92 COMPARATIVE ANATOMY. 



Mammalia. Nasal cavities complicated by sinuses occu- 

 pying superior maxilla, sphenoid and frontal bones; eth- 

 moidal scrolls greatly developed. Turbinated bone long and 

 comparatively simple in Ruminantia, but elaborately convo- 

 luted in many Carnivora. Exterior openings of nostrils 

 valvular in diving animals, as in Castor (beaver). The space 

 including the nostrils, when not prolonged beyond the lips, 

 is called the muzzle; when moderately extended, the snout, 

 as in Sus (hog); when conspicuously developed, the proboscis, 

 as in Elephas (elephant). 



EYE. 



The eye is the organ of vision. To adapt itself to the 

 function of receiving upon a nervous expansion a series of 

 external images or diffused rays of light, the anterior portion 

 of the capsule is transparent, convex, and very generally 

 placed upon the periphery of the body. The wall of the cap- 

 sule is the sclerotica; its anterior arc, the cornea; the liquid 

 contents, the humors; the nervous expansion, the retina ; and 

 the nerve filament, the optic nerve. These characters are es- 

 sential to an eye. Accessories both within and without the 

 organ add to its complexity. Eyes may be single, agglom- 

 erate, or compound. 



In the lowest animals the essential features of eye and ear 

 resemble one another so intimately that it is with difficulty 

 that the organs are distinguished. As a rule, the constantly 

 vibratile action sustained by the otolithes may serve to iden- 

 tify an ear, since the analogous body in the eye (crystalline 

 lens) is more fixed. 



PROTOZOA. Eyes wanting. 



RADIATA. Many radiate animals possess capsules upon the 

 edges of the body, as in Aurelia (jelly-lish), or near tentacles, 

 as in Syncoryne. Immediately behind the corneal portion, 

 and within the chamber, are occasionally lodged movable 

 polygonal crystals of carbonate of lime (crystalline lens), 

 which are generally of a red color. 



MOLLUSCA. Polyzoa and Brachiopoda. Eyes wanting. 



