ECIIINODKRMATA. 191 



terises the Pxhinoderms commonly called *' Sea-urchins," and Echinoi 

 by tlie Greeks. 



Asterias. 



The Echinoderms of the order Holothicrioidea may be described as 

 being constituted by a softening of the calcareous skin of the spheroidal 

 species, and by the reduction of the earthy matter to a greater or 

 less number of reticulate calcareous corpuscles, the globe being then 

 drawn out by the two opposite poles into an elongated cylindrical 

 form. These vermiform Echinoderms seem to lead, by tlie concluding 

 order Sipunculoidea, to the true worms, which stand on the lowest 

 step of the Articulate division of the Animal Kingdom. 



The name Echinodermata 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 foramina, through which a multitude of small tubes or 

 hollow suctorial tentacles (' tube-feet') can be protruded and Retracted, 

 and these constitute the common organs of adhesion and locomotion 

 in the Echinoderms. 



