228 



WHALES 



Figure ii6. Cross-section through 

 the eye of man. Sc = sclerotic coat; 

 C = cornea; I — iris; L = lens; 

 Cb = ciliary body; V = vitreous 

 humour; Ch = choroid; R = 

 retina ; On = ocular nerve ; Om = 

 ocular muscle. {IJsseling and Schey- 

 grond, 1 95 1.) 



in the Marineland Aquarium certainly rely on their eyesight to capture 

 prey, and at the Woods Hole Biological Station a one-eyed Bottlenose was 

 repeatedly seen turning its head so that its good eye was towards the fish 

 it was pursuing. Now all these dolphins are diurnal animals, but the Pilot 

 Whales in the Marineland Aquarium, which feed on slow-moving cuttle- 

 fish and which are primarily nocturnal, definitely appear to rely on vision 

 to a much smaller extent. Further evidence for the assumption that cuttle- 

 fish-eating and deep-diving whales do not rely on eyesight is that Sperm 

 Whales have very small eyes indeed. Quiring, comparing the weight of 

 the eye of a Humpback Whale with that of a Sperm Whale (both animals 

 weighed approximately forty tons), found that the former weighed 

 980 grams whereas the latter only weighed 290 grams. Right Whales, too, 

 have relatively small eyes compared with Rorquals, whose eyes are one 

 and a half times as big. According to Gilmore (1958), Grey Whales use 

 their eyes to some extent for finding their bearings in clear water. The 

 smallest Cetacean eye is that of the Gangetic Dolphin, a lead-black 

 animal, some eight feet long, with a forceps-like beak (Fig. 117). It is 

 found in the Ganges and its tributaries, and in the only full description of 

 this animal, Anderson (1878) states that its eye is as big as a pea and has 

 no lens at all. The eye muscles are diminutive, while the optic nerve is 

 exceptionally thin. The Gangetic Dolphin feeds on fish and crabs off the 

 muddy bottom of these rivers, where eyesight would be no advantage to it; 

 besides, it is a nocturnal animal. In captivity, these dolphins generally 

 refuse all food during the day. Other fresh-water dolphins from the same 

 region (species of Orcaella) and those relatives of the Gangetic Dolphin 



