Chap. III.] 



ECHINODERMS. 



61 



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

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

 which specially support and protect the tube-feet as 

 the ambulacral plates or ossicles. Accom- 

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



i*. 23. Diagram of a Cross-section of an Avin of a Common Starfish 



(wisterias rubens). 



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

 ossicles, but on the right side through one of them (no); ag, ambulacra! 

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

 anjpullaa ; t, tentacles or suckers; ap, adambulacral plates ; sp, spines : par, 

 paxillrs, arising from limestone plates; or, ovary; gp, genital pore ; gr, 

 genital blood-vessel ; br, respiratory processes ; pc, ca?ca 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 



