534 



INVERTEBRA TE MORPHOLOO T. 



separated from one another so that the body-wall has a more 

 or less leathery consistency, but more frequently placed 

 almost or quite in contact with each other, and uniting in 

 most of the Echinoids or Sea-urchins to form a firm test en- 

 closing the principal vegetative organs, a small area or peri- 

 stome around the mouth alone remaining but partially calci- 

 fied and retaining a leathery consistency. Spinous elevations 

 are frequently developed upon these dermal plates (whence 

 the name of the type) and may assume various forms, being 

 in some cases quite long, movably articulated with the plates, 

 and supplied with muscles so that they may aid in locomo- 

 tion. 



The arrangement of the calcareous plates differs greatly 



in the different classes which com- 

 pose the type, but certain of them, 

 distinguishable by their position 

 and relative arrangement, reappear 

 in the majority of the classes. 

 These plates are situated at the oral 

 and aboral surfaces of the body. 

 The oral plates are not so constant 

 nor so numerous as the aboral or 

 apical, and show a tendency, even in 

 those groups in which they are most 

 highly developed, to undergo a 

 greater or less amount of resorption 

 during development, being frequent- 

 ly more pronounced in larval than 

 in adult life. Typically the oral 



Zoroaster, SHOWING THE system consists of a central oral 

 SYSTEM OF PLATES plate> the orocentra ^ un t nown i u 



recent forms, but occurring in cer- 

 tain fossil genera, and this is sur- 

 rounded by a ring of five plates, 

 which may be termed the oral plates, 

 and which have an interradial posi- 

 tion. The apical system has as a 

 central plate the so-called centro- 

 dorsal (Fig. 247, CD), which in some forms is replaced by a 



FIG. 247. DISK AND ARM OF 



(after SLADEN). 

 an au us. 

 cd = centro-dorsal. 

 mt =r madreporite. 

 T -= terminal plate. 



2 = under-basals. 



3 = basals. 



4 == radials. 



