376 ZOOLOGY 



6. A cloaca occurs, into which the anus and the ducts from 

 the excretory and genital organs open. 



7. Development usually by a metamorphosis. Segmenta- 

 tion total but unequal. 



396. Form. Amphibia differ much as to the shape of the 

 body. The newts and salamanders are elongated, slender and 

 eel-like; the frogs and toads have large, flat heads, stout trunk, 

 and muscular limbs. Among the former groups there may be 

 as many as two hundred and fifty body segments, in the latter 

 the vertebras behind the head are reduced to ten. The neck is 

 usually inconspicuous, the head being poorly movable. 



397. Appendages. There may be two pairs of appendages, 

 one pair, or none at all. In most forms except the Anura (tail- 

 less) the limbs are small and weak as compared with the body 

 (Fig. 191). The limbs have a distinct dorsal and ventral (palmar 

 surface, as well as an anterior and a posterior border. The 

 digits are enumerated from the anterior border which terminates 

 in the first, or thumb. In many forms there is a reduction of 

 the digits on the anterior appendage from five to four. The 

 digits are almost universally destitute of claws. The feet are 

 often webbed, and in the climbing toads the digits may end in 

 discs by which they cling to objects. 



398. The skin is normally soft, and slimy by reason of a 

 glandular secretion. It is composed of two layers, epidermis 

 and dermis. In the frog the epidermis is in two layers, the 

 outer of which may be shed at intervals. In toads, and other 

 forms frequenting dry places, the epidermis may form warty 

 thickenings. The skin is often highly colored owing to the 

 presence of pigment cells in the deeper layers. There are two 

 chief kinds, black and yellow pigment cells. The pigment 

 cells are much branched, and the pigment may either be con- 

 centrated or diffused through the cells. The different colors 

 the due to different concentration and different proportions of 

 are various pigments. In some cases the tones of color may be 

 changed in accordance with the surroundings by direct action of 

 light on the pigment cells, or by the reflex nervous action of the 



