HIGHER CHORDATES 



283 



external gill: 



caudal Fin 



branchial opening 



NECTURUS 



\nostril 



ip fold 



CRYPTOBRANCHUS 



Figure 16.18 Two aquatic salamanders, Necturus, the mudpuppy, and Cryptobranchus, the hell 

 bender. (From Malcolm Jollie, Chordate Morphology, Reinhold Publishing Corp., New York, 1962.) 



lationships are concerned, because frogs and toads 

 occur in the same genus. In general, frogs are aquatic 

 forms having smoother skins, shmmer bodies, and 

 longer hind legs than terrestrial toads. 



Anura protective devices against enemies are simi- 

 lar to those of salamanders in that they are primarily 

 defensive rather than offensive. When cornered, frogs 

 and toads usually bloat and assume a squatting posi- 



Figure 16.19 A toad, Rulo (to 7 inches long), and a frog, Rano (to 

 8 inches long). (After Dickerson.) 



tion tight to the substrate, or rely upon skin poisons 

 to discourage predators. American frogs tend to be 

 restricted to such habits. However, there are African 

 frogs with poison and fangs to discourage predators 

 and perhaps kill some food. 



The sexes of our adult anurans can usually be de- 

 termined by various external sexual differences. Only 

 the male tailed-frog has a tail, an organ for internal 

 fertilization. In most anurans only the breeding 

 males have horny pads on the thumbs, fingers, and/ 

 or forearms. In many toads and tree frogs the breed- 

 ing males have a darkening of the throat. 



North American species lay their eggs in water. 

 Adult toads and many frogs migrate from land during 

 rains to their aquatic breeding habitats. The time of 

 pond entrance is a noisy period (in many species the 

 males croak or produce other breeding calls that 

 attract the females), because hundreds of individuals 

 congregate at localized breeding sites. In many 

 species, the males clasp the females and sperm is shed 

 simultaneously with the eggs. The egg hatches into a 

 tadpole, a vaguely fish-like, highly aquatic larval 

 stage. The tadpoles, or polliwogs, in contrast to the 

 adults are usually plant feeders, but many are scaven- 

 gers, and spadefoot toad larvae are carnivorous. Tad- 

 pole life extends from two weeks in some spadefoots 

 and other toads to two years in some frogs. Meta- 

 morphosis involves marked changes: the tail and gills 

 are lost, and the gill slits close; the digestive tract is 

 shortened; the eyes are shifted in position; the jaws 

 and skin are modified; and the forelimbs appear. In 



