XIII 



PHYLUM CHORDATA 



463 



Cetacea, an underlying layer of fat performs the function of the 

 hairs as a heat-preserving covering. 



In Manis (Fig. 1050) the greater part of the surface is covered 

 with large rounded overlapping horny scales of epidermal origin, 

 similar in their mode of development to those of Reptiles. A 

 similar phenomenon is seen in the integument of the tail of 

 Anomalurus a Flying Rodent. The Armadillo is the only Mam- 

 mal in which there occurs a bony dermal exoskeleton (vide infra). 



Also epidermal in their origin are the horny structures in the 

 form of nails, claws, or hoofs, with ' which the terminations of the 



g.m 



d. 



FIG. 1040. Echidna hystrix. A, lower surface of brooding female; B, dissection showing 

 a dorsal view of the poxich and mammary glands ; t t, the two tufts of hair in the lateral 

 folds of the mammary pouch from which the secretion flows ; bm, pouch ; cl. cloaca ; g. m. 

 groups of mammary glands. (From Wiedersheim's Comparative Anatomy, after W. Haacke.) 



digits are provided in all the Mammalia except the Cetacea. And 

 the same holds good of the horny portion of the horns of Ruminants. 

 The horns of the Rhinoceros are also epidermal, and have the 

 appearance of being formed by the agglutination of a number of 

 hair-like horny fibres. 



Cutaneous glands are very general in the Mammalia, the most 

 constant being the sebaceous glands (Figs. 1037, JJ ; 1038, H, B, D\ 

 which open into the hair- follicles, and the sweat, glands (Fig. 1037, 

 SD). In many Mammals there are, in addition, in various parts 

 of the body, aggregations of special glands secreting an odorous 

 matter. 



