354 CLASSIFICATION OF VERTEBRATES. 



other forms (insectivores, marsupials, etc.). More rarely the 

 whole body may be encased in them as in the pangolins. Again, 

 as in the armadillos, dermal bones are developed in connection 

 with the scales, while in some embryonic cetaceans similar 

 features are seen. 



Glands are far more abundant than in the sauropsida, and 

 include, besides the common sebaceous and sweat glands, numer- 

 ous modifications, usually in the line of scent glands. These 

 are mostly modifications of sebaceous glands, and in many car- 

 nivores, rodents, and edentates, are most abundant in the anal 

 or inguinal regions. In other groups they may have widely 

 diverse positions ; in the occipital region (camel), in the lachry- 

 mal bone (many ruminants), upon the face (bats), on the legs 

 (swine), in the temporal region (elephants), etc. Here, too, 

 belong the problematical glands connected with the spur on 

 the hind legs of the monotremes. 



The mammary or milk glands of the mammals are also 

 modified dermal glands, those of the monotremes most closely 

 resembling sweat glands, those of other mammals sebaceous 

 glands. In their development a marked ridge, the ' milk line,' 

 appears along the side of the body, certain portions of which 

 become developed into the glands, the intervening portions 

 aborting. Connected with the glands are the teats or nipples, 

 which are of two kinds ; the one produced by a protrusion of 

 that part of the surface upon which the lacteal glands open ; or 

 (ungulates) by a similar elevation of the surrounding surface, 

 the openings of the ducts remaining at the bottom of the tube 

 thus formed (Fig. 97). The number of teats varies between 

 one and eleven pairs (Centetes~). These may be distributed 

 along the length of the trunk, or may be restricted to either 

 thoracic or abdominal region. 



Except in a very few forms (hares) there is a layer of fat 

 (panniculus adiposus) between the skin and the muscles. Be- 

 sides, there is usually a layer of skin muscles (panniculus carno- 

 sus, p. 115). This is distinct from the isolated smooth muscles 

 connected with the hair follicles. 



In the skeleton there never occurs that pneumaticity found 

 in birds and some extinct reptiles, the cavities of the bones being 



