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PHYLUM CHORDATA 289 



on it by the male, and each sinks into a little pouch of skin 

 covered by a gelatinous film. The embryos, which have a large 

 yolk-sac, develop in these pouches; they never possess external 

 gills, and are hatched in the adult form. Another Anuran, Pseudis 



FIG. 902. Pipa americaxia. Female. (From Mivart.) 



paradoxa, is remarkable for the fact that the Tadpole is many 

 times larger than the adult. 



Some Salamanders (S. maculosa and S. atra) and a species of 

 Ccecilia are viviparous. The young of the Black Salamander 

 (S. atra) possesses long plume-like external gills during its 

 existence in the oviduct, shedding them before birth. If, how- 

 ever, the unborn young is removed from the oviduct and placed in 

 water, it swims about like an ordinary aquatic larva, losing its 

 long gills and developing a new and shorter set. Most Gymno- 

 phiona lay their eggs in burrows, but the larvae in some cases lead 

 an aquatic life for a time, and during this period possess, like 

 Tadpoles, a tail with a tail-fin which afterwards undergoes absorp- 

 tion. The larvae of most Gymnophiona have long external gills 



(Fig. 903). 



A very interesting case of pcedogenesis is furnished by the 

 Axolotl (Amblystoma tigrinum). This animal frequently under- 

 goes no metamorphosis, but breeds in the gilled or larval state 

 (Fig. 904). But under certain circumstances the gills are lost, the 

 gill-slits close, and a terrestrial salamandrine form is assumed. It 

 is to the branchiate stage that the name Axolotl properly applies ; 

 before the metamorphosis was discovered its connection with 

 Amblystoma was not suspected, and it was placed in a distinct 

 genus, Siredon, among the Perennibranchiata. 



VOL. II U 



