536 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Fig. 221. Pentacrinoid Larvse of 

 the Feather - star (Antedon 

 rosacea). 



A, Quite young, before the opening of 

 the cup, and the appearance of 

 the five radial plates; B, nearly 

 imture ; 6, basal ; o, orals ; r, first 

 radials. (After Carpenter. ) 



Among the Echinozoa 

 we often find arrangements 

 which exhibit more com- 

 pletely the characters of a 

 true metamorphosis, and 

 which are of especial inter- 

 est because they present a 

 bilateral symmetry, such as 

 is ordinarily obscured in the 

 adult. The simplest con- 

 ditions obtain in the Holo- 

 thurians. After passing 

 through the early stages 

 of development, the body, 

 which was originally co- 

 vered with cilia, has these 

 processes arranged in a 

 sinuous band at its edges. 



The anterior portion of 

 the enteric tract, before 

 uniting with the hinder in- 

 volution, the orifice of 

 which forms the permanent 

 anus, buds off a vesicle, 

 which becomes completely 

 separated from the enteric 

 tract, and the cavity of part 

 of which forms the I>od3* 

 cavity. The vesicle elong- 

 ates, and sends outwards 

 a process which comes into 

 contact with the dorsal sur- 

 face of the body, or that 

 which is opposite to the 

 surface on which the mouth 

 opens ; this process, or di- 

 vert! julum, has an opening 



