The third outgrowth of the archenteron gives rise to the water-vascular vesicle. It first grows round the region of the future œsophagus and so forms the water-vascular ring. The wall of the ring then grows towards the body wall so as to divide the oral (left) peritoneal vesicle into two distinct vesicles, an anterior and a posterior, shewn in [fig. 253], lp´ and lp. Before this division is completed, the water-vascular ring is produced in front into five processes—the future tentacles ([fig. 252], wv)—which project into the cavity of the oral vesicle (lp). After the oral peritoneal space has become completely divided into two parts, the anterior dilates ([fig. 253], lp´) greatly, and forms a large vestibule at the anterior end of the body. This vestibule (lp´) next acquires a communication with the mesenteron, shewn in [fig. 253] at m. The anterior wall of this vestibule is finally broken through. By this rupture the mesenteron is placed in communication with the exterior by the opening at m, while at the same time the tentacles of the water-vascular ring (t) project freely to the exterior. Such is Götte’s account of the præ-oral body space, but, as he himself points out, it involves our believing that the lining of the diverticulum derived from the primitive alimentary vesicle becomes part of the external skin. This occurrence is so remarkable, that more evidence appears to me requisite before accepting it.
The formation of the anus occurs late. Its position appears to be the same as that of the blastopore, and is indicated by a papilla of the mesenteron attaching itself to the skin on the ventral side ([fig. 253], an). It eventually becomes placed in an interradial space within the oral disc of the adult. The water-vascular ring has no direct communication with the exterior, but the place of the madreporic canal of other types appears to be taken in the larva by a single tube leading from the exterior into the body cavity, the external opening of which is placed on one of the oral plates (vide p. [571]) in the next interradial space to the right of the anus, and a corresponding diverticulum of the water-vascular ring opening into the body cavity. The line of junction between the left and right peritoneal vesicles forms in the larva a ring-like mesentery dividing the oral from the aboral part of the body cavity. In the adult[220] the oral section of the larval body cavity becomes the ventral part of the circumvisceral division of the body cavity, and the subtentacular canals of the arms and disc; while the aboral section becomes the dorsal part of the circumvisceral division of the body cavity, the cœliac canals of the arms, and the cavity of the centro-dorsal piece. The primitive distinction between the sections of the larval body cavity becomes to a large extent obliterated, while the axial and intervisceral sections of the body cavity of the adult are late developments.
Fig. 253. Longitudinal section through the calyx of an advanced Pentacrinoid Antedon larva with closed vestibule.
(From Carpenter; after Götte.)
ae. epithelium of oral vestibule; m. mouth; al. mesenteron; an. rudiment of permanent anus; lp. posterior part of left (oral) peritoneal sack; lp´. anterior part of left (oral) peritoneal sack; wr. water-vascular ring; t. tentacle; mt. mesentery; rp. right peritoneal sack; rp´. continuation of right peritoneal sack into the stalk; r. roof of tentacular vestibule.
The more important points in the development indicated in the preceding pages are as follows:
(1) The blastosphere is usually elongated in the direction of the axis of invagination, but in Comatula it is elongated transversely to this axis.
(2) The blastopore usually becomes the permanent anus, but it closes at the end of larval life (there being no anus in the adult) in Ophiuroids and some Asteroids, while in Comatula it closes very early, and a fresh anus is formed at the point where the blastopore was placed.
(3) The larval mouth always becomes the mouth of the adult.