THE EYE OF PECTEN. 7 



vexities. Fig. 6 represents an isolated rod in this con- 

 dition. 



The anterior ends of the rods contract considerably, and 

 again expand into spindle-shaped bodies, each of which con- 

 tains a nucleus ; so that in P.jacohceus, where the retinal 

 elements of this region are difficult to distinguish, there may 

 be seen a single row of nuclei running from end to end of the 

 retina, and following its sinuosities (Plate II, fig. 11 h). 



In some of the rods at the side of the retina a second 

 spindle-shaped body follows the first one, as represented in 

 the isolated rods in figs. 5, 6, but usually the anterior end 

 of the spindle is drawn out into a delicate thread, which 

 occasionally possesses nuclear swellings. Finally, this thread 

 breaks up into a network, which bears a number of nuclear- 

 like bodies at its nodes, and several round molecular bodies 

 appear to be caught in its meshes. These bodies are so 

 much like the ordinary nuclei of the network that I am in- 

 clined to believe that they are, in reality, merely modifica- 

 tions of them, and in some way connected with the network 

 (fig. 6 a). Anteriorly the fibres of the network bend at right 

 angles and enter the nerve layer, which covers the anterior 

 surface of the retina. This nervous layer will be described 

 with the description of the optic nerves. 



The above is a description of the retina as I found it in 

 P. maximus, and I believe it holds good for the other mem- 

 bers of the genus. The elements of the retina are so much 

 larger in this species, and the spaces between the rods and 

 network, &c., so much more considerable, that it is a great 

 deal easier to investigate ; but I believe careful examination 

 of the other species would show that they do not differ from 

 this in any important detail. 



The tapetiitn — is placed immediately behind the retina, and 

 may help in its support. When fresh,^ the tapetum exhibits 

 a display of colours, and it is this membrane which gives 

 the eyes their beautiful metallic lustre. When examined 

 with a -i-th-inch obj. it seems to be composed of a great 

 number of little black specks separated by a fine yellow 

 membrane, but careful examination with a higher power 

 shows that it is composed of a great number of fine fibrils 

 crossing at right angles. 



The space between the tapetum and the posterior part of 

 the eye-cavity is filled with a red fiuid pigment. In the fresh 

 condition the pigment readily flows on to the slide when the 

 eye is pricked, but in sections of the eye which has been 



* I have one series of sections stained in osmic acid, and mounted in 

 Canada balsam, wliich has retained tliis display of colours. 



