294 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



set in regular paired rows, at a more or less acute 

 angle to one another ; these ossicles are not perforated 

 like the ainbulacral plates of an echinid, but the 



lr 



ao 



Fig. 123 A. Cross Section of an Arm of a Starfish (Asterias riibcn?). 



On the left side the section is supposed to pass between two of the ambulacra! 

 ossicles, but on the right side through one of them (ao) ; act, ambulacra! 

 groove; n, radial nerve; 6, radial blood-vessel; w, radial water-vessel; a, 

 ampullae; t, tentacles or suckers ; ap, adamlnilacral plates; s.p, spines ; pa~r, 

 paxillae, arising from limestone plates; or, ovary; gp, genital pore; yi\ 

 genital blood-vessel ; fcr, respiratory processes ; pc, caeca of the intestine. 

 (After P. H. Carpenter.) 



tube-feet pass out between them ; attached to each 

 ambulacral ossicle is a smaller ad-ambulacral ossicle 

 (Fig. 123 A; ap), which completes the side of the 

 groove and carries spines ; the rest of the wall 

 of the arm is strengthened by irregular plates, 

 which may be so formed as to leave considerable 

 interspaces, as in the starfish, or they may be larger 

 and more closely packed, and have only minute pores 

 between them, as in Linckia ; sometimes, as in 

 Oreaster, the plates that form the margin of the 

 arm form two regular rows of much stronger supero- 

 and infero-marginal plates. All, some, or none 



