148 FIRST BOOK OF ZOOLOGY. 



stead of air-cavities, the crustaceans show a marked difference 

 between themselves and spiders. 



Now, there are himdreds of minute crustaceans in which 

 it would be hard to recognize any close relations between 

 them and the crustaceans already studied. 



Among these odd forms may be mentioned certain little 

 animals abundant in ditches and pools throughout the coun- 

 try. Some of these creatures are smaller than a pin's-head ; 

 others are as large as an ordinary white bean. 



At first sight they might readily be mistaken for bi- 

 valve moUusks, as the body is covered with a bivalve shell, 

 which partly opens and shuts, and is firmly attached to 

 the body within. If the pupils are fortunate enough to 

 collect some of these creatures in a net and watch them as 

 they actively dart about in a jar of water, they will at once 

 see the difference between them and the clam or mussel. 



Instead of the animal's projecting a soft and fleshy foot 

 with which to creep slowly along, as in the mussels, they will 

 see numbers of little jointed, swimming legs partly protruded, 

 and jointed antennae thrust out in front ; and, if their eyes 

 are keen enough, may detect a little black speck just above 

 the antennae, which represents the eye. 



The following figures represent a species collected in 

 Dubuque, Iowa, and another form from Lynn, Massachusetts. 



135. The concentric lines on the shell appear like lines 

 of growth, and such they really are ; but they are not made 

 like the lines of growth on the mussel. When the creature 

 moults, the delicate skin covering the antennce and swim- 



