The Small Crustaceans 



SECOND PAPER. 



W. C. Allee. 

 Small Entomostracans. 



The other crustaceans to be considered are those usually 

 called entomostracans. This group contains the smaller, less 

 highly developed crustaceans. To be exact, the fairy shrimp 

 would have to be included in this class, but on account of the 

 obvious difference in size it is better to consider them separately. 

 These entomostracans make up the majority of the animal plank- 

 ton of our lakes and ponds. Plankton, it may be explained, is 

 the term which includes all those plants and animals that live in 

 the water independent of the bottom, that is the floating or swim- 

 ming organisms that do not come to rest on the bottom. Since 

 these small crustaceans are easily compared as to habits and 

 habitats, they will be considered mainly as a single group. 



In the first place all are very small; some are microscopic, 

 while the largest are not over one-fourth of an inch long. The 

 three groups may be distinguished as follows : 



I. Segments showing plainly on the surface. 



Copepods (Fig. 6, 7, 8). 

 II. Segments not showing on the surface. 



1. The body completely enclosed in a bivalve chitonous 



"shell." 



Ostracods (Fig. 9 and 10). 



2. Body not completely enclosed in such a shell. 



Claderocera (Fig. 11, 12, 13). 

 The copepods are more or less cylindrical crustaceans with 

 a plainly segmented body and usually with long well-developed 

 antennae, and poorly developed feet. The antennae serve as 

 organs of locomotion and the animal swims by a series of rapid 

 darting movements. Copepods have a single eye which is placed 

 near the center of the head. There are three main groups which 

 can be easily recognized by the differing lengths of antennae : 

 I. Antennae very long, 23-25 jointed. 



Family Calanidae, Representative genus, Diaptomus 

 (Fig. 61). 



II. Antennae shorter, not reaching to the abdomen, 8-18 seg- 



ments. 

 Family Cyclopidae, Representative genus, Cyclops (Fig. 



V. 



104 



