400 CRIXOIDEA. 



(tig. 253, lp) greatly, and forms a large vestibule at the anterior end 

 of the body. This vestibule (lp") next acquires a communication with 

 the mesenterou, shewn in fig. 253 at m. The anterior wall of this 

 vestibule is finally broken through. By this rupture the mesenterou 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 Gb'tte's account of the prae-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 

 mesenteroii 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 iuto the 

 body cavity, the external opening of which is placed on one of the oral 

 plates (vide p. 476) 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 



o *> o 



aboral part of the body cavity. In the adult 1 the oral section of the larval 

 body cavity becomes the ventral part of the circum visceral division of the 

 body cavity, and the subtentacular canals of the arms and disc ; while the 

 aboral section becomes the dorsal part of the circuni visceral division of the 

 body cavity, the cosliac canals of the arms, and the cavity of the centro- 

 dorsal piece. The primitive distinction between the sections of the larval 

 body cavitv becomes to a large extent obliterated, while the axial and 

 intervisceral sections of the body cavity of the adult are late developments. 



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. 



(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 



1 Vide P. H. Carpenter, "On the genus Actinometra." Linntan Tnm., 21111 Series, 

 Zo:)loj?y, Vol. ii.. Tart i.. 1871). 



