STRUCTURE OF THE STARFISH. 75 



Fig. 32. — Diagram of the transverse section of the ray of a star- 

 fish. 



a. The layer of epithelium which invests the outer surface. 6. The 

 ossicles of the ab-oral surface, c. The layer of epithelium which lines 

 the perivisceral cavity, d. The layer of epithelium which lines the water 

 sj'stem. e. The epithelial lining of the digestive tract. /. PediceUariae. 

 g. Spines of ab-oral surface, h. Ab-oral tentacles, i. Hepatic-coeca. 

 k. Ambulacral vesicles. /. Pladiating water-tube. m. Radiating nerve, 

 n. The ambulacra, o. Movable spines, carried by the inter-ambulacral 

 ossicles, p. Inter-ambulacral ossicles, q. Ambulacral ossicles, r. Me- 

 senteries, s. Their inner spaces, t. Perivisceral cavity, ri. Peri-4i8emal 

 spaces. 



the joints between these parts, and the muscular fibres 

 which move them. 



c. Notice that the spines (Fig. 32, o) which are carried 

 by the inter-ambulacral ossicles (p) are furnished with a 

 movable joint. 



(L The ab-oral tentacles (h) which project throuorh the 

 spaces between the ossicles, and open into the perivisceral 

 cavity (t). 



e. The la^er of epithelial cells, which covers the outer 

 surface of the body, and which is represented in the dia- 

 gram by the heavy line a. It covers the pedicellariae and 

 the spines, and foiins the outer surface of the wall of the 

 ab-oral tentacles. 



f. The layer of epithelium which lines the perivisceral 

 cavity, and which is represented by the shaded line c. 

 This layer runs into and lines the alvoral tentacles, and is 

 reflected out so as to cover all the organs which project 

 into the perivisceral cavity. 



7. Between this layer and the inner surface of the ab- 

 oral ossicles, a layer of naiTOW spaces (not shown in the 

 di;i<rram), lined by another layer of epithelium. In a 

 transverse section these spaces appear as naiTow slits. ' 



h. The cross-sections of the hepatic coeca (e) suspended 

 to the bodv wall ])v the mesenteries. 



fe 



