142 Comparative Morphology of Chordates 



subcutaneous electric mantle of Malapterurus is unique. Electric or- 

 gans, so far as is known, exist in no vertebrates other than fishes, and 

 in only a very few of the vast number of species of fishes. This fact 

 makes it the more remarkable that organs of such peculiar nature 

 should have had so many separate origins. Paleontologists have sug- 

 gested that some of the ancient ostracoderms may have had electric 

 organs in the branchial region. The suggestion is based on evidence of 

 the existence of a group of large cranial nerves in a position correspond- 

 ing to that of the electric nerves of Torpedo. 



III. Class Amphibia 



This Class, taken as a whole, is amphibious, but many of its mem- 

 bers are not. Some spend their whole lives in the water; others spend 

 it on land. The great majority begin life in the water but, as adults, 

 live more or less after the maimer of the common frog, never far from 

 the water's edge, sometimes in and sometimes out. None inhabits salt 

 water. Its unique relations to both land and water give the Class 

 special interest and importance. 



The most highly characteristic feature of amphibians is the larval 

 stage. In most of them the egg develops into an aquatic fishlike "tad- 

 pole" having a long locomotor tail supported by a notochord. The 

 larva acquires three pairs of gills which, however, are unlike fish gills 

 in that they are tufts of vascular filaments projecting externally from 

 the surfaces between successive gill-clefts, of which four pairs usually 

 perforate to the exterior (Fig. 341). In frogs and toads these external 

 gills are later absorbed and replaced by internal gills which are more 

 like those of fishes (Figs. 342, 343). Meanwhile, a fold of skin grows 

 back over the external gill-apertures on each side, forming an opercu- 

 lum similar to that of fishes. In most genera the right and left folds 

 become joined so that the gill-clefts of both sides open into one common 

 opercular space, and this eventually has exit to the exterior through 

 only one aperture, which is usually on the left side but is sometimes 

 median and ventral. The opercular space is very similar to the peri- 

 branchial (atrial) cavity of Amphioxus and the tunicates. In larvae of 

 amphibians other than frogs and toads, the opercular folds are rudi- 

 mentary or absent. In some amphibians the larval period is passed, 

 not in external water, but in some internal cavity of one of the parents 

 — in the oviduct or in a pouch of the skin (Fig. 344) or even in the oral 

 cavity of the male (see p. 239). Under these circumstances, the gills 

 become variously modified. In certain terrestrial frogs the larval stage 

 is entirely omitted and the egg develops directly into a very small frog. 



In addition to gills and other piscine features, the larvae of amphib- 

 ians possess lateral-line sensory organs which differ from those of 



