AQUATIC AMPHIBIA; SIRENIANS 407 



large-eyed stage, the 'macrophthalmia', in preparation for their transfer 

 from fresh water to the sea, where their adult lives are spent. Even in 

 some of the non-parasitic lampreys which remain always in fresh water, 

 there are traces of a macrophthalmia stage — as a remembrance of the 

 more complicated life history of their ancestors. 



Aquatic Amphibia — Those salamanders which are permanently 

 aquatic live in shallow water, and have little use, or special adaptation, 

 for underwater vision. Many of these forms are secretive, living in mud 

 or under flat stones — for example Necturus, Cryptobranchus, Siren, and 

 Amphiuma. In such species the eye is extremely crude and disharmon- 

 iously developed, and vision is no more than a mere directional light- 

 sense. As would be expected, the eyes of cave forms (Proteus, Haide- 

 otriton, Typhlomolge, adult Typhlotriton) are microscopic, concealed, 

 and functionless. Some newts and axolotls, however, have quite present- 

 able eyes. Less complex than good anuran eyes, their simplicities are not 

 all attributable with certainty to the aquatic mode of life. But at least the 

 spherical lens, the absence of iris folds and of the canal of Schlemm, and 

 the emmetropic refraction in water are as probably positive adaptations 

 as they are mere evidences of primitiveness. The few terrestrial salaman- 

 ders so far studied are emmetropic in air, and hence (at least when 

 adult) become hypermetropic in water, at breeding time. 



In permanently aquatic anurans, such as the aglossal toads (Pipa, 

 Xenopus, Hymenochirus, etc.) and the pseudine bufonid Telmatobius 

 microphthalmus, the eyelids never develop as they do, at metamorphosis, 

 in other frogs and toads. The eyes are very small, with round pupils. 

 Externally they give the appearance of being almost as degenerate as 

 those of the Central American termitivorous toads which live under- 

 ground. But little seems to be on record concerning the anatomy and 

 histology of the eyes of the above-mentioned genera. 



Sirenians — Two groups of mammals have become secondarily adapted 

 to water so completely that they are even able to breed in that medium 

 and, unlike the seals, never need to return to the land. These are the sea- 

 cows and the whales. Not that these animals never put their heads out 

 of water — supposedly, it was a distant glimpse of an upreared manatee, 

 its nursing baby cradled in its flippers, which gave some ancient sailor 

 the raw material from which the legend of the mermaid was constructed. 

 The old superstition is commemorated in the modern scientific name of 

 the order Sirenia. 



