NORTH-CAROLINA GEOLOGICAL SURVEY. 303 



CHAPTEE XIX. 



RADIATA. 



Considerations relative to animals belonging to this type. Aberant forms 

 of the Echinodermata. Species described. Bryozoa, Polyparia, etc. 



Echinodermata comprehends a class in the Kingdom, Ra- 

 diata, whose organization belongs to the stellate type. Thit 

 sub-class derives its name from the character of the integu- 

 ment, and its appendages, which remotely resemble that of 

 the hedge-hog. Some are called sea-urchins, others star-fishes. 

 In most of the families of this great class, the integument is 

 protected by calcareous spines. The integument itself is co- 

 riaceous, but it takes into its composition a large quantity of 

 lime which imparts to it firmness and durability. The skin 

 is complicated in its structure. It is made up of an immense 

 number of plates of a polygonal form. They amount to 600 

 pieces in all. These are dove-tailed together in the most per- 

 fect manner, and yet they are so invested in living membrane, 

 that additions of carbonate of lime are constantly made to 

 each. By this arrangement, the animal within grows without 

 inconvenience to itself, which it could not do, if the integu- 

 ment or dwelling was composed of one piece. 



The forms of the Echinoderms differ much among them- 

 selves, and yet it is apparent that they all belong to one type, 

 and are constructed upon one plan. One of the most aber- 

 rant of this type is the sea cucumber, (Holothuria,) which is 

 a firm fleshy bag, destitute of plates, composed of carbonate 

 of lime. In another upon our coast, we find the star-fishes 

 with five arms extending from a common center ; and in an- 

 other^ the globular sea-urchin, in which the five arms are 

 folded and soldered together so as to form a ball. Another 

 interesting form has the stellate type, but differs considerably 

 from the star-fish, and most strikingly in the fact that the stel- 1 



