TYPES OF VERTEBRATE AND INVERTEBRATE EYES 19 



Fig. 14. — Semidiagrammatic 

 Section through the 

 Right Pineal Eye, 

 Pineal Nerve and Right 

 Habenular Ganglion 

 of Ammoccetes. (After 

 Gaskell.) 



Showing an optic vesicle of 

 the simple upright type 

 which resembles in some 

 respects types found in 

 certain of the inverte- 

 brates, e.g. Acilius (see 

 Fig. 13). The outer or 

 superficial segment of 

 the vesicle consists chiefly 

 of elongated clear cells 

 which form an imperfect 

 lens. The inner segment 

 is the retinal layer, which 

 on the right side of the 

 diagram is represented as 

 seen in a bleached speci- 

 men and on the left in 

 the natural condition 

 with the pigment in situ. 

 It consists of elongated 

 sensory cells, the distal 

 segments of which are 

 rodlike, while the proxi- 

 mal ends of the cells 

 appear to be continuous 

 with the component fibres 

 of the main optic nerve, 

 which ends in the right 

 habenular ganglion. In 

 addition to the sensory 

 cells of the retina are 

 certain oval and rounded 

 cells which are peripheral 

 in position and lie be- 

 tween the columnar cells 

 of the retina and the 

 capsule, and others 



between the retina and the lenticular cells. The pigment appears to lie 

 partly within the substance of the sensory cells and partly in specialized 

 intermediate pigment cells between the sensory cells. Superficial to the 

 optic vesicle are the connective tissue fibres of the cranial capsule, above 

 this but not shown in the diagram is the parietal plug, which intervenes 

 between the vesicle and the sub-epidermal stratum of connective tissue, 

 the latter corresponds to the substantia propria of the cornea of the lateral 

 eyes of vertebrates in general. 



c.t. : connective tissue. 



?i. sh. : nuclei of nerve sheath. 



pig. : pigment. 



pig. c. : pigment cells. 



p.n. : pineal nerve. 



ret. c. : sensory cells of retina. 



r.h.g. : right habenular ganglion. 



