Fig. 196.—Dissection of Ctenodiscus to show the Polian vesicles. amp, Ampullae of the tube-feet; nerv.circ, nerve-ring; Pol, Polian vesicle; sept, interradial septum; stone c, stone-canal; T, Tiedemann's body; w.v.r, water-vascular ring. × 1.

The stone-canal is rarely repeated, but this occurs in the aberrant genus Acanthaster, where there may even be several in one interradius, and each stone-canal has an axial sinus, genital stolon, and madreporite annexed to it. According to Cuénot, in Asterias, when 6-rayed specimens occur in a species normally 5-rayed, there are two stone-canals, suggesting that the repetition of stone-canals is a suppressed effort at multiplication by division. This is also true of Echinaster, but in Ophidiaster two madreporites may occur in an individual with five arms. In the Asterinidae the Y-shaped fold which projects into the cavity of the stone-canal is feebly developed, whereas in the Pentacerotidae it meets the opposite side of the stone-canal, and in Culcita gives out branches which reduce the cavity of the canal to a series of channels. In Echinasteridae and some Asterinidae, and in Astropectinidae and Pentacerotidae the ampullae become so deeply indented as to be almost divided into two, so that each tube-foot has virtually two ampullae.

The alimentary canal has a remarkably constant structure. The only important variation from the type, as described in Asterias, is found amongst the Astropectinidae and Porcellanasteridae, where the anus is wanting. In Astropecten the rectum and the rectal caeca still persist, but in Luidia even these have disappeared. The rectal caeca are remarkably variable structures. In Asterias there are two, but in Pentacerotidae there are five forked caeca, in Asterina five simple caeca, and in the Echinasteridae and Astropectinidae one large flat slightly 5-lobed caecum. In the Asterinidae the pyloric caeca are remarkable for the size of the enlarged basal portion in each radius, which serves as a reservoir for the juices secreted by the branched forks of the caecum. In Porcellanaster pacificus the pyloric caeca are vestigial, and in Hyphalaster moseri they are absent.[[454]]

The genital organs are, as we have seen, outgrowths from radial branches of the genital rachis. In most species, as in Asterias, they are limited to a single cluster of tubes on each branch of the rachis, but in the Astropectinidae and Pentacerotidae each branch gives rise to a large number of clusters, arranged in longitudinal series, each cluster having its independent opening to the exterior.

Asexual reproduction, as a regular occurrence, is not common amongst Asteroidea. If, however, a Starfish loses some of its arms, it has the power of regenerating the missing members. Even a single arm will regenerate the whole Starfish. Now in some cases (Astropectinidae, Linckiidae) Starfish will readily snap off their arms on irritation. In Linckia this occurs at regular intervals and the separated arm forms a new individual. In one of the Asterinidae, Asterina wega, a small Starfish with seven arms, transverse fission regularly occurs, a portion with three arms separating from one with four. The same is believed to occur in two species of Asterias, and as has already been pointed out, the repetition of the madreporite and stone-canal is, in many cases, possibly connected with this tendency to transverse fission.

Classification of Asteroidea.

Whilst there is considerable agreement amongst the authorities as to the number of families, or minor divisions of unequivocal relationship, to be found in the class Asteroidea, there has been great uncertainty both as to the number and limits of the orders into which the class should be divided, and also as to the limits of the various species. The difficulty about the species is by no means confined to the group Echinodermata; in all cases where the attempt is made to determine species by an examination of a few specimens of unknown age there is bound to be uncertainty; the more so, as it becomes increasingly evident that there is no sharp line to be drawn between local varieties and species. In Echinodermata, however, there is the additional difficulty that the acquisition of ripe genital cells does not necessarily mark the termination of growth; the animals can continue to grow and at the same time slightly alter their characters. For this reason many of the species described may be merely immature forms. In proportion, however, as the collections from various localities increase in number and size, difficulties connected with species will tend to disappear.

The disputes, however, as to the number of orders included in the Asteroidea proceed from a different cause. The attempt to construct detailed phylogenies involves the assumption that one set of structures, which we take as the mark of the class, has remained constant, whilst others which are regarded as adaptive, may have been developed twice or thrice. As the two sets of structures are often of about equal importance it will be seen to what an enormous extent the personal equation enters in the determination of these questions.

Where, as in Asteroidea, the internal organisation is very uniform, the best method of classification is to take as our basis the different methods in which the demands of the environment have been met. It is in this way, we hold, that divergence of character has been produced, for whilst species may differ in trifling details, families and orders differ in points of functional importance. The fact that one of the orders may have sprung from several allied species instead of one may be admitted, and at the same time the hopelessness of trying to push phylogenetic inference into details asserted.

Sladen, in his Monograph of the Asteroidea collected by the "Challenger" expedition, took for the basis of his system the presence or absence of distinct pavement-like marginal plates along the edges of the arms and the restriction of the papulae to the aboral surface, or their distribution over the whole surface of the body. What connexion, if any, the presence of these pavement-like plates has with the habits it is impossible to say, but it is unlikely to be of the high importance with which it was regarded by Sladen, for in the same family we have genera with inconspicuous marginals (Asterina) and others with conspicuous marginals (Palmipes). The restriction of the papulae to the back also varies within the same family (Linckiidae), and whilst, on the whole, it is perhaps a primitive arrangement, it is in many cases connected with burrowing habits, which can scarcely be deemed to have been the original mode of life of the class.