446 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



sirenians and whales, but it is not 'nocturnal' in size. In fact, it has about 

 the same relative diameter equatorially as that of man, being 37% of the 

 vertical diameter of the eyeball in Phoca vitulina and 38.5% in Eumeto- 

 pias. These ratios are quite distant from those obtaining in nocturnal 

 carnivores (about 50%) and are closer to those in the twenty-four-hour 

 ungulates (around 40%) . The lens being spherical, it can be thought of 

 as being greatly thickened; but this is an adaptation to the 'loss' of the 

 cornea in aquatic vision, and not to nocturnality; for, the seals are diur- 

 nal. Its refractive index, like its shape, is on a par with that of fishes, and 

 Matthiessen's ratio (p. 264) probably holds for its relationship to the 

 retina. In consequence of the sphericity, the border of the lens epithelium 

 reaches around past the equator of the lens onto its posterior face, as it 

 does in fishes, aquatic amphibians and tadpoles, and toothed whales. 



cornea 



pectinate ligament 



ciliary process 



lens 



adipose pad 

 optic nerve 



Fig. 150 — Seal eyes, x 1. After Putter, 

 a, an eared seal, Otaria jubata. b, a true seal, Phoca vituli, 



In view of the diurnality of the group, the apparent nocturnal adap- 

 tations of the pupil, retina, and chorioid seem paradoxical at first 

 thought; but this paradox is the very heart of the method by which the 

 seal accomplishes amphibious vision. The retinal rods are exceedingly 

 long and they are commonly said to have no cones amongst them; and 

 the retina is backed by a bright chorioidal tapetum over much of its area. 

 These features bespeak a sensitivity which appears totally unnecessary, 

 at least to the shallow-swimming Otariidas. The reason for it is complex 

 but fascinating : 



In both seals and sea-lions the pupil gives some evidence of being 

 under voluntary control, but it is ordinarily a very large circle as long as 

 the eye is under water. The dilatator is so conspicuously developed that 



