AMPHIBIA. 373 



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

 the excretory and genital organs open. 



7. Development usually by a metamorphosis. Segmentation 

 total but unequal. 



389. 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 strong muscular limbs. Among the former groups there 

 may be as many as two hundred and fifty body segments, in 

 the latter the vertebrae behind the head are reduced to ten. The 

 neck is usually inconspicuous, the head being poorly movable. 



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

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

 (tailless) the limbs are small and weak as compared with the 

 body (Fig. 188). 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. 



391. 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. In some cases 

 the tones of color may be changed in accordance with the sur- 

 roundings, by the reflex nervous action of the animal, resulting 

 from impressions on the retina of the eye. In the extinct 

 Labyrinthodonts external protective plates were developed in 

 the dermis. Minute dermal scales are found in some of the 

 lowest present forms ( " blind-worms " ) . 



392. The Skeleton. The points of contrast with the skele- 



