The members of this remarkable order, like those of the preceding one, are burrowers; but though their feelers are larger, the rest of their anatomy has undergone much more profound modification than that experienced by the Molpadiida. The loss of the radial canals, which must be practically functionless in Molpadiida, is not a great step, but the change in the mode of respiration is a greater modification. Respiration appears to be effected by diffusion through the body-wall, which is always comparatively thin. The circulation of the body-cavity fluid is assisted by a number of stalked, ciliated cups placed on the mesenteries near the line of their insertion on the body-wall. In dealing with Asteroidea it was pointed out that the ends of the tube-feet are the only places where numerous sense-hairs are to be found, and which, therefore, can be called sense-organs. This is true generally throughout Echinodermata. Now in Synaptida, where the tube-feet are lost, the surface of the body has scattered over it little sense-organs consisting of hillocks of ectoderm with an aggregation of sense-cells. These may be regarded as representing the discs of the missing tube-feet. One is involuntarily reminded by the ciliated cups and scattered sense-organs of the ciliated urns and sense-organs of the Sipunculidae, which lead a similar life; and taking into consideration the general superficial likeness in appearance of the two groups, the epigram is almost justified that "if the Synaptida were not extremely careful they would become Gephyrea."
This order is represented in British waters by three species of the genus Synapta, which is remarkable for possessing, as ossicles, only the peculiar anchors attached to anchor plates. The present author has dug up the commonest species (S. inhaerens) from its burrows in the sand at low water in the Clyde. These animals seem to seek their food at the surface; the feather-shaped feelers are used to seize small algae and zoophytes, of which the food apparently consists. If seized, S. inhaerens readily amputates the posterior part of the body, whilst the head with its feelers immediately buries itself. The other genera of the order (except Anapta) are characterised by the possession of wheels with spokes as their characteristic ossicle, as the names Trochodota, Trochoderma, Acanthotrochus bear witness.
The only fossil remains of Holothuroidea consist of isolated ossicles—wheels, gratings, anchors, etc.—which first make their appearance in the Carboniferous limestone and tell us practically nothing of the evolution of the group. From a comparison with one another of the living families, certain conclusions can be drawn. The Aspidochirote feeler and the method of using it recall forcibly the shape and function of the buccal tube-feet of Spatangoidea. It is probably safe to assume that it is the primitive form from which the other forms of feeler have been derived. Secondly, the anal respiration and the curious internal madreporite have been developed in correlation with one another, and are like nothing found elsewhere among the Eleutherozoa. Hence we may with high probability assume a Protoholothuroid stock with shield-shaped feelers but devoid of respiratory trees, and with an external madreporite. From this stock the Elasipoda developed by migrating into deeper water, whilst the Pelagothuriida sprang from the same root by taking to swimming; the Aspidochirota constituting the main line. The Dendrochirota were developed from a stock with respiratory trees and internal madreporite—in a word, from Aspidochirota. From them the Synaptida and the Molpadiida have developed as offshoots at different periods through taking to a burrowing life. These relationships are shown by the following diagram:—
CHAPTER XX
ECHINODERMATA (CONTINUED): PELMATOZOA—CRINOIDEA = SEA-LILIES—THECOIDEA—CARPOIDEA—CYSTOIDEA—BLASTOIDEA
SUB-PHYLUM II. PELMATOZOA
The Pelmatozoa differ from the Eleutherozoa in several important respects. They are fixed (at any rate in the young stage) by the centre of the aboral surface, and this portion of the body usually takes on the form of a stem supported by a definite series of ossicles, so that we can discriminate a "calyx"—the main part of the body—from the "stem." Further, the podia and the ambulacral grooves seem to be always covered with powerful cilia, which are employed in producing a current which sweeps small organisms to the mouth. The podia are never locomotor in function; their use is similar to that of the tentacles on the lophophore of Polyzoa and Brachiopoda.
The living Pelmatozoa are very few in number compared with the extinct forms. It may with justice be said that the group is nearly extinct; indeed, out of its five classes one alone, and that the most highly specialised class, survives till the present day. Now we have already seen that, in the case of the Eleutherozoa, if the annectant fossil types were taken into consideration, the definition of the classes would be difficult, so that it is not to be wondered at if the classes of the Pelmatozoa are also somewhat difficult to define; and it must be added that this difficulty is not only due to the fact that intermediate types occasionally occur, but also to our ignorance of the functions of many structures found in fossil types, speculations regarding which are to be received with caution. Bearing in mind, then, the provisional nature of the classification, we may give the diagnoses of the principal divisions as follows:—
Class I. Crinoidea.—Pelmatozoa provided typically with a well-marked stem; calyx consisting of an aboral "patina" of two or three circles of plates, and a flexible "tegmen" or oral surface with small plates or none; radial canals supported by long branched arms, which are developed as direct prolongations of the uppermost circle of plates in the patina.