CLASS ECHINODERMATA. I 7 



green ; and it attains a diameter of an inch or an inch 

 and a half. 



Recapitulation of Essential Characters. — The 

 body is made up of two distinct layers, enclosing a 

 " body - cavity " which communicates freely with the 

 outer world through the mouth. There intervenes, 

 however, between the mouth and the body -cavity a 

 globular stomach. The integument is furnished with 

 "nettle-cells." There is no nervous system (with few 

 exceptions), and distinct organs of respiration and circu- 

 lation are not developed. These characters distinguish 

 the class Actinozoa as a whole. 



CHAPTER VI. 



class ECHINODERMATA. 



The commonest of the animals which are included in 

 this class are generally known as Sea-urchins, Star-fishes, 

 Brittle-stars, Sand-stars, and Sea-lilies. Most of these com- 

 mon names refer to the fact that the body in these animals 

 is generally more or less star-like in shape. The name 

 "Sea-urchin" and the technical name " Echinoderviata," 

 on the other hand, refer to the fact that the skin in these 

 animals is generally rendered prickly, like that of a hedge- 

 hog, with numerous spines, tubercles, and grains of lime 

 (Greek, echinos, a hedgehog ; derma, skin). As an exam- 

 ple of this class we may take the common Star-fish or 

 Cross-fish ( Uraster ruhens) of British seas. 



The most conspicuous point about the form of the Star- 

 fish is its strikingly star-like shape (fig. 6). It consists, 

 namely, of a by no means well-marked central body or 

 disc, from which spring five (sometimes four or six) blunt 

 finger-like processes or "arms." The arms, in fact, form 

 a star, and the body looks as if it were composed of the 



B 



