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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. 00--'. Pipa americana. 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 

 Coecilia are viviparous. The young of the Black Salamander 

 ($. 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 larvse 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 



