STRUCTURE OF THE STARFISH. 79 



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

 of the ray is greatly thickened, and is generall}' 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- 

 tiniious with the gi'anular 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. 35 and 36, y). Ac- 

 cordinof to Ludwiw, the fine louoritudinal 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 (i). 



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



3. The epithelium (/^) which lines the pei-i%'iscei"al 

 cavity (6). 



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

 (c?r). 



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

 cles of adjacent rays with each other. 



6. The circum-oral peri-h?emal 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 (I) in the 



