FOURTH SUB-KINGDOM 



RADIATES. (Lat. radius, a ray.) 



THIS includes animals so named from the arrangement of the 

 parts round an axis somewhat as in plants ; hence they were 

 called by Cuvier, ZOOPHYTES, Plant-like animals. These intro- 

 duce us to the 



NINTH BRANCH OF ZOOLOGY. 



ACTINOLOGY, (Gr. axTlv, aktin, a ray ; Aoy6?, logos, a discourse. 



RAY-LIKE ANIMALS. 



The terms which are here employed are not, by any means, 

 equally applicable to all the beings included in this last sub-divis- 

 ion of the Animal Kingdom. While in some of them the radiated 

 arrangement of the parts is very easily seen, in others it can 

 be traced only by a close microscopic examination. Agassiz says 

 it can be perceived in all with "sufficient observation." Such 

 are the differences of form and in the degree of organization 

 among the Radiates, that instead of undertaking to present addi- 

 tional characters, describing them as a whole, we shall at once 

 proceed to consider the four classes into which they are divided. 



I. ECHINODERMS, (Gr. s^o?, echinos, a sea-urchin ; 

 derma, skin.) 



These are all marine, and characterized by having a well or- 

 ganized skin, under which or attached to which are often found 

 plates of solid matter, forming a kind of skeleton, and some- 

 times joined together like the stones of a pavement. They 

 have a digestive and x vascular system. A circular nervous sys- 



