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 

 system, e. The epithelial lining of the digestive tract. /. Pedicellariae. 

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

 k. Ambulacral vesicles. I. Radiating water-tube, m. Radiating nerve, 

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

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

 senteries, s. Their inner spaces, t. Perivisceral cavity, u. Peri-haemal 

 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. 



if. The ab-oral tentacles (h) which project through the 

 spaces between the ossicles, and open into, the perivisceral 

 cavity (t). 



e. The layer 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 pedicellarire and 

 the spines, and forms 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 ab-oral tentacles, and is 

 reflected out so as to cover all the organs which project 

 into the perivisceral cavity. 



</. Between this layer and the inner surface of the ab- 

 oral ossicles, a layer of narrow spaces (not sho\vn in the 

 diagram), lined by another layer of epithelium. In a 

 transverse section these spares appear as narrow slits. 



7^. The cross-sections of the hepatic coeca (i) suspended 

 to the bodv wall bvthe mesenteries. 



