258 



CLASSIFICATION OF ANIMALS 



angles to that of the other Echinoderms. The principal axis 

 is also much longer than the radii. The skeletal parts are re- 

 duced to minute hooks and plates, but the integument is very 

 thick and leathery. The animals feed on organic detritus 

 which they collect by means of a circle of branching tentacles 



Fig. 148. — A sea-urchin (Clypeaster). The spines have been removed. The 

 five ambulacral areas are clearly shown. The test shows marked bilateral 

 symmetry. 



surrounding the mouth. The intestine is a coiled tube and 

 ends at the aboral pole of the animal in a large cloacal cavity. 

 Lying in the body cavity and connected with the cloaca is a 

 very peculiar organ called the respiratory tree. It is a tubular 



