354 



COMPARATIVE ANATOMY 



CHAP. 



round a central axis : into the proximal ends of each of these cells 

 a fibre from the optic nerve enters. The central axis is a tubular 

 rod called the rhabdom. The 5 cells of the retinula contain pigment 

 in the immediate neighbourhood of the rhabdom, and this pigment 

 occurs in such large quantities in the thicker nucleated portions of the 

 cells, that in each retinula a distal pigment layer is formed. Above 



FIG. 238. Compound Crustacean eye. A, 2 single eyes (ommatidia) of Palaemon Squilla. 

 The pigment is removed from the ommatidium on the right hand. C, Isolated crystal body 

 of an ommatidium, consisting of 4 pieces. D, Transverse section through a retinula, about 

 the middle of its length ; re, retinal cells ; rh, rhabdom consisting of 4 pieces. B, Section 

 through the stalked eye of Branchipus. E, 2 ommatidia of the same animal, on a larger 

 scale (figs. A, C, D, after Grenacher, B and E, after Claus); c, cornea (cuticle) ; d, corneal lens ; 

 hy, hypodermis cells ; fc, crystal cone ; fcj, outer crystal body ; Jcz, crystal cells ; p, pigment ; pj, in 

 fig. A, pigment strands running between the retinulse from the layer of nerve fibres ; re, retinula ; 

 rh, rhabdom ; nf, nerve fibres ; 6m, basal membrane ; m, muscle ; rg, retinal ganglion ; go, ganglion 

 opticum ; no, nervus opticus. 



this layer, terminal unpigmented portions project. These 5 ends 

 together embrace the proximal ends of 



B. the refractive crystal cells. These are 4 in number and 

 together form a cone, which contains in its distal portion a solid crystal 



