ADNEXA IN MAMMALS 427 



does also the reindeer). The bear-like giant panda also has a prominent 

 nictitans. 



Ungulates all have the nictitans, though with great interspecific vari- 

 ations, and usually with no apparent usefulness. In the horse family, 

 however, the nictitans is as extensive and as rapid in action as in many 

 sauropsidans. Its retention here is attributed to the need for special pro- 

 tection of the eye when feeding in deep grass, and an analogous useful- 

 ness would explain its persistence in the Sirenia. It has however not 

 been reported as being particularly well developed in the antelopes, most 

 of which have horse-like feeding habits. 



The retractor bulbi is well distributed in the lower orders of mammals, 

 and occurs in scattered species among the higher orders. In some mam- 

 mals, including all rodents, the globe is pulled back somewhat into the 

 orbit directly by its action. In other instances, especially among the 

 'edentates', the eye seems rather to be pressed back passively by the lids 

 during their periodic closures. In the hairy armadillo (Dasypus villostis) , 

 and also in the echidna, the lids simply swing together like a pair of 

 gates whenever the eye is retracted, instead of sliding over the globe. 



A most peculiar arrangement is seen in the opossum. As the eye closes, 

 two vertical folds form in the conjunctiva, one at either canthus; and 

 these close tightly over the cornea so that if the lids were then forced 

 open, one might think the eye had been replaced by a white tumor. The 

 writer has been fooled by a similar concealment of the retracted and 

 rotated eyeball of a snapping turtle by proptosed conjunctiva and mus- 

 cles. Another unusual phenomenon occurs in the rhinoceroses and, less 

 conspicuously, in one species of bear {Melursus labiatus) . Here the eye- 

 ball, every few seconds, is flicked temporally and retracted at the same 

 time, all with lightning speed. The action appears to be a clumsy sub- 

 stitute for the kind of rhythmic blinking we humans perform, for it takes 

 place too quickly to seem a means of sweeping the horizon for the detec- 

 tion of possible approaching enemies, 



Inter-Relat'wns of Globe and Adnexa — The evolution of lids and 

 their associated muscles and glands by the air-breathing, air-seeing verte- 

 brates represented primarily an effort to protect the eye by keeping it in 

 a local aquatic environment. This method has been highly successful — 

 too much so, in a sense, in secondarily aquatic forms, which have appar- 

 ently found it impossible to dispense with as many of the concerned parts 

 as we might think they could easily discard. An even better protection 



