i ) Shape , size and Colour :
Asterias has a radially symmetrical and pentamerous body. The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrical spaced projections, the rays or arms. In some genera , the number of arms may be more than five, for example, there are 7-14 arms in Solaster and more than 40 arms in Heliaster.
The size varies from 10-20 cm in diameter though some forms may be much smaller or longer. The colour is variable having shades pf yellow, orange, brown and purple. The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface.
The lower surface is flat, less pigmented and is called the oral or actinal surface. The oral and aboral surfaces are not the ventral and dorsal surfaces but correspond to the left and right sides of the bilaterally symmetrical larva. The axes occupied by the arms are known as radii and the regions of the central disc between the arms are inter-radii. A well defined head is entirely absent.
ii ) Oral Surface :
The side of body, which in natural conditions remains towards the substratum and contains the mouth or oral opening, is flat and a dark orange to purplish colour, is called oral or actinal surface.
The oral surface bears the following structures :
1. Mouth :
On the oral surface, in the centre of the pentagonal central disc is an aperture, the actinosome or mouth. It is a pentagonal aperture with five angles, each directed towards an arms. The mouth is surrounded by a soft and delicate membrane, the peristomial membrane or peristone and is guarded by five groups pf oral spines or mouth papillae.
2. Ambulacral Grooves :
From each angle of the mouth radiates a narrow groove called the ambulacral groove which runs all along the middle of oral surface of each arm.
3. Tube Feet or Podia :
Each ambulacral groove contains four rows of locomotory, food capturing, respiratory and sensory organs called tube feet or podia. The tube feet are soft, thin- walled, tubular, retractile structures provided with terminal discs or suckers. The suckers function as suction cups to afford a firm attachment on the surface to which they are applied.
4. Ambulacral Spines :
Each ambulacral groove is bordered and guarded laterally by 2 or 3 rows of movable calcareous ambulacral spines which are capable of closing over the groove. Near the mouth, these spines often become larger, stouter, assemble in five groups, one at each inter-radius of disc and are called mouth papilla.
Outside the ambulacral spines are three rows of stout immovable spines, beyond which occurs another series of marginal spines along the borders of the arms demarcating the oral from the aboral surface.
5. Sense Organs :
Sense organs include five unpaired terminal tentacles and five unpaired eye spots. The tip of each arm bears a small median, non-retractive and hollow projection, the terminal tentacle. It acts as a tactile and olfactory organ. At the base of each tentacle occurs a bright red photosensitive eye spot made up of several occulli.
iii) Aboral Surface :
The side of the body, which remains directed upwards or towards the upper surface, is convex and of light orange to purplish colour, is called aboral or abactinal surface.
The aboral surface bears following structure :
1. Anus :
A minute circular aperture, called the anus, is situated close to the central disc of aboral surface.
2. Madreporite :
At the aboral surface or the central disc occurs a flat, sub-circular, asymmetrical and grooved plate called madreporite plate or madreporite between the bases of two of the five arms. The surface of madreporite is marked by a number of radiating, narrow, straight or slightly wavy grooves wit pores in them. The madreporite is, thus, a sieve-like porous plate and it leads to the stone canal of water vascular system.
The number of madreporite to an individual though remains one, but the presence of more than one madreporite in some species is due to the increase in number of arms beyond the normal number of five.
The two arms having madreporite between their bases are collectively referred to as a bivium and the other three arms as a trivium. The symmetrical position of madreporite, thus, converts the radial symmetry of Asterias into bilateral symmetry.
3. Spines :
The entire aboral surface is covered with numerous short, stout, blunt, calcareous spines or tubercles. The spines are variable in size and are arranged in irregular rows running parallel to the long axes of the arms. The spines are supported by the irregularly-shaped calcareous plates or ossicles which remain buried in the integument and form the endoskeleton.
4. Papulae and Gills :
Between the ossicles of integument are present a large number of minute dermal pores. Through each dermal pore projects out a very small, delicate, tubular or conical, finger-like or thread-like, thin-walled, membranous and retractile projection called the dermal branchia of gill or papula.
The papulae are hollow evaginations of the body wall and their lumen remains in continuations with the coelom. They are internally lined by coelom. They have respiratory, as well as excretory functions.
5. Pedicellariae :
Besides the spines and gills, entre aboral surface is covered by many whitish modified spine-like tiny pincers or jaws called pedicellariae. The oral surface also bears pedicellariae. Each pedicellaria consists of a long or short, stout, flexible stalk having no internal calcareous support.
The stalk bears calcareous ossicles or plates a basilar piece or plate at the extremity of the stalk and jaws or valves which remain movably articulated with the basilar piece and serrated along their apposed edges. pedicellariae having three calcareous pieces and a stalk are called forcipula repedunculate pedicellariae.