrapod limb from the archipterygium is now discarded.
II. From Crossoptergians Fins:
The link between the fins of fishes and limbs of
tetrapoda is found in fossil crossopterygians. The lobed fins of extinct
Sauripterus and Eusthenopteron were like paddles. They are called
ichthyopterygium and suggest the beginnings of tetrapod limbs.
Their fin had a well-developed fleshy lobe covered
with scales and a paddle-like fin blade. In the lobe the skeleton had in the
first row a single basal comparable to the humerus (femur), in the second row
were two radials comparable of the radius (tibia) and ulna (fibula). The rest
of the radials are comparable to carpals (tarsals).
Terrestrial animals have acquired jointed limbs for
locomotion on land. The limb of an ancient extinct amphibian Eryops is known as
a cheiropterygium in which there is a humerus in the first row, radius and ulna
in the second row, followed by several carpals in diagonal rows, after which
there is a prepollex and five digits which are not present in the fin blade and
have been formed as new distal outgrowths.
If the skeleton of an ichthyopterygium is compared to
that of a cheiropterygium, it is easy to see how the transformation of a fish
fin into an amphibian limb took place. The former shows that even before
locomotion on land was adopted the arrangement of its skeleton had already come
to resemble that of land animals. Thus,
the crossopterygian fin or ichthyopterygium gave rise to a pentadactyl limb by
an elongation of the basal lobe and its skeleton, by a loss of the fin blade
and supporting fin rays and by formation and separation of terminal radials to
form digits.
Girdles
and Limbs in Tetrapoda:
1.
Amphibia (Frog):
Pectoral girdle is an arch enclosing the chest. The
two halves are united midventrally and closely related to the sternum. (In
toads the two halves overlap). Each half has a coracoid, precoracoid, clavicle,
paraglenoid cartilage, scapula and
suprascapula.
The two precoracoid catilages fuse ventrally. The clavicle is the only dermal bone. It is fused to and reinforces a cartilaginous precoracoid. Glenoid cavity is bordered by coracoid, paraglenoid, and scapula. A partly bony suprascapula reinforces the scapula. On the inner margin of the coracoid is an epicoracoid cartilage. Anura have two kinds of pectoral girdles-(a) firmisternal in which the two epicoracoid cartilages are fused mesially along their entire lengths, (b) arciferal in which the two epicoracoids are fused only along their anterior edges. A sternum having two separate parts is fused to the middle of the pectoral girdle. The pectoral girdle is reinforced anteriorly by an omosternum and posteriorly by a mesosternum and their cartilaginous expansions, the episternum and xiphisternum respectively.
Forelimb:
Humerus has a prominent deltoid ridge. Radius and
ulna fuse into a radioulna. There are five carpals in two rows, in the first
row the intermedium and ulnare are fused together, and there is a radiale.
There is a single centrale. The three carpals of the second row are fused
together. Thus, the number of carpals is reduced. Only four digits are present
besides a rudimentary pollex enclosed in the skin.
Pelvic
Girdle:
It is V-shaped and has a long ilium on each side. It
articulates with the transverse processes of the sacral (9th) vertebra. The
ilia act as shock absorbers when the animal lands after a jump. Pubis and
ischium are small. The pubis is only partly ossified. The two pubes and two
ischia are fused together to form a solid fulcrum.
All three bones join at the acetabulum cavity. Pelvic
girdle is different from that of any other vertebrate; with very long ilia it
forms a long lever for transferring the force from the hindlimbs to the
vertebral column in jumping.
Hindlimb:
The tibia and fibula fuse into a tibiofibula or
crural bone. There are five tarsals in two rows, the promixal row has two long
bones, an inner astragalus (ulnare) and outer calcaneum (fibulare) which make
the ankle very long for jumping. There are
five digits of which the first is called hallux. There is an additional digit
called calcar or prehallux enclosed in the skin. It probably represents an
additional tarsal and may be regarded as a remainder of a fin ray of the
ancestral fin. It helps in stretching the web of the foot.
2.
Reptilia (Varanus):
In reptiles the girdles and limbs are well-developed,
except in snakes and limbless lizards, but vestigial pelvis and hindlimbs are
found in some snakes.
Pectoral
Girdle:
It is well formed and joined to the sternum. Each half
has a bony coracoid and scapula, between which is a glenoid cavity. There is an
expanded suprascapula of calcified cartilage. Between the coracoids of the two
sides is an expanded epicoracoid cartilage formed by the fusion of two. In each
half there are three fenestrae between the coracoid and epicoracoid.
A T-shaped unpaired dermal bone called interclavicle
(episternum) lies midventrally. On the arms of the interclavicle are two
clavicles bracing the girdle, but they do not meet ventrally. Clavicles and
interclavicle are dermal bones. Attached to the coracoids is a sternum made of
cartilage.
In turtles the coracoids and scapulae are slender
rods, while the clavicle and interclavicle form dermal bony plates of the plastron.
