688 Comparative Morphology of Chordates 



In sea cows and whales the lacrimal gland is much reduced or absent 

 (but a rudiment of it may occur in the embryo), and the large Harde- 

 rian gland is said to produce an oil in whales and a thick mucus-like 

 substance in sea cows. That the secreted substances applied to the 

 external surface of the eyeball in aquatic animals should be oily instead 

 of watery is intelligible. Whales lack Meibomian glands, but are said 

 to have small oil-glands in the conjunctiva of the lids, and they have 

 no nasolacrimal duct. 



In moles (insectivores) and some burrowing rodents the eyes are 

 more or less degenerate. In the marsupial, molelike, burrowing 

 Notoryctes there are only quite functionless vestiges of eyes. 



The immediate connotation of the word "ear" is sound and hear- 

 ing. But the vertebrate ear is physiologically two organs, one serving 

 for equilibration, the other for hearing. The specialized structures 

 concerned with these two very unlike functions tend to become sepa- 

 rately localized in the ear. The at first simple otic sac which invaginates 

 from the superficial ectoderm of the side of the embryo's head (see p. 

 207) differentiates into a dorsal and a ventral enlargement. The former, 

 the utriculus, produces the semicircular canals (Figs. 180, 182), 

 three in all vertebrates except cyclostomes; and from the ventral 

 sacculus grows out a more or less elongated tubular structure, the 

 lagena. The semicircular canals are equilibratory. The lagena, becom- 

 ing the longer and spirally curving cochlea of Amniota, is concerned 

 exclusively with hearing. In vertebrates other than mammals, the 

 utriculus and sacculus communicate freely through the intervening 

 narrower region of the original otic sac. In mammals the passage 

 between them becomes very narrow, or, as in man (Fig. 514), they are 

 commonly described as connecting only indirectly via the bifurcated 

 proximal end of a tubular structure, the endolymphatic duct, whose 

 distal end pierces the inner cranial wall and terminates in an enlarged 

 endolymphatic sac which lies in the space between the cranial wall 

 and the brain. The sac and duct probably allow for diffusion of sub- 

 stances between the endolymph and the lymphlike cerebrospinal Quid 

 which bathes the outer surfaces of the brain and spinal cord. These 

 mammalian endolymphatic ducts are not exact homologs of the 

 similarly named ducts which, in sharks, open by a pair of pores on top 

 of the head. The latter are the persisting and much-elongated distal 

 parts of the original embryonic otic invaginations. In vertebrates other 

 than elasmobranchs, the primary connection with the external surface 

 is obliterated and a secondary endolymphatic duct develops by out- 

 growth from the otic sac, but it may involve at least the stump of the 

 primary invaginated duct. 



