CRUSTACEA. 



127 



CRUSTACEA. 



In their habits most of the Crustacea are aquatic ; and although there 

 are a few forms which live on land, in these the presence of moisture is 

 necessary for respiration. The great majority of the aquatic forms live 

 in the sea, the fresh waters having but few, and the land but an inconspicu- 

 ous number of representatives. How many species there are in the whole 

 world is very uncertain ; it is usually put at two or three thousand, but 

 this number is far too small. Of the single group of ten-footed forms, the 

 North American seas alone contain about a thousand species, which would 

 indicate not far from five thousand species of decapods from the whole 

 world. With this as a basis, it seems safe to estimate that the whole 

 number of species of Crustacea in the world cannot fall far short of ten 

 thousand. 



The central stem from which all the Crustacea seem to branch are 

 some small forms known as Phyllopods, a name given in allusion to the 

 leafy character of their feet. These forms are largely inhabitants of fresh 

 water, and are especially well represented in the United States. West 

 of the Mississippi River some of the species almost fill the ponds, while 

 in the east their numbers, or variety, is not so considerable. In the 



Fig. 115. — Fairy-shrinip (Branchipus vernalis), enlarged. 



east the most familiar species is a delicate form occurring in early spring 

 or even in midwinter, in pools in the fields and by the roadsides. It 

 is about an inch in length, colored reel, }^ellow, green, and purple, in 

 the most attractive shades, while its outlines are so graceful and its 

 texture so delicate, that no artist has yet been able to draw its portrait 

 in a satisfactory manner. It swims slowly through the water with its 

 back downward, and its leafy feet waving backwards and forwards with 

 n perfect poetry of motion — turning now this way, now that, rising to 

 the surface or descending to the bottom. It is, indeed, worthy the name 



