(2) The Ophiopluteus, the larva of the Ophiuroidea. In this type the prae-oral lobe remains small, and the primitive ciliated band is undivided. The processes into which it is drawn out are very long, and are supported by calcareous rods. Of these processes we may distinguish prae-oral, postero-dorsal, postero-lateral, and post-oral. The postero-lateral are always much longer than the rest, so that the larva when swimming appears to the naked eye as a tiny V. In the case of Ophiothrix fragilis (Fig. 284, A) the postero-lateral processes are many times longer than the rest of the body. The Ophiopluteus was the first Echinoderm larva to be recognised. It was discovered by Johannes Müller,[[518]] who also discovered the other three types of Dipleurula. He named this one Pluteus (easel), from a fancied resemblance, when turned upside down, to a painter's easel. The same name was bestowed on the next type, to which it presents a superficial resemblance, and hence the distinguishing prefix "Ophio-" was added to the original name by Mortensen.
(3) The Echinopluteus, the larva of the Echinoidea. This type is strikingly like the preceding one in possessing a very small prae-oral lobe and in having the processes of the ciliated ring supported by calcareous rods, but a close inspection of these shows that they do not exactly correspond to those of the Ophiopluteus. Thus we have prae-oral, postero-dorsal, and post-oral processes (Fig. 285), but usually no postero-lateral process, and when it does occur it remains short. On the other hand, an antero-lateral process unrepresented in the Ophiopluteus is constantly present, and in its later stage the Echinopluteus develops, out of parts of the ciliated ring, horizontally-placed crescentic ridges of cilia, which are termed ciliated epaulettes (Fig. 285, a.cil.ep). There may even be, as in the larva of Echinus esculentus, a second posterior set of these (Fig. 285, p.cil.ep). In the older larva at the apex of the prae-oral lobe there is an ectodermic thickening, at the base of which are developed nerve-cells and nerve fibres constituting a larval brain (Fig. 285, ap).
Fig. 285.—Dorsal view of larva of Echinus esculentus. × 45. a.cil.ep, Anterior ciliated epaulette; ap, apical plate or larval brain; ech, rudiment of Sea-urchin; l.a.c, left anterior coelom; l.oes, larval oesophagus; l.p.c, r.p.c, as in Fig. 284; p.cil.ep, posterior ciliated epaulette; r.a.c, right anterior coelom.
(4) The Auricularia, the larva of the Holothuroidea. This type strikingly resembles the Bipinnaria in its external features. The prae-oral lobe is well developed, and has on its under surface a backwardly projecting loop of the ciliated band, which is not, however, as in the Bipinnaria, separated from the rest of the band. The processes of the band are much more faintly marked than in the Bipinnaria, the anterior median, prae-oral, and median dorsal processes being absent; but a pair of intermediate dorsal processes are developed in the interspace between anterior and posterior dorsal.
Fig. 286.—Three views of metamorphosis of Auricularia of Synapta digitata. A, fully grown Auricularia; B and C, stages in the metamorphosis. hy, Hydrocoel; Int, intestine; l.p.c, left posterior coelom; O, fragments of ciliated band which are invaginated into the stomodaeum, and coalesce to form a ring round the mouth; oss, ossicle; pod, rudiment of feelers which here spring directly from the hydrocoel; r.p.c, right posterior coelom; st, stomach; w.v.r, rudiment of water-vascular radial canals; 1-5, corresponding pieces in the three figures of the longitudinal ciliated band. (After Bury.) × 40.
In the Bipinnaria, Ophiopluteus, and Echinopluteus the coelomic vesicle, after separation from the archenteron, divides into right and left halves. The left then sends out a short dorsal process, which, fusing with the ectoderm, acquires an opening to the exterior. This opening is the primary madreporic pore, and the process of the left coelomic sac, which is ciliated, is the pore-canal. In the Auricularia the pore and pore-canal are formed before the division of the coelom. In the Bipinnaria the right and left sacs subsequently fuse in the front part of the prae-oral lobe. In the first three types of larva the coelomic sac on each side then undergoes a segmentation into anterior and posterior portions. At the hinder end of the anterior sac on each side a swelling occurs. That on the left side is the "hydrocoel," or rudiment of the water-vascular system (Fig. 287, A3, l.hy); it quickly assumes a crescentic form, and gives off five blunt outgrowths, which are the rudiments of the radial canals, and the terminal tentacles. It remains in connexion with the anterior coelom by a narrow neck, which later becomes the stone-canal. That on the right side separates completely from the right anterior coelom; it remains small, and forms the madreporic vesicle (Fig. 287, A3, r.hy) of the adult. In the Ophiopluteus and in the larva of Asterina gibbosa (v. infra) it occasionally takes on a form similar to that of the hydrocoel; from which circumstance, as well as from the similarity in its mode of origin, it is here regarded as a right hydrocoel, i.e. a rudimentary fellow of the organ which develops into the water-vascular system.
Fig. 287.—Diagrams of the mode of formation and division of the coelom in Echinodermata. a.c, Anterior coelom; coe, primitive coelomic rudiment; int, intestine; l.a.c, left anterior coelom; l.hy, left hydrocoel; l.p.c, left posterior coelom; oes, oesophagus; p.c, posterior coelom; r.a.c, right anterior coelom; r.hy, right hydrocoel; r.p.c, right posterior coelom; st, stomach; stom, stomodaeum.