no 



THE PINEAL ORGAN 



inwards as a tract to the central nervous system. In invertebrates, how- 

 ever, the optic tract is formed by a series of ganglia, with intervening 

 plexiform zones, the tissue of which is known as the neuropil or punctate 

 substance. The ganglia appear as localized swellings in the stalk of the 

 eye, which differ in number in different types of Crustacea. In Astacus, 

 if the retinal ganglia are included, there are four ganglia ; in others there 

 are three or two. Thus in Astacus we have a compound retina and a 

 continuation of the nerve-chain as a tract in the form of a series of ganglia. 

 The compound retina of crustacean eyes is, moreover, complicated by 



xS*-c.l.e 



Fig. 72. — Head of Phyllosoma Larva, showing Lateral and Median Eyes 



(Palinurus) (R. J. G.). 



c.l.e. : compound lateral eye. 

 f.m.e. : fused median eyes. 



the modification of the outer ends of certain of the cells to form refractive 

 elements, namely, the crystal-cones — cylinders or prisms which are isolated 

 by separate pigment cells. Moreover, there is a further modification both 

 in the retina itself, as in Palcemon (Fig. 37, p. 52), and in the optic stalk 

 beyond the basement membrane in the form of spindle-shaped swellings 

 which are highly refractile and are either transversely striated or show 

 spiral markings. These were named " rhabdomes " by Grenacher, and 

 are especially well seen in the retina of Palcemon, which besides the corneal 

 facets or lenses and the principal crystal cones CC, which are intermediate, 

 has distal cc and proximal refractile elements cc" . This type of eye is 



