Fig. 295.—Four diagrams to explain the metamorphosis of the larva of Antedon rosacea. a.c, Anterior coelom; gen.st, genital stolon; l.hy, left hydrocoel; l.pc, left posterior coelom; mp, madreporic pore; r.p.c. right posterior coelom; stom, stomodaeum. (A, B, and C after Korschelt and Heider; D after Perrier.)
Now in reviewing this life-history we cannot fail to be struck with resemblances to the development of Asteroidea, and especially to that of Asterina gibbosa. The absence of a connexion between stomodaeum and gut is due to the embryonic mode of life. On the other hand, the presence of a long prae-oral lobe, containing an extension of the anterior coelom and having a fixing organ at its apex, can only be paralleled among Asteroidea. In broad outlines, then, up to the period of fixation the two developments are parallel, but after this point a divergence takes place, which points clearly to the splitting of the Echinoderm stem into two main branches, corresponding with two different sets of habits. In the Eleutherozoan stock, represented by the development of the Asteroidea, the disc became flexed ventrally on the stalk, so that the mouth and podia were brought within reach of material drifting along the bottom, which the podia were employed to seize. As a consequence the base of the stalk was brought near the mouth, and so it came about that the hydrocoel, when it became a ring, encircled both. In the Pelmatozoan stock, on the other hand, the podia and mouth are rotated upwards and backwards from the stalk, which thus came to have an aboral position (Fig. 296, B). The podia are thus placed in a favourable position for capturing free-swimming organisms, which their cilia sweep toward them. It is worthy of note that a similar change of position of the mouth occurs in other groups of animals which have similar habits (Polyzoa Entoprocta, Tunicata).
Fig. 296—Figures to show the supposed connexion of Eleutherozoa and Pelmatozoa. A, common fixed ancestor of the two stocks, still bilaterally symmetrical; B, primitive Pelmatozoon; C, primitive Eleutherozoon. a, Anus; a.c, anterior coelom; l.p.c, l.p.c1, left posterior coelom; m, mouth; p, primary pore-canal; r.p.c, right posterior coelom.
The division therefore of the phylum must have occurred at an extremely remote epoch, before the hydrocoel was a closed ring, and before, therefore, radial symmetry was completely attained.
Turning now to the question of the origin of the classes of Eleutherozoa, we find that the study of development strongly reinforces the views gained from the study of adult anatomy. The Asteroidea are the most primitive group; only in their case is the fixed stage retained, and both Ophiuroidea and Echinoidea pass through an Asteroid stage in development. The only serious competitors for the position are the Holothuroidea, which many have imagined to have been directly derived from Cystoidea (in the old sense; better Thecoidea). This view, though adopted by Semon,[[525]] Haeckel,[[526]] and Bather,[[527]] is open to many objections. The type of Holothuroid development referred to in these discussions is that of the extremely aberrant Synapta digitata, in which the radial canals are vestigial structures which disappear in the adult. In this species, where the feelers are multiplied, some originate in the larva directly from the water-vascular ring, and thus alternate with the canals. From this circumstance Semon drew the conclusion that the radial canals of Holothuroidea are not homologous with those of other Echinoderms, but this conclusion is contradicted by the development of more normal species, in which all the feelers spring from the radial canals. The meridional course of these canals, the closure of the ambulacral grooves, coupled with the retention of a nervous ectoderm, are all features found in Echinoidea. So is also a reduction in the number of the genital organs, on which Bell[[528]] laid such stress that he separated Holothuroidea from all other Echinoderms. But if in Spatangoidea a reduction to four and even three can take place (Fig. 246, p. [552]), why should a reduction to two or one excite surprise? The primitive outer appearance of the Auricularia is counterbalanced by the development of the coelom, which is much modified, so that the primitive bilateral arrangement is obscured. If, then, Asteroidea are the most primitive Eleutherozoa, we may imagine that primitive Echinoidea were derived from Asteroidea through adaptation to life in crevices, where an upward bending of the radii was of advantage, in order to enable the animal to attach its podia above as well as below itself; and that Holothuroidea arose from primitive Echinoidea in which the plates of the corona were still movable, through a further adaptation to narrow crevices, where worm-like wriggling would be the most successful method of adapting themselves to their environment.
The final result, then, of all our inquiries leads us to a view of the mutual affinities of the classes of Echinoderms, which may be indicated in the following table:—
We shall hazard the prophecy that if ever pre-Cambrian Echinoderms are found, there will be amongst them small stalked forms which may be superficially classed with "Cystids," but which are in reality the fixed ancestors of Asteroidea. They should have an irregular skeleton, and be devoid of arms, which are secondary formations; but they should indicate, by the proximity of the mouth to the stalk and by the relation to the stalk of the grooves for the podia, that they have diverged from the Pelmatozoan stock, and are the ancestors of Eleutherozoa.