538 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



connection between the peritoneal vesicle and the outer 

 world is permanently retained, and forms the so-called 

 madreporic canal. 



A few Echinoderms (Hemiaster, Ophiacantha 

 vivipara, Chirodota rotifera) do not pass through any 



Fi.j 223. Pluteus paradoxvs, the Larva of an Ophiuroid, at a late stage, 

 in which both the Larval Arms with their supports and the rudi- 

 ments of the Disc and Radial Skeleton of Adult are to be seen. 

 (After J. Miiller.) 



larval stages ; the eggs are received into incubatory 

 pouches, or are developed in the ccalom without pass- 

 ing through any larval stages, or leading a free- 

 swimming independent existence. 



A very common form of free-swimming larva is 

 that which is known as the Trocliospliere, and 

 which essentially resembles the adult condition of a 

 Rotifer; it is found among the marine Chsetopocla, 

 some of the Gephyrea and Mollusca, and in the 



