A LOBSTER OR A CRAYFISH. 



Phylum X, ARTHROPODA; class i, CRUSTACEA; sub-class 2, 

 MALACQSTRACA; order 3, DECAPODA. 



HABITAT. Crayfishes-of the genera Cambarus or Astacus are com- 

 mon in the fresh-water streams and ponds of this country, and allied 

 species or genera inhabit the streams of other continents. The 

 ordinary lobster (Homarus americanus] is at home in the shallow seas 

 washing the rugged, irregular outlines of our New England coast. 

 Crayfish and lobster are so similar in external form and internal anat- 

 omy, as well as in habit and general method of living, that the same 

 directions for study may be made to apply to either. Both move about 

 at or near the bottom of the water, preferring localities which are 

 rocky or stony; both are carnivorous, feeding on smaller animals and on 

 carrion. 



Technical Note. When the season will permit, live crayfishes should be 

 kept in aquaria or in a cool, moist chamber of some sort. Methods of 

 locomotion and respiration, and perhaps also habits connected with the 

 care of the eggs and young, can then be studied. For dissection 

 fresh material is the better, but specimens preserved in alcohol or four 

 per cent formalin will do very well. A dissecting pan must be sup- 

 plied to the student, as much of the work will have to be done under 

 water. 



EXTERNAL STRUCTURE. Note the following details: 



a) The hard exoskeleton, composed, as in the insects, of chitin, but 



with the addition here of calcareous salts. In the adult this outer 

 covering is moulted periodically, perhaps once a year; in the 

 young moulting occurs more frequently. 



b) The division of the body into segments or metameres, the exact 



number of which will be determined later in a study of the 

 appendages. 



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