When the series of ambulacral and adambulacral ossicles is followed inwards towards the mouth it is seen that the first ambulacral ossicle is closely fixed to the second, but is widely separated from its fellow, remaining, however, connected with the latter by a powerful adductor muscle. In consequence of the separation of this pair of ossicles each is brought into closer contact with the corresponding ossicle in the adjacent radius, to which it is connected by a muscle called the abductor. The first adambulacrals in adjacent radii are also brought into closer contact and carry long spines which, when the ambulacral grooves are contracted, project like a grating over the mouth. In the order of Asteroidea to which Asterias belongs, the adambulacrals themselves do not project much, but in all other cases they form prominent mouth-angles, so that the opening of the mouth becomes star-shaped (Fig. 211, p. [483]).
Except in the case of the ambulacral and adambulacral plates little regular arrangement is to be detected in the ossicles of the skeleton which, as has already been mentioned, form a mesh-work. If, however, the arm be cut open and viewed from the inside it will be seen that the edge is strengthened above and below by very thick, powerful, rod-like plates. These are called the "supero-marginal" and "infero-marginal" ossicles; they are not visible from the outside, since they are covered by a thick layer of the body-wall containing other smaller plates (Fig. 190, marg). In many genera, however, they are exposed, and form a conspicuous edging to the arm above and below. In many genera, also, there are three conspicuous series of plates on the back of each arm, viz. a median row, called "carinals" (car., Fig. 191), and two lateral rows, termed "dorso-laterals" (d.lat., Fig. 191). These three rows, with the two rows of marginals, one of ambulacrals, and one of adambulacrals on each side (11 rows in all), constitute the primitive skeleton of the arm, and appear first in development.
The structure of all these elements of the skeleton is the same. They may be described as scaffoldings of carbonate of lime, interpenetrated by a mesh-work of cells fused with one another, by which the carbonate of lime has been deposited. The matrix in which the ossicles lie is a jelly-like substance traversed by a few bands of fibres which connect the various rods with one another. This jelly is almost fluid in the fresh state, but when heated forms a hard compound, possibly allied to mucin, which will turn the edge of a razor.
When the covering of the back is dissected off the coelom is opened. This is a spacious cavity which apparently surrounds the alimentary canal and extends into the arms. It has, however, its own proper wall, which is called the "peritoneum," both on the outer side, where it abuts on the skin, and on the inner side, where it comes in contact with the wall of the alimentary canal. The outer wall is called the "somatic peritoneum," and it is possible to dissect off the rest of the body-wall and leave it intact; the inner wall, from its close association with the alimentary canal, is termed the "splanchnic peritoneum." This wall can only be distinguished in microscopic sections from that of the alimentary canal, to which it is closely applied.
The coelom is filled with a fluid, which is practically sea water with a little albuminous matter in solution. Through the thin walls of the papulae oxygen passes into this fluid, whence it easily reaches the inner organs, since they are all in contact with some part of the coelomic wall. Similarly CO2 is absorbed by the coelomic fluid from all parts of the body, and diffuses through the papulae to the surrounding water.
The Starfish possesses no definite kidney for getting rid of nitrogenous waste. In most of the higher animals with a well-developed coelom it has been proved that the kidney is simply a specialised portion of the coelom, and in many cases some parts of the coelomic wall still retain their excretory functions, which apparently the whole originally possessed. In the Starfish and in Echinodermata generally this primitive state of affairs is still retained. From the cells forming the coelomic wall, cells are budded off into the fluid, where they swim about. These cells from their movements are called amoebocytes. If a substance such as indigo-carmine, which when introduced into the tissues of the higher animals is eliminated by the kidney, is injected into the Starfish, it is found soon after to be vigorously absorbed by the amoebocytes. These later accumulate in the dermal branchiae, through the thin walls of which they make their way[[442]] to the outside, where they degenerate.
The coelom is indented by five folds, which project inwards from the interradii. These folds are called the "interradial septa"; they are stiffened by a calcareous deposit, which is not, however, sufficiently dense to constitute a plate. In one of the septa the axial sinus and stone-canal (see below) are embedded. These septa are to be regarded as areas of lateral adhesion between the arms.
Fig. 188.—View of upper half of a specimen of Asterias rubens, which has been split horizontally into two halves. ax.c, Axial sinus; g.d, genital duct; oe, cut end of the oesophagus, the narrow neck of the stomach; py, pyloric sac; py.c, pyloric caeca; r, rectum; r.c, rectal caeca; sept, interradial septum; st.c, stomach lobe.
The alimentary canal consists of several distinct portions. The mouth leads by a narrow neck called the "oesophagus" into a voluminous baggy sac termed the "stomach," which is produced into ten short pouches, two projecting into each arm. The stomach leads in turn by a wide opening into a pentagonal flattened sac, the "pyloric sac," which lies above it. Each angle of the pyloric sac is prolonged into a tube—the so-called "pyloric duct"—running out into the arm, where it immediately bifurcates into two forks, each beset by a large number of small pouches and attached to the dorsal wall of the coelom by suspensory bands of membrane called mesenteries. These ten forks are called "pyloric caeca"; they are of a deep green colour owing to the pigment in their wall. Beyond the pyloric sac the alimentary canal is continued as the slender "rectum" to the anus. The rectum gives off two small branched pouches of a brown colour called "rectal caeca." This comparatively complicated form of alimentary canal is related to the nature of the food of the animal and the method it employs to capture its prey.