CEAYFISH OE LOBSTEE: LABOEATOEY WOEK. 



EXTEK^AL. 



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 fit- 

 ting them for dissection. 



Can you distinguish two regions in the body ? How 

 many joints (segments or somites) can you distinguish in 

 the posterior region or abdomen ? Can you see segments 

 in the anterior region (cephalothorax) ? 



Examine a segment (the third) of the abdomen. How is 

 it joined to the segments in front and behind ? Are the 

 parts between the segments as hard as the walls of the seg- 

 ment ? What is gained by this arrangement ? How does 

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

 of the ring are the appendages (swimmerets) attached ? 

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

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

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

 (endopodite), the other outside of this (exopodite). Draw 

 the segment and appendages from in front. 



Compare the segments behind the third with that one. 

 Do all have the two-branched appendages ? How are the 

 swimmerets of the sixth segment modified ? How does the 

 last segment (telson) differ from the others ? Where is the 

 vent? Compare the appendages of the first and second 



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