148 Comparative Morphology of Chordates 



little or no use to the animal. In one of these "eels" (Siren), hindlegs 

 are entirely absent. At the other extreme are the frogs and toads, whose 

 legs are strongly developed and modified for vigorous leaping locomo- 

 tion (Fig. 345). A median tail-fin is present in larvae and in gill- 

 breathing adults, but it differs from the tail-fin of fishes in having no 

 skeletal fin-rays. 



The skeleton is mainly bony, although more or less of the embry- 

 onic cartilage persists in the skeleton of the head (Figs. 345, 346) and in 

 the appendages. In tailed amphibians remnants of the larval noto- 

 chord may persist in the vertebral column. The skull is much simpler 

 and contains fewer bones than that of a bony fish. In modern amphib- 

 ians ribs are always poorly developed — short, mere rudiments, or 

 entirely lacking. 



The skin is of the aquatic type — thin and richly mucous (Fig. 347) 

 — even in those that live on land, although in toads it is thicker and 

 less mucous. In modern amphibians there are usually no scales of any 

 sort. In certain frogs, especially some having a burrowing habit, there 

 are bony plates in the skin of the back, and in some of the externally 

 snakelike caecilians there are very small bony scales buried in the skin 

 and invisible externally. In early amphibians, the Stegocephalia, the 

 animal was more or less completely covered by an armor of heavy bony 

 plates, sometimes especially large and heavy over the head. 



The amphibian heart, like that of lungfishes, has two auricles and 

 one ventricle, the right auricle receiving blood from the general circula- 

 tion, the left receiving from the lungs. In frogs and toads the partition 

 between right and left auricles is complete, but in other amphibians 

 it is more or less incomplete. 



No intestinal spiral valve is found in any modern amphibian. 

 In all of them the anus and urinogenital ducts open into a cloaca. 



Between the nervous organs of amphibians and those of fishes 

 there are no very striking anatomic differences other than the relatively 

 small size of the cerebellum (Fig. 348). The amphibian brain, however, 

 has one feature which, although not conspicuous, is functionally very 

 significant. In most fishes the dorsal wall (pallium) of the most anterior 

 (telencephalon) of the five regions of the brain contains no nervous 

 tissue. This is in utmost contrast to the fact that in mammals, including 

 man, the pallium, forming the thick roof of the cerebral hemispheres, 

 becomes elaborated into a vast complex of nervous elements whose cell- 

 bodies (i.e., "bodies" in distinction from the nerve-fibers which are 

 outgrowths from them) are massed into a sharply delimited superficial 

 layer of "gray substance," the cerebral cortex. This cortex con- 



