94 



THE PINEAL ORGAN 



from the tip of each of the five rays of a starfish (Fig. 60). They are of 

 a bright red colour and form a slight elevation which is called the optic 

 cushion (Fig. 61). On microscopic examination, the cuticular epithelium 

 is seen to be specially modified in this area, and to line a series of small 

 conical pits or ocelli. The cells are elongated and form a single layer. 

 Their outer ends are directed towards the hollow of the pit, and are clear 

 and rod-like. The middle portion of each cell is deeply pigmented, while 

 the outer part which contains the nucleus is in relation with a subepidermal 

 plexus of nerve-fibres ; this is continuous with the radial nerve of the 



^-C. 





Wkr P c - 







$&M 



-cut. 



ep.t.c.' 



I.n.f. 



Fig. 61. — Section through the optic cushion at the base of the terminal 

 tentacle of an asteroid. 



c. : cuticle of the optic cup. ep. t. : epithelium of tentacle. 



p.c. : pigment cells. I.n.f.: layer of nerve fibres. 



cut. : cuticle of the epithelium of c.t. : connective tissue. 



the tentacle. ep. t. c. : epithelium of tentacle 



canal. 



corresponding arm which terminates in the central pentagonal nerve- 

 ring surrounding the mouth. 



In the sea-urchins slight elevations are present on the " ocular plates." 

 These are situated in a position which corresponds to that of the " optic 

 cushions " of star-fishes, namely at the tips of the five ambulacral zones 

 of the shell, in the " periproctal " region, round the anus. The ambulacral 

 zones are homologous with the rays of a star-fish ; the rays having, as 

 it were, been bent backwards towards the anal or aboral pole, which lies 

 opposite the oral pole on which the mouth is found. 



