ECHmODEKMS. 91 



ECHINODERMS. 



OuB illustrations and descriptions of Echinoderms are scanty 

 in comparison to those of the preceding class ; for while, in con- 

 sequence, perhaps, of the combined influence of the Gulf Stream 

 and the cold arctic current on the New England shore, Acalephs 

 are largely represented in our waters, our marine fauna is 

 meagre in Echinoderms. But although we have few varieties, 

 those which do establish themselves on a coast seemingly so 

 ungenial for others of their kind, such as the Echinus, and our 

 common Star-fish, for instance, thrive well and are very abun- 

 dant. The class of Echinoderms includes five orders, viz. Cri- 



NOIDS, OpHIUKANS, StAE-PISHES, SEA-URCfflNS, and HOLOTHURIANB. 



The animals composing these orders differ so widely in appear- 

 ance that it was very long before their true relations were de- 

 tected, and it was seen that all their external difiFerences were 

 united under a common plan. Let us compare, for instance, the 

 worm-like Holothurians (Figs. 124, 126, 127) with all the host of 

 Star-fishes (Figs. 142, 146, 147) and Sea-urcMns (Figs. 131, 139), 

 or compare the radiating form of the Star-fish, its arms spreading 

 in every direction, with the close spherical outline of the Sea- 

 urchin, or the Crinoid floating at the end of a stem (Fig. 152) 

 with either of these, and we shall cease to wonder that naturalists 

 failed to find -at once a unity of idea under all these varieties of 

 execution. And yet the fundamental structure of the class of 

 Echinoderms is represented as distinctly by any one of its five 

 orders as by any other, and is absolutely identical in all. , They 

 difler only by trifling modifications of development. 



In Echinoderms as a class, the body presents three regions dif- 

 fering in structure, and on the greater or less development of 

 these regions or systems, as we may call them, their chief difiFeren- 

 ces are based. Take, for instance, the dorsal system, the nature 

 of which is explained by the name, indicating of course the back 

 of the animal, though it does not necessarily imply the upper 

 side of the body, since some of the Echinoderms, as the stemmed 

 Crinoids, for example, carry the dorsal side downward, while 



