(4) The archenteron always gives rise to outgrowths which form the peritoneal membrane and water-vascular systems. In Comatula there are three such outgrowths, two paired, which form the peritoneal vesicles, and one unpaired, which forms the water-vascular vesicle. In Asteroids and Ophiuroids there are two outgrowths. In Ophiuroids both of these are divided into a peritoneal and a water-vascular vesicle, but the right water-vascular vesicle atrophies. In Asteroids only one water-vascular vesicle is formed, which is derived from the left peritoneal vesicle. In Echinoids and Holothuroids there is a single vaso-peritoneal vesicle.
(5) The water-vascular vesicle grows round the larval œsophagus in Holothuroids, Ophiuroids, and Comatula; in these cases the larval œsophagus is carried on into the adult. In other forms the water-vascular vesicle forms a ring which does not enclose the œsophagus (Asteroids and Echinoids); in such cases a new œsophagus is formed, which perforates this ring.
Development of the larval appendages and metamorphosis.
Holothuroidea. The young larva of Synapta, to which J. Müller gave the name Auricularia ([fig. 255]), is in many respects the simplest form of Echinoderm larva. With a few exceptions the Auricularia type of larva is common to the Holothuria.
Fig. 254. A. The larva of a Holothuroid. B. The larva of an Asteroid.
m. mouth; st. stomach; a. anus; l.c. primitive longitudinal ciliated band; pr.c. præ-oral ciliated band.
It is ([fig. 254] A and [fig. 255]) bilaterally symmetrical, presenting a flattened ventral surface, and a convex dorsal one. The anus (an) is situated nearly at the hinder pole, and the mouth (m) about the middle of the ventral surface. In front of the mouth is a considerable process, the præ-oral lobe. Between the mouth and anus is a space, more or less concave according to the age of the embryo, interrupted by a ciliated ridge a little in front of the anus. A similar ciliated ridge is present on the ventral surface of the præ-oral lobe immediately in front of the mouth. The anal and oral ridges are connected by two lateral ciliated bands, the whole forming a continuous band, which, since the mouth lies in the centre of it ([fig. 255]), may be regarded as a ring completely surrounding the body behind the mouth, or more naturally as a longitudinal ring.