ECHINODERMA. 



113 



and general, whence the species have received the common appellation 

 of star-fish. 



Asterias. 



In certain StelleridcB we trace a shortening, flattening, and expan- 

 sion of the rays, until the body assumes a pentangular discoid form. In 

 the next family (Echinidce), the angles disappear, and the disc expands 

 until a spheroid or globular form is obtained, which characterises the 

 Echinoderms commonly called " Sea-urchins," and Echinoi by the 

 Greeks. 



The third tribe of Echinoderms {HolothuriidcB) may be described as 

 being constituted by a softening of the calcareous skin of the sphe- 

 roidal species, the globe being then drawn out by the two opposite 

 poles into an elongated cylindrical form. These vermiform Echino- 

 derms conduct to the true worms, which stand on the lowest step of 

 the Articulate division of the Animal Kingdom. 



The name Ecldnoderma has been applied to these diversified forms 

 of the higher organised Zoophytes of Cuvier, because in many of the 

 species the integument is defended by spines : they, however, possess, 

 and are associated together by, another and more general tegumentary 

 character ; the skin is perforated in most of the species by minute 



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