EC HI NO DERM A TA . 



461 



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 peri- 

 toneal 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. M tiller 

 gave the name Auricularia (fig. 255), is in many respects the simplest 

 form of Echinoderm larva. With 



a few exceptions the Auricularia A. B 



type of larva is common to the 

 Holothuria. 



It is (fig. 254 A and fig. 255) 

 bilaterally symmetrical, present- 

 ing a flattened ventral surface, 

 and a convex dorsal one. The 

 anus (an] is situated nearly at 

 the hinder pole, and the mouth 

 (ra) about the middle of the ven- 

 tral surface. In front of the 

 mouth is a considerable process, 

 the prse-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 prse-oral lobe immediately in front of the mouth. The 



FIG. 254. A. THE LARVA or A HOLO- 

 THUROIP. B. THE LAKVA OF AN ASTEROID. 



m. mouth; .9*. stomach; a. anus; I.e. 

 primitive longitudinal ciliated band; pr.c. 

 prae-oral ciliated band. 



FIG. 255. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN AURICULARIA 



FROM THE SIMPLEST ECHINODERM LARVAL FORM, (Copied from Mttller.) 



The black line represents the ciliated ridge. The shaded part is the oral side of 

 the ring, the clear part the aboral side. 



in. mouth: an. auus. 



