ECHINODERMATA. 553 



(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 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 

 oesophagus in Holothuroids, Ophiuroids, and Comatula ; in 

 these cases the larval oesophagus is carried on into the adult. 

 In other forms the water-vascular vesicle forms a ring which 

 does not enclose the oesophagus (Asteroids and Echinoids) ; 

 in such cases a new oesophagus is formed, which perforates this 

 ring. 



Development of the larval appendages and metamorphosis. 



Holothuroidea. The young larva of Synapta, to which J. 

 Muller 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. 



It is (fig. 254 A and fig. 255) bilaterally symmetrical, pre- 

 senting 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 prae-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 



