160 HOMOLOGY 



grouped together as a class; mammals, reptiles, etc., form other 

 and fairly homogeneous classes, five classes in all, in the race of 

 vertebrates. 



Further subdivision is necessary for the complete classifica- 

 tion of animals. The classes which form the most comprehen- 

 sive groups within the phyla are frequently broken up into sub- 

 classes, and these into orders, the basis of classification being 

 structures or mode of life, or some other pronounced characteris- 

 tic or aggregate of characteristics. The sub- class Oligochaeta 

 for example includes a group of worms inhabiting fresh water, and 

 another group which burrow into the earth. The former are 

 classified as an Order Limicola, while the latter are placed in the 

 Order Terricola. Orders in turn are sub-divided into sub-orders 

 and families and the families into genera, the basis of classifica- 

 tion as before, being structures where possible, or some promi- 

 nent characteristic. Thus Megascolex, Allolobophora, etc., are 

 similar to Lumbricus the earthworm, forming different genera in 

 the common family Lumbricidae. According to such a scheme 

 therefore, the animals studied here are classified as follows: 



GENUS SPECIES FAMILY ORDER CLASS PHYLUM 



Amoeba proteus Gymnam- Rhizopoda Sarcodina Protozoa 



oebidae 

 Euglena viridis Euglenidae Euglenida Mastigo- Protozoa 



phora 

 Paramecium caudatum Parame- Holotrichida Infusoria Protozoa 



cidae 



Hydra fusca, Hydridae Leptolina Hydrozoa Coelen- 



viridis terata 



Taenia solium Taeniidae Polyzoa Cestoda Platyhel- 



minthes 

 Lumbricus terrestris Lumbri- Oligochae- Chaetopoda Annelida 



cidae tida 



Homarus Americana Astacidae Decapoda Malacos- Crustacea 



traca 



Callinectes hastatus Cancridae Decapoda Malacos- Crustacea 



traca 



At first glance it is often difficult to classify animals even to 

 the phylum, and in some cases only a prolonged study furnishes 

 the key to relationships. Animals that fly, for example, includ- 



