Fig. 255. Diagrammatic figures representing the evolution of an Auricularia from the simplest Echinoderm larval form. (Copied from Müller.)
The black line represents the ciliated ridge. The shaded part is the oral side of the ring, the clear part the aboral side.
m. mouth; an. anus.
The bilateral Auricularia is developed from a slightly elongated gastrula with an uniform covering of cilia. The gastrula becomes flattened on the oral side. At the same time the cilia become specially developed on the oral and anal ridges, and then on the remainder of the ciliated ring, while they are simultaneously obliterated elsewhere; and so a complete Auricularia is developed. The water-vascular ring in the fully developed larva has already considerably advanced in the growth round the œsophagus ([fig. 256] w.v.r).
Most Holothurian larvæ, in their transformation from the bilateral Auricularia form to the radial form of the adult, pass through a stage in which the cilia form a number of transverse rings, usually five in number, surrounding the body. The stages in this metamorphosis are shewn in figs. 256, 257, and 258.
Fig. 256. Full-grown larva of Synapta. (After Metschnikoff.)
m. mouth; st. stomach; a. anus; p.v. left division of perivisceral cavity, which is still connected with the water-vascular system; w.v.r. water-vascular ring which has not yet completely encircled the œsophagus; l.c. longitudinal part of ciliated band; pr.c. præ-oral part of ciliated band.
The primitive ciliated band, at a certain stage of the metamorphosis, breaks up into a number of separate portions ([fig. 256]), the whole of which are placed on the ventral surface. Four of these ([fig. 257] A and B) arrange themselves in the form of an angular ring round the mouth, which at this period projects considerably. The remaining portions of the primitive band change their direction from a longitudinal one to a transverse ([fig. 257] B), and eventually grow into complete rings ([fig. 257] C). Of these there are five. The middle one (257 B) is the first to develop, and is formed from the dorsal parts of the primitive ring. The two hinder rings develop next, and last of all the two anterior ones, one of which appears to be in front of the mouth ([fig. 257] C).