SPECIAL ORGANS. 95 



fly), they are 400 in number; in LibeUuta, 12,000; in Mwdella, 



25,088. 



VERTEBRATA. Pisces. Eye compressed antero-posteriorly; 

 sclerotica thick, sometimes bony, as in Cephaloptera (devil- 

 fish); crystalline lens globular; iris fixed: pupil in Anableps ^c 

 (star-gazer), double. Surrounding the entrance of the optic 

 nerve into the eye of osseous fishes is seen a crescentic body, | 

 called the 'choroid gland.' It is not glandular, but erectile, 

 being composed of congeries of blood-vessels. Its use is not 

 certainly known. It may assist in accommodation, by push- 

 ing the retina forward during turgescence. The eye of the 

 fish is adapted to focusing through a dense medium under 

 great pressure. 



Reptilia. General features of eye those of other air-breath- 

 ing vertebrates. Certain extinct animals, as Ichthyosaurus -, 

 had a more complex sclerotica than any extant forms. It was 

 composed of a number of distinct plates, by which the focus- 

 ing distance could be regulated by muscular action. (See 

 Aves.) Rudiments of this structure can be detected in the 

 eyes of certain Chelonia and Sauria. The Chameleon can 

 i|move eyes in opposite directions. Pupil, as a rule, fixed, 

 slit-like. 



Aves. Most peculiar feature witnessed in rapacious bird, 

 where the sclerotic coat is composed of a number of plates 

 arranged in the long diameter of the organ, capable, by 

 mutual compression, of elongating, or, by relaxation, of 

 shortening the axis of the organ. By this means an object 

 of prey can be distinguished at a great distance while it is 

 equally visible at the moment it is seized. Within the choroid 

 tunic a peculiar body, called from its appearance the pecten, 

 is situated : it is a fold of the choroid, and is often attached 

 to the capsule of the lens. Its function is unknown. 



Mammalia. Eye closely resembles that of man. Is slightly 

 compressed antero-posteriorly (as in fishes) in certain aquatic 

 animals, as in Phoca (seal). Eye rudimentary in Talpa (mole). 



Appendages to eye. Conjunctiva well defined, excepting in 

 fishes, where it is rudimental and yields no secretion. 



In Reptilia lachrymal gland makes its appearance for the 