Forelimb:
It is typical with no unusual feature. Humerus is
broad, radius and ulna are separate, carpals are in two rows: four in the first
row including a centrale, and five in the second row. There are five digits
ending in claws.
Pelvic
Girdle:
All three bones are fused inseparably to form a single
innominate bone in each half. Ilium points backwards and is firmly united with
two sacral ribs of the sacrum so that the articulation is post-acetabular. Both
pubis and ischium meet their fellows to form pubic and ischiac symphyses. At
the pubic symphysis is a epipubis cartilage and at the ischiac symphysis is a
hypoischium cartilage.
The pubis has a small foramen for the obturator
nerve. Between two innominate bones is a large cordiform (or ischiopubic)
foramen divided by a ligament into two obturator foramina. Acetabulum is
bordered by all three bones.
Hindlimb:
It is typical, but the tibia begins to become larger
than the fibula. The tarsals are reduced by fusion, the proximal row has two
bones and distal row has three, between the two rows of tarsals is an
intratarsal ankle joint where bending takes place. There are five digits ending
in claws.
In some reptiles, but not in Varanus, two structures
appear for the first time in the hindlimbs, first the calcaneum bears a
posterior calcaneal process or heel, secondly a sesamoid bone forms a patella
or knee cap in front of the tibia.
3.
Aves (Gallus):
The appendicular skeleton of birds is remarkably
uniform. The pectoral girdles and forelimbs are modified for flight. The pelvic
girdles and hindlimbs are modified to support the weight of the body in
standing and walking. Bones are tubular, spongy, and pneumatic so that they are
light, but lightness is achieved without any sacrifice of strength. The marrow
cavity has no marrow but there are supporting bony struts which make the bones
strong to bear stresses.
Pectoral
Girdle:
It lies far backwards towards the centre of the body.
It has a large, stout coracoid attached to the sternum at the coracoid groove
by means of a synarthrosis or immovable articulation. Coracoid is joined at its
other end at right angle with a sword-like scapula lying over and bracing the
ribs. A glenoid cavity is formed by an imperfect union of scapula and
coracoid.
It is greatly displaced above the centre of gravity,
consequently the coracoids are elongated. In front of and attached to each
coracoid is a thin clavicle. The two clavicles are joined ventrally to a small,
flat interclavicle forming a furcula or “wishbone”. In flying birds the furcula
is joined to coracoids and is well developed. In non-flying birds it is reduced
or absent.
The arched clavicles and strong coracoids brace the
sternum and resist the inward pressure of the powerful down-stroke of the
wings. At the junction of coracoid, scapula, and clavicle is a foramen
triosseum through which the tendon of a pectoralis minor muscle passes and is
inserted on the humerus.
Forelimb:
Forelimb and its skeleton are modified for flight. The
wings are attached high up on the thorax towards the centre of the body for
balancing the weight in flying. The forelimb has a stout humerus with a deltoid
ridge. Humerus has a pneumatic foramen through which an air sac enters making
the bone spongy and light.
Air in pneumatic bones oxygenates blood and adjusts
air pressure when a bird descends from a great height. Such bones have no
marrow. Radius and ulna are well formed. The ulna is more strongly developed
than the radius; the ulna is large for attachment of flight feathers. Carpus
has four carpals in two rows-in the first row are radiale and ulnare, but the
carpals of the second row fuse to three fused metacarpals to form a
carpometacarpus.
Of the three metacarpals the first is much reduced,
all three are fused proximally, but distally the second and third are free
forming two rods. There are three digits- the first has one phalanx, second has
two phalanges, and third has one phalanx. The digits are reduced to almost a
one-fingered condition. Muscles of the upper arm are large and strong, those of
the forearm are reduced, and those of the hand have atrophied.
Pelvic
Girdle:
It has a longitudinal curvature which distributes the
body weight evenly in bipedal locomotion. Ilia and ischia have become long and
wide and lie longitudinally. Large expanded ilium extends in front and behind
the acetabulum, and is fused to the synsacrum along its entire length. Ischium
is broad and extends posteriorly. It lies parallel to and is fused with the
ilium, an ilioischiatic foramen lies between the two bones.
A slender pubis extends backwards and ends freely.
Between the ischium and pubis is an obturator foramen. All three bones are fused
together to form an innominate bone which has an incompletely ossified
acetabulum with a foramen.
Pubic and ischiac symphyses are absent so that
large-shelled eggs can be laid. But in ostrich (Struthio) there is a pubic
symphysis, while in Rhea, the South American ostrich, there is an ischiac
symphysis.
Hindlimb:
These are modified for bipedal locomotion. A strong
femur articulates at the acetabulum. Fibula is much reduced and is short. The
tibia has a chemical crest, in front of the junction of femur and tibia. A
sesamoid bone forms a patella. The tibia is fused with the proximal tarsals
(astragalus and calcaneum) to form a tibiotarsus.
