5OC RADIATES I ECHINODERMS. 



tough, leathery envelope, capable of great dilation and 

 contraction, and generally containing more or less of cal- 

 careous particles. There are at least four families : 

 Pentactidae, with the locomotive suckers in five regular 

 rows ; Thyonidae, with suckers scattered over the whole 

 body ; Psolidae, with suckers in three rows on an oblong 

 disk ; and Synaptidae, destitute of suckers. The different 

 species vary from an inch to a foot or more in length. 



SUB-SECTION II. 



THE ORDER OF ECHINOIDS, OR SEA-URCHINS. 



THE Order of Echinoids contains echinoderms which 

 have a more or less spherical or discoidal shell composed 

 of definitely formed and symmetrically arranged plates, 

 which are firmly bound together, and which bear tubercles 

 crowned with spines. These plates are so arranged as to 

 divide the shell into more or less distinctly marked zones 

 radiating from the oral opening. In every alternate zone, 

 the plates are perforated for the passage of the locomotive 

 suckers or ambulacra, and are called ambulacral plates ; 



Fig. 484. 

 Fig. 483. 



Sea-Urchin, Toxopneustes drobachiensis, Ag. 

 Both coasts of the United States, at the Top view of Sea-Urchin, spines re- 



North, moved. Shows ambulacral and 



interambulacral plates. 



and the plates of the other zones are not perforated, and 

 are called interambulacral plates. 



Echinoids may be divided into two great groups, 

 Regular and Irregular Echinoids, and these, according to 

 Desor's Synopsis, into families and genera, as given below. 



