PHYSIOLOGY OF THE INVERTEBRATA. 369 



eyes of other Invertebrata. Highly-developed simple eyes 

 are also present in the Branchiogasteropoda. 



In the Pteropoda the visual organ is either absent or it is 

 rudimentary. 



In the Cephalopoda the organs of vision are large and 

 highly developed ; in fact, the eyes of the typical Dibranchiate 

 Cephalopoda are more highly organised than those of any 

 other Invertebrate animal. A pair of eyes are situated in the 

 orbital cavities at the sides of the head in all the Dihranchiata. 

 The eye (Pig. 73) is more or less of globular form and con- 

 sists of the following parts : cornea, tapetum, ciliary body, 

 crystalline lens, vitreous humour, sclerotica, retina, white 

 glandular body, and the optic ganglion* and nerve. 



* A great part of the eyeball is occupied by the optic ganglion. 



2 A 



