Chap. III.] 



ECHINODER MS. 



61 



tube-feet were placed as the "walks" or ambulacra, 

 and we may, therefore, speak of the ossicles or plates 

 which specially support and protect the tube-feet as 

 the ambulacra! plates or ossicles. Accom- 

 panying the radial water-vessel is a nerve-trunk and 



an 



Fig. 23. Diagram of a Cross-section of an Arm of a Common Starfish 

 (Asterias rvbens). 



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) ; ag, ambulacra! 

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

 ampullae ; t, tentacles or suckers; ap, adambulacral plates ; sp, spines ; pax, 

 paxillie, arising from limestone plates ; or, ovary ; gp, genital jpore ; gv, 

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

 (After P. H. Carpenter.) 



a blood-vessel ; while in the arm of the starfish we 

 find also generative sacs, and processes of the digestive 

 tract ; all of which enter, like the water-system, into 

 the cavity of the disc. 



If, therefore, we make a transverse section (Fig. 

 23) throughout the arm of a starfish at a short 

 distance from the disc we should cut through digestive, 

 circulatory, ambulatory, generative and nervous 



