410 PRACTICAL ZOOLOGY 



SMELL. - - The nose is the organ of smell. Its cavities are 

 lined with a membrane that is supplied with nerve endings from 

 the olfactory nerve. These are stimulated by substances in 

 the air that enter the nose during inspiration. 



TASTE. Organs of taste are present on the tongue and 

 enable mammals to determine the nature of the food they eat. 



The Skeleton. - - The skeleton of mammals (Fig. 280) con- 

 sists almost entirely of bone. It serves the same purposes as 

 does that of the frog (p. 254), but of course it differs some- 

 what in details of structure. The bones are similar in number 

 and position in all mammals, but they are modified according to 

 the habits of the species. A comparison of the bones in various 

 kinds of mammals and in other vertebrates makes a very 

 interesting study. 



Reproduction. - - Mammals are separated into male and fe- 

 male individuals. The essential organs of the male are two tes- 

 tes in which the spermatozoa arise and the ducts which carry the 

 spermatozoa to the outside. The female organs in which the 

 eggs are produced are the two ovaries. Connected with these 

 ovaries is an egg duct, the oviduct, into which the fully grown egg 

 passes. Here it is fertilized by a spermatozoon. In most cases 

 the eggs develop within the egg tubes of the mother. The young 

 embryo becomes connected with the wall of the egg tube by a 

 strand of membranes and blood vessels called the placenta. 

 Through the placenta, nourishment from the blood of the 

 mother is carried to the growing young. The interval between 

 iertilization and the birth of the young which develop from the 

 fertilized egg is known as the period of gestation. This period 

 varies in different species; in the rabbit it is thirty days. From 

 one to eight or ten young may be produced at a birth, and, in 

 the case of rabbits, several litters may be born during the year. 



Animal Tracks. - - The study of mammals in their native 

 haunts is rather difficult since most of them are so badly perse- 

 cuted by man that they conceal themselves as soon as they be- 

 come aware cf the presence of human beings. We have already 



