CKUSTACEA. 



The crayfish and sow-bug may be taken as types of the 

 Crustacea, or crab-like forms. These all have two pairs of 

 antennae or appendages in front of the mouth ; they have a 

 varying number of segments at the front of the body, cov- 

 ered by a common shell or carapax, and, excepting gill-less 

 microscopic forms, they all breathe by means of gills at- 

 tached to some of the feet. 



The number of segments in the body varies ; in the higher 

 groups it is constantly twenty, but in the lower it may fall 

 far short of, or far exceed, that number. The regions also 

 vary in extent and cannot be compared throughout the 

 group. Taking the segments connected with the senses 

 and with eating as constituting the head, this region may 

 contain as few as five or as many as eight segments. Not 

 infrequently the head and the next region of the body are 

 united so that they are called a cephalothorax. The abdo- 

 men is usually well developed, but it may be reduced to a 

 mere stump, as in the barnacles. Any of the segments ex- 

 cept the last one may bear appendages. Those most usually 

 present are two pairs of antennae,* a pair of mandibles, two 

 pairs of maxillae, and a varying number of maxillipeds and 

 walking-feet. 



If we study these appendages in the young, or in the 

 adult of some forms, we find that they each consist of a 



* One pair is very small in the sow-bug, but it can be seen with a 

 lens. 



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