408 ADAPTATIONS TO MEDIA AND SUBSTRATES 



The existing sirenians comprise the manatees and the dugongs, the 

 genera Trichechus and Dugong respectively. These animals are Uttoral, 

 cropping grasses in shallow water, salt or brackish. In great contrast to 

 the whales the sea-cows have an acute olfactory sense, and excellent hear- 

 ing as well. Their eyes are relatively small considering the size of the 

 animals and the turbidity of their visual medium. The eye of a six-foot 

 Dugong dugon is about man-sized, with horizontal and vertical diam- 

 eters of 25mm. and an axial length of 23mm. The eyes of manatees are 

 somewhat smaller (Fig. 140). 



The alterations of the eye for aquatic activity relate chiefly to the 

 adnexa. While these structures have specialized about as far as those of 

 whales, the globe on the other hand has lost the organization which 

 would make it a good organ for vision through air, without taking on 

 those characteristics which would make it really valuable under water. 

 As compared with the whales, and particularly as compared with the 

 seals, the sirenians have been most half-hearted in their ocular mod- 

 ifications for life in the water — no doubt because they were already 

 placid herbivores (their ancestral roots are in the pro-ungulate stock) 

 before ever they took to the sea. The condition of the modern hippo- 

 potamus, whose eyes are not his pride, affords an analogy for the prob- 

 able half-way stage in the derivation of the sirenian type from a strictly 

 terrestrial one (see p. 443). 



An unusual area of the sclera shows through the lid opening — as in 

 man, where it is also the result of a small cornea coupled with great 

 mobility of the globe. The lids have practically lost their lashes, but 

 they have well-developed muscles; and a retractor bulbi muscle is present 

 so that the eye is protectible from mechanical injury. When the globe 

 is retracted, the lids can be closed ahnost completely. There is disagree- 

 ment as to whether a nictitating membrane is present. The tear-gland 

 has vanished, but the Harderian gland has been retained. Its secretion 

 is apparently not the usual sebaceous sort — Dexler and Freund describe 

 a continuous flow of tough egg-white-like material from the eyes of a 

 landed dugong. The cessation of this flow, as in a specimen which has 

 drowned in a submerged net, promptly leads to a severe damaging of 

 the cornea by the sea-water. The mucous Harderian secretion is aug- 

 mented by the products of a regular pavement of special oil-glands lining 

 the eyelids. Obviously the Sirenia are not interested in trying to recruit 

 aqueous humor from the outside water, for they effectually prevent the 

 latter from actually touching the cornea. Like land animals, they secrete 



