502 



THE EYE IN EVOLUTION 



an oily substance to protect the keratinized cornea against the sea-water ; 

 in the absence of naso -lacrimal canals, this secretion j^ours copiously 

 over the face when the animal is on land (Fig. 663). In Sirenians 

 (sea-cows) the lashes are extremely scanty but the lids freely mobile, 

 closing completely over the small eye when it is pulled backwards by 

 the well-developed retractor muscle. There is no lacrimal gland but 

 the harderian gland is well developed, as in Pinnipedes, secreting a 

 copious thick mucoid secretion like egg-white (Fig. 664). 



The Cetaceans (whales and dolphins) are completely adapted to 

 aquatic life : the lids are small, without tarsal plates or tarsal glands ; 



Figs. 663 and 664. — Aquatic Placental^. 



Phoca 



Showing the upwardly directed eyes, 

 as an adajDtatioii for swimming (Zool. 

 Soc, London). 



Fig. 664.— The Head of the Manatee, 

 Trichechus manatus. 



Showing the small retractable eyes 

 (photograph by Michael Soley). 



lashes are lacking ; a " lacrimal " gland is present but secretes not 

 tears but a fatty water-repellant secretion, and the same hypertrophy 

 of the harderian gland is seen, the oily secretion of which is augmented 

 by that of numerous oil-glands distributed over the palj)ebral conjunc- 

 tiva. The naso-lacrimal conducting mechanism is absent as also is 

 the nictitating membrane. The extra-ocular muscles are, however, 

 enormous, more in keeping with the size of the animal than that 

 of the small eye ; each rectus is comparable to the biceps of man. 

 In the whale this seems curious in view of the immobility of the 

 downward-looking eye fixed firmly on its immensely rigid accessory 

 optic nervo sheath and situated low down on a level with the 

 angk- '' the mouth about one-third of the length of the huge animal 

 away : a its anterior extremity. It has been said that the enormous 



