526 ZOOLOGY. 



mollusca. In structure, these animals differ widely from each 

 other; and, externally, some more resemble plants than 



Fig. 552. The Echinus, or Sea Hedgehog.* 



animals. They have been divided into five classes the 

 echinodermata, the acalepha, the polyps^ the infusoria 

 polygastria, and the sponges. 



CLASS OF THE ECHINODERMATA. 



617. The echinodermata (Figs. 159 arid 186) are radiated 

 animals whose skin is thick, and often supported by a solid 

 skeleton (Fig. 552), with a very complex internal structure. 

 They are formed to creep along the bottom of the waters, 

 and are in general provided with a number of small retractile 

 tentacula, which pass through pores in the integuments, and 

 act by their extremities like suckers. In most zoophytes, 

 the sea urchin and holothuria for example, the digestive 

 cavity has the form of a tube, open at its two extremities ; and 

 in others (the sea stars) it consists only of a sac, furnished 

 all around with a number of appendages, more or less branched, 

 with a single aperture communicating externally. Theechino- 



* On the left side the spines have been removed to show the shell. 



