INTRODUCTION. 



The Echinodermata constitute one of the three great 

 classes into which the Radiata are divided. The radiate 

 type presents us with animals which either have their parts 

 arranged in a ray-like manner round a common centre, or 

 have their hilaterality so modified as to give them a star- 

 like form. The Zoophytes, the Medusa?, and the creatures 

 to which this volume is devoted, constitute the type. The 

 Echinodermata are most highly organized, much more so 

 than the Polypes ; they are almost all free animals, creep- 

 ing ahout at the bottom of the sea ; and as the greater 

 number of species are covered with a coriaceous skin, which 

 is commonly strengthened by calcareous plates or spines, 

 they have derived their general appellation from that re- 

 markable character, which at once distinguishes them from 

 the Medusa;, free swimming animals of the most delicate 

 and membranous texture. 



Throughout animated Nature forms and structures merge 

 into each other. While the central groups of a type pre- 

 sent its essential characters, the more distant families 

 approach in appearance and habits to the members of some 

 other great class of forms. This is equally true respecting 

 small as well as large groups. Thus, the class of Radiata 

 before us presents examples, at one extreme, of animals 

 truly symmetrical, and, at the other, of species which ap- 

 proach either in general form or in their early life to the 

 Aniorphozoa, the lowest of animal types. For example, 



