CRUSTACEA. 



EITHER the lobster, or the fresh water crayfish, is 

 common, and of sufficiently generalized structure to 

 he taken as the type of the class, and one of these 

 should become familiar to the scholars. Lobsters are 

 found below low- water mark along the New England 

 coast, and boiled specimens may be obtained in all 

 the markets. 



Fresh water crayfishes are abundant in the rivers 

 of the Middle and Western States, and can sometimes 

 be procured in Eastern markets.* On account of their 

 small size they are more convenient to use, if the class 

 is a large one. With the exception of a few important 

 modifications, the structure of the two animals is the 

 same, so that a description of the lobster will answer for 

 that of the crayfish. t 



Every lobster should be placed for examination upon 

 its lower or ventral side, with the head turned from the 

 pupils. The body will then be above, and the organs 

 known as the lobster's appendages will project from the 

 lower surface on either side (Fig. 6). This position 

 is very favorable for making instructive comparisons 



* Woodcut No. 22, p. 56. 



t Huxley's book on the "Crayfish" contains a full and accu- 

 rate anatomy of this interesting form. 



