THE ORDER AMONG ANIMAL TYPES 123 



Order VII. Cetacea. The Whales. 



Order VHI. Sirenia. The Sea Cow, Manatee. 



Order IX. Ungulata. Hoofed animals, Horse, Cow, Elephant, 

 Giraffe, etc. 



Procedure in Classification. Identifying an animal as a mem- 

 ber of one of the larger groups is in most cases a relatively simple 

 matter; distinctions are very pronounced. The assignment of the 

 organism to its proper ultimate group, the species, is often a mat- 

 ter of some difficulty, for in some cases the differences between 

 species are slight or obscure. For purposes of illustration, let us 

 assume that we have at hand a living object. It is noted that it 

 moves rapidly and responds actively to light and to touch; obviously 

 we may clasisfy it as a member of the Animal kingdom. Only a 

 brief examination is necessary to discover that it is composed of 

 many cells; therefore we know it to be one of the sub-kingdom 

 Metazoa. We note that it is segmented, the body being composed 

 of rings that are externally visible. Only three phyla of the Metazoa 

 show this external segmentation so distinctly, namely, Arthropoda, 

 Annelida, and Chordata. The facts that the segments are covered 

 with a tough armor and that its walking appendages are jointed 

 enable us to place it in the phylum Arthropoda. The animal has 

 three pairs of walking legs and also is equipped with wings; obvi- 

 ously it agrees with the description of the characters that are com- 

 mon to the class Insecta. The wings are a single pair, transparent 

 and membranous. Insects that have wings of this sort and number 

 are grouped in the order Diptera, the Flies. Examination shows 

 that the antenn.e are three jointed, with a bristle on the third seg- 

 ment, and that the head bears a proboscis. These facts, together 

 with certain characteristic arrangements of the veins in the wings, 

 conform with descriptions of the flies that are grouped in the fam- 

 ily Muscidae. The abdomen of the fly we are examining is not 

 bristly, as in some members of the family Muscidae; hence it may 



