CRAYFISH OR LOBSTER. 



Each pupil will require at least two specimens. One of 

 these should be opened along the back, as described below, and 

 placed for some days in alcohol in order that the internal parts 

 may become hardened, thus better fitting them for dissection. 

 Injection is easiest performed by cutting into the dorsal surface 

 of the abdomen and inserting the canula into the dorsal abdom- 

 inal artery and injecting forwards. This specimen preserved 

 in alcohol will answer for all internal structures. 



EXTERNAL. 



Can you distinguish two regions in the body? How 

 many joints (segments, somites, or metameres) can you 

 distinguish in the posterior region or abdomen? Can you 

 see somites in the anterior region (cephalothorax)? 



Examine a somite (the third) of the abdomen. How 

 is it joined to the somites in front and behind? Are the 

 parts between the somites as hard as the walls of the 

 somite? What is gained by this arrangement? How does 

 the wall of the somite differ from a ring? To what part 

 of the ring are the appendages (swimmerets) attached? 

 How many of these are there on the somite? In a swim- 

 meret make out the basal joint (basiopod), having two 

 leaf -like branches, one towards the median line of the 

 body (endopod), the other outside of this (exopod). Draw 

 the somite and appendages from in front. 



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