136 



URODELA 



either capable of long fasts or live on infusoria in the water." 

 It seems more reasonable to suppose that these newts live upon 

 Crustacea, four kinds of which, all new to science, also came up 

 with the water. 



Fam. 4. Sirenidae. The three pairs of fringed external 

 gills persist throughout life. The body is eel-like. Hind-limbs 

 are altogether absent, while the fore-limbs are short and have 

 three or four fingers. The maxillary bones are absent. With 

 .the exception of small teeth on the vomer the mouth is tooth- 

 less, but the jaws are furnished with horny sheaths. The eyes 

 are devoid of lids, but shine through the skin. 



The Sirenidae are the most degraded members of the Urodela 

 and are represented by two closely-allied genera, each with one 



FIG. 27. Siren lacertina. x . 



species, in the south-eastern parts of the United States. Their 

 most interesting feature, which bears upon the question of 

 neoteny, is their retrograde metamorphosis as described by 

 Cope. 1 The gills atrophy in the young and are subsequently 

 redeveloped. Cope therefrom concludes rightly that the ultimate 

 or persistent gills of Siren are signs of maturity and not a larval 

 character. In young specimens of Siren of 5 to 6 inches in 

 length the gills are functionless ; in one of 3 inches they were 

 found to be entirely vestigial and " subepidermal," i.e. covered by 

 a common dermal investment. Unfortunately really young 

 larvae are still unknown. Old Sirens can live without gills, as 

 has been shown by aquarium-specimens. In the adult Pseudo- 

 branchus all the gills are normally covered up by an investment 

 of the skin so as to be quite without function and movability. 

 Siren lacertina, the " mud-eel," is distinguished by the 

 1 Amer. Natural, xix. 1885, p. 1226. 



