Chap. IX.] 



SKELETON OF ECHINODERMS. 



295 



a.o 



of these ossicles may bear spines of varying size and 

 strength ; where they are best developed we rarely 

 find that any of the arms have suffered injury, and 

 they are, no doubt, of very considerable importance 

 as protective organs. The disc is formed chiefly of 

 irregularly arranged intermediate plates, but the 

 radials and basals are, in some cases, to be clearly 

 detected in young specimens ; near or at the centre of 

 the disc there is an anal perforation which is rarely 

 wanting ( Astropecten). On the 

 lower surface the large central 

 mouth is to a slight extent 

 aided in its work by the modi- 

 fication of the most central 

 ambulacral ossicles, which pro- 

 ject inwards at the angles of 

 the mouth. 



In the Ophiuroidea a 

 cross section of the arms (Fig. 

 123 B) shows that the ambu- 

 lacral ossicles (o) are covered 

 in on all sides, so that no 

 groove is apparent ; pores in 

 the lower plate allow of the 



passage of the tube feet ; the side plates (s) ordinarily 

 bear spines (t), which are never of great length or much 

 size, and can be of little use as organs of defence ; 

 above, a single plate (u) roofs in the ambulacral ossicles, 

 but tliis is rudimentary in Neoplax, and absent in 

 Ophioscolex. The plates in the disc are propor- 

 tionately larger than in starfishes, are ordinarily set 

 in a close mosaic, and not unfrequently exhibit the 

 essential parts of the typical calyx, the dorsocentral 

 even being often apparent, owing to the fact that 

 there has been no resorption of calcareous tissue to 

 make room for an anal orifice. The plates around the 

 mouth are so arranged as to give rise to five radially 



Fig. 123 B. Cross Section of 

 an Arm of an Ophiuroid. 

 (after P. H. Carpenter.) 



