Now amongst the lowest types of animal in which traces of Vertebrate structure can be detected, there is one group, the Hemichordata (Vol. VII. p. 3), in which there is a larva which strikingly recalls the Dipleurula. This larval form belongs to Balanoglossus and is called the Tornaria. It possesses a well-marked prae-oral lobe and a folded longitudinal ciliated band, which resembles that of Auricularia. Its peculiarity is that in addition there is a posterior ring of cilia (Fig. 293, p). The coelom is in five divisions:—a median anterior sac (a.c) opening to the exterior by a short ciliated canal on the left side; and paired middle divisions (m.c) and posterior divisions (p.c). At the apex of the prae-oral lobe there is a plate consisting of sensory epithelium, with nerve-fibres at its base, which acts as a brain. Tornaria undergoes metamorphosis, assumes a worm-like form, and takes on a burrowing life. The five divisions of the coelom are retained, and it can be proved that the pore-canal, like the madreporite of Echinodermata, is used for taking in water. Further, there are two aberrant sessile members of the group (Cephalodiscus and Rhabdopleura), in which the middle divisions of the coelom which would correspond to the hydrocoels are produced into long arms, each with a double row of ciliated tentacles, which strikingly recall the radial canals and podia of the Pelmatozoa. Taking all these facts into consideration, it seems probable that Vertebrata and Echinodermata both arose from Protocoelomata.
When we turn to the developmental history of Echinodermata for light on the question as to how the bilaterally symmetrical ancestor became converted into the radially symmetrical Echinoderm, it seems probable that only in the development of the Asteroidea can we hope to find the solution of the problem. The abrupt changes of habits shown in the metamorphoses of the other types are clearly secondary phenomena. No species of animal could suddenly change its habits from swimming by means of cilia to walking with tube-feet. In the development, however, of Asterina gibbosa we get a hint of the way in which a free-swimming life could alternate with periods of temporary fixation, gradually passing into a condition in which the fixation was permanent. This period in the history of the race when ancestral Echinodermata were sessile would mark the point at which Eleutherozoa diverged from Pelmatozoa, and the former existence of a fixed ancestor explains the tendency first to asymmetry and later to radial symmetry. Bilateral symmetry is characteristic of most free-swimming animals which have to pursue a straight course through the water, but in fixed forms no disadvantage arises from want of symmetry. A radial disposition of organs is, however, valuable to them, since food must be sought and danger avoided from all points of the compass; and hence we can understand, when fixation became permanent, how one hydrocoel could grow larger than the other, and finally assume the form of a ring.
The last question which arises is the vexed one of the mutual relationships of the various Classes constituting the Phylum. Before attempting to seek for light on this problem from development, it will be necessary to sketch the life-history of Antedon rosacea, the only Pelmatozoon whose development is known.
Fig. 294.—Three views of the development of Antedon rosacea. A, free-swimming larva; B, longitudinal section of free-swimming larva; C, oral view of young fixed form. a.c, Anterior coelom; amb, ambulacral groove; ap, apical plate of sensory and nervous tissue; cil, ciliated ring; hy, hydrocoel; l.p.c, left posterior coelom; mad, primary pore-canal; pod, podia; r.p.c, right posterior coelom; stom, larval stomodaeum. (A and B after Bury; C after Perrier.)
The eggs are comparatively large and full of food-yolk, and they adhere for a considerable period to the pinnules. They pass through a large portion of the development within the egg-membrane. The blastula and gastrula are formed in the usual way, but the formation of the coelom is most remarkable (Fig. 287, E1, E2). The archenteron divides into anterior and posterior divisions. The posterior divides into right and left, posterior coelomic sacs, but before the division is complete a dorsal and a ventral tongue grow out from the anterior division and unite posteriorly, encircling the band of connexion between right and left posterior coelomic sacs like a ring. This band of connexion becomes solid and is absorbed, and pari passu the ring becomes converted, by the disappearance of its central opening, into a sac, which is the definitive gut (Fig. 287, E). The rest of the anterior division divides into a thick-walled sac, the hydrocoel, on the left, and a median thin-walled anterior coelom, which sends a long extension into the anterior portion of the larva, which we may compare to the prae-oral lobe of the Bipinnaria. The anterior coelom communicates with the exterior by a short pore-canal, and later forms a connexion, the stone-canal, with the hydrocoel. At the apex of the prae-oral lobe there is formed a thickened patch of ectoderm, bearing stiff sensory hairs, and having at their bases nerve-fibres and ganglion cells. This larval brain corresponds to that of the Tornaria and Echinopluteus. Behind the brain there is a glandular pit, which is used for fixation, and recalls the similar organ in the Bipinnaria. A series of ciliated rings is then formed, and between the second and third of them an oval depression appears. This is the stomodaeum; but as the larva takes no food it does not communicate with the gut (Fig. 295, A, stom).
The larva next escapes from the egg-membrane and swims freely for a day or two, and then, like the Bipinnaria, fixes itself by the apex of the prae-oral lobe, which is converted into a stalk. The larval stomodaeum closes, and the oesophagus of the adult appears as a solid peg of cells abutting against it; round this peg the hydrocoel grows like a ring.
The closed stomodaeum and the underlying hydrocoel are now rotated backwards until they come to be at the end of the animal opposite the stalk (Fig. 295, C). The left posterior coelom, which has also, as in the Asteroid larva, assumed a hoop-like form, is carried along with them; but the right posterior coelom becomes shifted forwards and sends out five outgrowths into the stalk, which form the rudiments of the chambered organ, and a central one as a continuation of the genital stolon (Fig. 295, D, gen.st), the extension of the anterior coelom (Fig. 294, B) having disappeared.
Then the outer wall of the stomodaeum splits into five valves—the future oral valves. The radial canals appear as freely projecting tentacles, which issue in the intervals of these valves and soon acquire two pairs of lateral branches. The skeleton consists of five oral plates in the oral valves, of a ring of five basals, of three small under-basals, and of a series of "columnals," i.e. stem-ossicles, as rings embracing the stalk. The area of attachment is supported by a "foot-plate." The radial plates next appear as a ring of small ossicles between the orals and basals, and simultaneously the arms make their appearance as five outgrowths supported by the first radials, and by the other radials when these appear. The free radial canals now become adherent to the arms, but these canals soon give off paired branches of unlimited growth, which are supported by bifurcations of the primitive arms, and in this way the ten arms of the adult are established. So far, then, as the water-vascular system is concerned, the apparent forking is not a true dichotomy, but results from the production of two opposite branches, whilst the main axis ceases to grow. The appearance of cirri marks the fusion of the uppermost stem-ossicles to form a centro-dorsal, and shortly afterwards the young Antedon snaps off its stem and swims away.
