STRUCTURE OF THE STARFISH. 79 



6. The epithelium of the outer surface (g) of the middle 

 of the ray is greatly thickened, and is generally regarded 

 as the true nerve of the ray. It is divided into two sharply 

 defined layers. 



1. An outer layer (g) made up of dark, granular, nu- 

 cleated cells. 



2. An inner layer (f) which consists of: 



(i.) Fibres perpendicular to the outer surface, and con- 

 tinuous with the granular cells. 



(ii.) Very fine longitudinal fibres, which appear as fine 

 dots in a transverse section (Fig. 33), but as fine parallel 

 lines in a longitudinal section (Figs. 3^ and 3$7jf). Ac- 

 cording to Ludwig, the fine longitudinal fibres are the true 

 nerve-fibres of the ray. 



7. Covering the outer surface, notice a very thin, trans- 

 parent structureless cuticle. 



0. Cut a number of vertical sections across the mouth- 

 pentagon, along the line of an inter-radial partition, and 

 mounting them, notice : 



1. The cut surface of the inter-ambulacral plate (Fig. 

 34, i a). 



2. The peristome (Fig. 34,^). 



3. The epithelium (h) which lines the peri viscera I 

 cavity (6). 



4. The cut sections of the circum-oral water-tube 

 (c iv). 



5. The muscle (m) which connects the ambulacra! ossi- 

 cles of adjacent rays with each other. 



6. The circum-oral peri-hremal vessel, which is divided 

 into an inner chamber (a) and an outer one (c) by a par- 

 tition. 



7. The circum-oral blood-tube in the partition. 



8. The thickened layer of pigmented cells (?) in the 



