164 



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 

 classification, as before, being structures where possible, or some 

 prominent 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 



