12 GENERAL ANATOMICAL CHARACTERS 



Equidce, the grasping under surface of the tail of the prehensile-tailed 

 Monkeys, etc. The greater part of the skin of both species of 

 one-horned Asiatic Ehinoceros is immensely thickened and stiffened 

 by increase of the tissue both of the derm and epiderm, con- 

 stituting the well-known jointed "armour-plated" hide of those 

 animals. 



Nails, Claws, and Hoofs. — With very few exceptions, the terminal 

 extremities of the digits of both limbs are more or less protected or 

 armed by epidermic plates or sheaths, constituting the various forms 

 of nails, claws, or hoofs. These are wanting in the Cetacea alone. 

 A perforated spur, with a special secreting gland in connection with 

 it, is found attached to the hind leg of the males of the three genera 

 of Monotremata, Ornithorhynehus, Proechidna, and Echidna. 



Odour - secreting Glands. — Besides the universally distributed 

 sebaceous glands connected Avith the pilose system, most mammals 

 have special glands situated in modified portions of the integument, 

 often involuted to form a shallow recess or a deep sac with a narrow 

 opening, situated in various parts of the surface of the body, and 

 secreting odorous substances, by the aid of which individuals 

 appear to recognise one another, and probably affording the princi- 

 pal means by which wild animals are able to become aware of 

 the presence of other members of the species, even at great dis- 

 tances. Although the commencement of the modifications of 

 portions of the external covering for the formation of special 

 secretions may be at present difficult to understand, the principle 

 of natural selection will readily explain how such organs become 

 fixed and gradually increase in development in any species, especi- 

 ally as there would probably be a corresponding modification and 

 increased sensibility of the olfactory organs. Such individuals as 

 by the intensity and peculiarity of their scent had greater power of 

 attracting the opposite sex would certainly be those most likely to 

 leave descendants to inherit and in their turn propagate the modi- 

 fication. 



To this group of structures belong the suborbital gland or 

 " crumen " of Antelopes and Deer, the frontal gland of the Muntjak 

 and of Bats of the genus Hipposiderus, the submental gland of the 

 Chevrotains and of Taphozous and some other Bats, the post-auditory 

 follicle of the Chamois, the temporal gland of the Elephant, the 

 lateral glands of the Musk-Shrew, the dorsal gland of the Peccary, 

 the inguinal glands of Antelopes, the preputial glands of the Musk- 

 Deer and Beaver (already alluded to in connection with the use 

 made of their powerfully odorous secretion in medicine and per- 

 fumery) and also of the Swine and Hare, the anal glands of Carni- 

 vora, the perineal gland of the Civet (also of commercial value), the 

 caudal glands of the Fox and Goat, the gland on the humeral 

 membrane of Bats of the genus Saccopteryx, the post-digital gland of 



