360 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



(ix.) The outer layer {h) of the retina is about as thick as 

 the layer of rods, and is made up almost entirely of gan- 

 glion cells, and is similar, in structure, to the surface layer 

 of the pedal ganglion (<:?). 



(x.) The inner layer of rods is separated from the outer 

 layer of ganglion cells by a thin third layer, the layer of 

 pigment. This is continuous at the anterior edge of the 

 retina with the layer of pigment on the internal surface of 

 the ciliary body. 



(xi.) Outside the retina there is a layer (r) of cartilage 

 and muscular fibres, which supports the retina. 



(xii. ) Outside this is the optic ganglion {g) . In the more 

 anterior sections this ganglion is small, and consists almost 

 entirely of gray matter, but in sections farther down the 

 head it is very large, and consists, like the pedal ganglion, 

 of an outer layer of gray ganglion cells, and a central axis 

 of white nerve-fibres, some of which pass through open- 

 ings in the cartilaginous eye-capsule (r) into the ganglionic 

 layer of the retina. 



e. In a section just posterior to the eyes (Fig. 189) no- 

 tice the cesophagus (Z), surrounded by a sheath of circular 

 fibres, which also enclose the ducts of the salivary glands. 



1 . Dorsal to this the middle line of the body is occupied 

 by the cerebral ganglia (7n) , and the commissure {g) be- 

 tween the optic ganglia. 



2. The sides of the head are filled by two great masses, — 

 the optic ganglia {h). Each of these consists of a surface- 

 layer of gray ganglion cells {j), of a second thin layer 

 of white matter (^), and of a great central mass of white 

 matter (Ji) in which are complicated radiating bands of 

 ganglion cells. 



3. On the ventral surface of the oesophagus, on the mid- 

 dle line of the head, t\iQ pedal ganglion (/"). 



