190 ANIMAL BIOLOGY 



are valuable on account of their fur. The rapacity of man, however, 

 has nearly exterminated these very interesting animals. 



G. Classification of Animals 



146. Vertebrates and invertebrates. — All animals may be divided 

 into two great groups, known respectively as vertebrates and inverte- 

 brates. To the first group belong the animals that have a " back- 

 bone " or spinal column composed of a series of bones known as 

 vertebrcB. To this group belong fishes, frogs, turtles, birds, rabbits, 

 and human beings, for all of them have a spinal column made up of 

 vertebrae. Insects, earthworms, and oysters, on the other hand, 

 have no backbone ; hence, they are called invertebrates (i.e. ani- 

 mals without vertebrae). 



147. Summary of the classification of the invertebrates. — While 

 the vertebrates, on account of their size, are more familiar to most 

 people, in reality there are a great many more kinds of inverte- 

 brates than vertebrates. For example, over 300,000 different 

 species of insects have been described, more than all other species 

 of animals put together. The invertebrates are divided by zoolo- 

 gists into ten or more branches or svbkingdoms, some of the most 

 common of which are named in the table on pages 192 and 193. 



148. Summary of the classification of the vertebrates. — The 

 vertebrate branch of the animal kingdom is divided into five distinct 

 classes. The striking characteristics of each of these classes will be 

 seen by studying the table on page 194. 



149. Reproduction among the vertebrates. — Among the ani- 

 mals belonging to the two lowest vertebrate groups, namely, the fish 

 and amphibia, the female forms eggs within the body and deposits 

 them in the water. Before these eggs can develop, however, they 

 must be fertilized by sperm-cells produced by the male, and this is 

 likewise true of all the higher animals and plants. The fertilized 

 eggs develop into embryos by the process of cell division, and enough 

 food is stored in the egg to supply the young animal until it can 

 secure its own food. Much the same is true also in the case of rep- 



