142 



Fourth Great Division of the Animal Kingdom. 



ANIMALIA RADIATA. 



ORGANIZATION more simple than in the three preceding 

 branches ; the parts arranged around an axis, and upon two 

 or more rays, or upon two or more lines extending from 

 pole to pole ; nervous system indistinct, generally not appa- 

 rent ; circulatory organs very incomplete, and, in the greater 

 part, not existing at all ; seldom a mouth ; sometimes an in- 

 testinal cul-de-sac ; in the greater number a simple cavity, 

 even in the thickness of the body, opening, sometimes by 

 several suckers ; often no distinguishable mouth. Five 

 classes. 



ECHINODERMATA. 



Skin well organized, sustained frequently by a sort of 

 skeleton armed with articulated and movable points or spines, 

 with an internal cavity in which viscera are floating ; vas- 

 cular system confined to different portions of the intestines, 

 and to the organs of respiration, which are, in general, very 

 distinct, as well as the viscera ; nervous system very incom- 

 plete, filiform, and, in many species, even altogether imper- 

 ceptible. Two orders. 



