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SUB-CLASS II. ENTOMOSTRACA.* 



THIS division contains animals which are immensely 

 numerous, but almost all of minute size, some being, 

 in fact, only discernible with the aid of the micro- 

 scope. They are all aquatic, and most of them are 

 confined to fresh waters. Generally, they are covered 

 with a thin, horny, and perfectly transparent shell, 

 which is open at the lower part. The feet, which vary 

 greatly in number, as do also the antennae, are in 

 many cases connected with the gills ; the antennas, or 

 organs answering to them, are often branched, and 

 are used as oars. The feet are also often branched. 

 Very many of them have but a single eye, rarely 

 placed on a footstalk. They appear to change their 

 skin as insects do. This section contains two numer- 

 ous Orders, in each of which, we shall notice a single 

 genus. 



Daphnia, the Waterflea. 



No one can have peered intently into any collec- 

 tion of fresh water in summer, however small, with- 

 out having observed hundreds of minute atoms, 

 scarcely larger than the mites of cheese, swimming 

 through the fluid in all directions, moving by little 

 leaps or jerks. These tiny creatures are the Wa- 

 terflea (D. Pulex), which we choose to represent 



* "EvrofAov, entomon, an insect, and oWgaxov, ostrakon, a shell. 



