BRANCH CHORDATA: CLASS BATRACHIA 293 



The skin is sometimes thrown up into folds or ridges, and 

 in some species is elevated to form a kind of fin on the 

 tail or back. This unpaired fin differs from the dorsal fin 

 (and other fins) of fishes in not being supported by rayed 

 processes of the skeleton. There are in some batrachians 

 traces of an exoskeleton in the presence of scale-like 

 structures in the skin or in the horny nails on the digits, 

 but these cases are rare. The skin contains pigment-cells 

 and many of the batrachians are brilliantly colored and 

 patterned ; some of the pigment is carried by special con- 

 tractile or expansile cells, the chromatophores (see 

 account of chromatophores of the Cephalopoda, p. 256), 

 so that the animal can change its tint and markings more 

 or less rapidly. All the batrachians possess external gills 

 in their aquatic larval stage, and in a few forms, as the 

 sirens and mud-puppies, gills are retained all through life. 

 These gills are branched folds of the skin abundantly 

 supplied with blood-vessels. 



In the organization of the batrachian body the usual 

 vertebrate characters appear, the body-organs being 

 arranged with reference to a supporting and protecting 

 internal bony skeleton. The head is plainly set off from 

 the rest of the body and bears the mouth and the organs 

 of hearing and sight. Certain so-called lateral sense 

 organs, the function of which is not exactly known, occur 

 arranged in three lines on each side of the body of some 

 of the forms. Both pairs of limbs are present and func- 

 tional in almost all of the species. In the coecilians the 

 limbs are wholly wanting ; in the sirens only the fore legs 

 are present. 



Structure. The most obvious skeletal differences 

 among batrachians are those due to variations in external 

 form. While there are as many as 100 vertebra,* in some 

 of the elongate long-tailed salamanders (even 250 in the 

 strange snake-like ccecilians), there are but 10 (the last 



