536 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



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 body 

 cavitv. The vesicle elong- 



/ 



ates, and sends outwards 

 a process which comes into 

 contact with the dorsal sur- 



Fig. 221. PentacrinoidLarvEeof face of the body, or that 



the Feather - star (Antedon w hi cn i s opposite to the 

 rosacea). rr 



A, Quite young, before the opening of SUl'f aC6 Oil which the mouth 



P ens ; this Process, or di- 

 :{' flrst verti julum , has an opening 



