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 

 Amoeba 



Euglena 



Paramecium 



Hydra 



Taenia 



Lumbricus 



Homarus 



Callinectes 



SPECIES FAMILY ORDER CLASS 



proteus Gymnam- Rhizopoda Sarcodina 



oebidae 

 viridis Euglenidae Euglenida Mastigo- 



phora 

 caudatum Parame- Holotrichida Infusoria 



cidae 



fusca, Hydridae Leptolina Hydrozoa 



viridis 

 solium Taeniidae Polyzoa Cestoda 



terrestris Lumbri- Oligochae- Chaetopoda 



cidae tida 



Americana Astacidae Decapoda Malacos- 



traca 

 hastatus Cancridae Decapoda Malacos- 



traca 



PHYLUM 

 Protozoa 



Protozoa 

 Protozoa 



Coelen- 



terata 

 Platyhel- 



minthes 

 Annelida 



Crustacea 

 Crustacea 



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

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