The distal tarsals fuse with the second, third, and
fourth fused metatarsals to form a compound bone, the tarsometatarsus. The
ankle joint is between the two rows of tarsals is intratarsal in position. The
first metatarsal is free and projects near the lower end of the
tarsometatarsus.
In the fowl and some other birds a bony spur projects
backwards from the tarsometatarsus, it has a horny epidermal covering. The spur
is more developed in male birds for fighting. The digits are never more than
four, having 2, 3, 4, 5 phalanges. The last phalanx of each digit bears a claw.
The first digit (hallux) usually points backward, while the other three digits
are directed forward.
4.
Mammalia:
The appendicular skeleton of mammals shows much
variation from a reptile like condition to a high degree of
specialisation.
Pectoral girdle of Ornithorhynchus, (a monotreme), has
a scapula, coracoid, and a cartilaginous precoracoid (epicoracoid) on each side
and there is an expanded suprascapula. Between coracoid and scapula is a
glenoid cavity on each side. Ventrally
the two coracoids are connected with the manubrium of the sternum. Between the
precoracoids is a median T-shaped episternum (interclavicle) forming a
connection between the girdle and sternum. Two clavicles rest on the episternum
and each is attached to a simple acromion of the scapula. Clavicles and
episternum are dermal bones. Such a pectoral girdle is like that of a
reptile. In most mammals the
precoracoids and interclavicle are lost, and the coracoids are much reduced,
the clavicle may be a strong bony arch from the scapula to the sternum or it
may be reduced or even lost. When the clavicles are lost all connection between
the pectoral girdle and axial skeleton (sternum) disappears.
Pectoral
Girdle (Rabbit):
Each half of the girdle has a triangular scapula or
shoulder blade, at the end of which is a glenoid cavity. Near the glenoid
cavity there is a coracoid process formed by the reduced coracoid. On the outer
surface of the scapula is a spine which has two projections, an acromion
process and a metacromion process for attachment of muscles.
There is a clavicle, a dermal bone forming a slender
rod attached by ligaments to the acromion process and manubrium of the sternum.
Clavicles are retained only in those mammals which have an extensive movement
of forelimbs, in others they are reduced or lost.
Forelimb:
The limbs of most mammals are capable of extensive
rotation, extension and bending. Humerus is stout with a deltoid ridge; the
distal end has two expanded condyles between which are a pulleys or trochlea
for articulation with ulna. Above the trochlea is an olecranon fossa perforated
by a supratrochlear (supracondyle) foramen for the passage of a nerve and
brachial artery. Radius and ulna are separate
but tightly bound together. In primates, with prehensile forelimbs, the radius
and ulna are not fixed but the radius can rotate over the ulna to turn the hand
to either a prone or a supine position.
The ulna is produced into an elbow or olecranon
process, in front of which is a sigmoid notch for articulation with the
humerus. Carpus has eight bones, three in the first row, then a small centrale
and four distal carpals. There are five metacarpals having five digits with 2,
3, 3, 3, 3 phalanges ending in claws. In
primates the first digit or pollex is independent of the others. It can be
brought opposite the other digits and palm. This opposable thumb can handle
minute objects. This factor has led to a superior position of primates.
Pelvic
Girdle:
In each half the ilium, ischium, and pubis fuse to
form an innominate bone. The two halves meet by a midventral pubic symphysis.
In rabbit and some others there is a small acetabular (cotyloid) bone at the
lower margin of the acetabulum. The acetabulum lies where all three bones meet
(except the pubis). There is a large obturator foramen between the pubis and
ischium on each side. The two ilia articulate with the transverse processes of
the first vertebra of the sacrum.
Hindlimb:
Femur is strong with a head for articulation with the
acetabulum. There are three trochanters for attachment of muscles, distal end
of femur has two smooth condyles. Tibia is strong and has a cnemial crest,
above is a sesamoid bone or patella formed in the tendon of triceps femoralis
muscle. It forms a knee cap which reinforces the ligaments of the knee
joint.
Fibula is reduced and fused distally to the tibia but
separate proximally. Tarsus has six bones, proximal row has astragalus and
calcaneum, its postior projection, the calcaneal process forms a heel. Centrale
lies in front of the astragalus, distal row has three tarsals.
There are four metatarsals bearing digits. Each digit
has three phalanges ending in claws. First digit or hallux is absent. In some primates
the hallux is opposable in the same way as the pollex of the hand.
Postures
of Limbs:
In terrestrial mammals there are three kinds of
postures of hands and feet in locomotion.
a. Plantigrade:
Plantigrade locomotion is the most primitive, in which
the entire hand and foot are in contact with the ground, as in bears, rabbit,
some insectivores, and man.
b. Digitigrade:
Digitigrade locomotion is used for increased speed. In
it only the digits are placed on the ground. The wrist and ankle are lifted
above, as in dogs, cats and wolves.
c. Unguligrade:
Unguligrade locomotion is an extreme condition in which only the tips of the digits are placed on the ground and the tips are covered with hoofs. This is the most specialised condition, as in deer, horse, and cattle.