422 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Though it is difficult to see how the pyramidaUs could have arisen as a 

 derivative of the retractor bulbi, it is supplied by the same cranial nerve, 

 the sixth or abducens. In fact, all of the conspicuous muscles specially 

 developed by land animals — retractors, levators of the upper lid, depres- 

 sors of the lower, operators of the nictitans — are irmervated by one or 

 another of the same three cranial nerves (third, fourth, and sixth) which 

 supply the six primitive oculorotatory muscles (the four recti and the two 

 obliques). Each of the newer muscles can be seen, with more or less 

 clarity, to have been derived from some member of the original set. 



The crocodilians have a lacrimal gland under the dorsal orbital roof — 

 the lacrimal, in vertebrates generally, is most often tucked under the 

 more mobile of the two lids — and they also have a large Harderian gland 

 with several outlets beneath the nictitans. This situation reflects the 

 greater importance of oily, than watery, secretions for the insulation 

 of an essentially terrestrial eye, in an animal which has secondarily 

 returned to water. The condition in the marine Crocodilus porosus is 

 particularly interesting, as a parallel to that in the Sirenia and Cetacea; 

 for here the conjunctiva of the lower lid is similarly paved with glands, 

 and the nasolacrimal duct, though present, has only one puncta instead 

 of the row of three to eight seen inside the lower lids of other crocodiles. 

 In Caiman sclerops (the spectacled cayman) the upper lid shows peculiar 

 variations, being swollen and wrinkled in some individuals and horny in 

 others, as it is also in C. latirostris. 



Turtles — The turtles have also 'gone back to the water', and their eyes 

 reflect the change of habit from terrestrial to amphibious — and back to 

 terrestrial, in the box turtles and desert tortoises. The adnexa have fol- 

 lowed all but the last of these vicissitudes. 



The lower lid is the larger, but has lost its tarsus since the wetted eye 

 needs none. The nictitans has a small cartilage and is operated, as in 

 crocodiles, by a pyramidalis, which sends a second tendon to the lower lid 

 and thus acts as a levator muscle for the latter (Fig. 143e). The retractor 

 bulbi is powerful, and may turn the eyeball almost completely over as it 

 retracts, the nictitans and lower lid closing the eye passively at the same 

 time. The palpebral fissure, or opening between upper and lower lids, is 

 canted more or less so that it runs dorso-temporally to ventro-nasally of 

 the eyeball. Though this same slant has been retained in strictly terres- 

 trial turtles, it seems most useful to the freshwater and marine turtles, 

 which float in a slanted position at the water surface; for when their 



