CLASS OF THE ECHINODEBMATA. 483 



other; and, externally, some more resemble plants than 

 animals. They have been divided into five classes the 

 echinodermata, the acalepka, the polyps, the infusoria 

 potygastria, and the sponges. 



CLASS OF THE ECHINODEEMATA. 



617. The echinodermata (Figs. 136 and 163) are radiated 

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

 skeleton (Fig. 466), 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, 



Fig. 468. Holothuria. 



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. The echino- 

 dermata have a circulatory apparatus sufficiently developed ; 

 and of all the zoophytes are those whose organization is most 

 complex and most perfect. They live in the sea, and when 

 young undergo some remarkable metamorphoses. The echino- 

 dermata form three principal groups the holothuria (Fig. 

 468), the echinus (Fig. 466), and the asteria or sea star (Fig. 

 136). Some species of this last family attach themselves by 

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