Fig. 290.—Brachiolaria fixing itself, × 60. Ast, rudiment of the body of the Starfish; fix, fixing processes. (After Johannes Müller.)
The metamorphoses of the other types of Dipleurula contain no fixed stage. They are what might be called "cataclysmal metamorphoses." That is to say, the outer form and habits of the larva are preserved till the last moment, whilst the organs of the adult are being gradually perfected; then in an hour or two all trace of larval structures disappears. The Ophiopluteus preserves the larval mouth, round which the hydrocoel grows; the long lateral ciliated processes are preserved till the animal has attained all the adult characters. Before this, however, it passes through what may be called an "Asteroid" stage in development, in which the ambulacral grooves are open. The Echinopluteus loses both larval mouth and anus. It develops the adult organs on the floor of a sac-like invagination of the ectoderm, situated on the left side within a loop of the ciliated band (Fig. 291, B, C). This invagination becomes completely closed. It is termed the "amniotic cavity," and its roof is termed the "amnion." On its floor are developed the primary tentacles, terminating the radial canals, as well as a number of spines. After taking on a creeping life and losing its larval appendages, the young Sea-urchin passes through an "Asteroid" condition, in which the arched dorsal surface, the future periproct, is greater in extent than the ventral, and the radial canals run horizontally out from the water-vascular ring and terminate in free movable podia (Fig. 291, C and D, pod), ending in suckers, in the centre of which are pointed sense-organs. These podia become later enclosed in grooves in the corona, and are reduced to vestiges in the adult.
Fig. 291.—Four views of Echinopluteus from the left side, to show the metamorphosis. A, B, and C are taken from the development of Echinus miliaris. D is a young Echinus esculentus. The rudiment of the oral disc of the Echinus is seen beginning in B and larger in C. ad.stom, Adult stomodaeum; cil.ad, adoral ciliated band; cil.ep, ciliated epaulette; coe, coelom; d.sp, prismatic spine of dorsal surface (periproct) of adult; ech, rudiment of Echinus; int, intestine; l.oes, larval oesophagus; mp, madreporic pore; nerve circ, nerve-ring of adult; pod, first paired tube-feet; st, stomach; t, terminal tentacle of the radial band; v.sp, pointed ventral spine of adult. A, B, and D magnified 45 diameters; C, 60 diameters.
The Auricularia is the only type of Dipleurula in which larval mouth and anus are retained. For this reason it has been supposed that its median plane of symmetry remains the median plane of the adult. The researches of Bury[[522]] have shown that this is not so. As in other types of Dipleurula (with the possible exception of the Ophiopluteus) the adult position of the mouth is on the left side of the larva, and in the commencement of the metamorphosis the mouth migrates into this position (Fig. 286, C). Then the rudimentary prae-oral lobe is rapidly absorbed, so that the mouth again acquires a terminal position. The hydrocoel (Fig. 286, A, hy) has by this time completely encircled the oesophagus, and from it grow out the radial canals which bud off the feelers[[523]] (buccal tentacles) into the larval stomodaeum. This, although it later flattens out to form the adult peristome, forms in these stages an almost closed sac, reminding us of the amniotic cavity in the Echinopluteus. The ciliated band breaks up into a number of pieces, which rearrange themselves so as to form a series of transverse rings of cilia; so that the free-swimming life can be carried on somewhat longer. The animal in this stage is called a "pupa" (Fig. 292); it eventually loses the rings, drops to the bottom, and develops tube-feet. From specimens which the author has seen, he has little doubt that in some cases the young animal passes through an "Echinoid" stage, for it possesses, besides the feelers, only median tube-feet, terminating the radial canals, and it is covered by a cuirass of plates, which recalls the Echinoid corona.[[524]]
Fig. 292.—"Pupa" of Synapta digitata. × 50. circ.cil, Ciliated rings; oss, calcareous ossicle; ot, otocysts; pod, feelers; w.v.r, radial water-vessel. (After Semon.)
Reviewing the development of the Eleutherozoa in the light of the facts so far presented, and using the same method of reasoning which is employed in the case of other groups of animals, we seem to be justified in concluding that the Echinodermata are descended from a simple free-swimming ancestor possessing the fundamental characters of the Dipleurula. These would include a longitudinal folded band of cilia as the principal organ of locomotion; a thickened plate of nervous epithelium at the anterior end serving as combined sense-organ and brain; a V-shaped band of cilia projecting into the oesophagus as the organ of nutrition; a wide, shallow stomodaeum and an alimentary canal consisting of three well-marked divisions, viz. oesophagus, stomach, and intestine; and finally a secondary body-cavity or coelom, consisting of three divisions on each side, though possibly the most anterior pair were confluent in the prae-oral lobe. On the left side the anterior coelom opened to the exterior by a short ciliated canal. To the hypothetical group so defined which were certainly not Echinodermata the name Protocoelomata may be given.
Fig. 293.—Tornaria larva. a, Anus; a.c, anterior coelom; a.p, apical plate; g.s, rudiments of gill-sacs; m, mouth; m.c, middle or "collar" coelom; p, posterior ciliated band; p.c, posterior coelom; pr, longitudinal ciliated band. (After Morgan.)