XII.J THE FRESH-WATER CRAYFISH. 145 



I 



a. Composed of large oval branched cells, each con- 

 sisting of a granular mass in which lies a clear 

 round nucleus, containing a nucleolus. 



21. The appendages. Beginning with the sixth abdomi- 

 nal segment, remove with forceps the appendages of 

 the body and arrange them in order on a piece of 

 cardboard. The abdominal appendages have been 

 already described ; note the following points in the 

 remainder, working from behind forwards. 



a. The four posterior thoracic appendages [ambu- 

 latory appendages). 



a. The most posterior : elongated and seven- 

 jointed, the joints working in different planes 

 so that the limb as a whole can move in any 

 direction : the joints have the following names ; 

 the proximal, short and thick, coxopodite ; the 

 next, small and conical, hasipodite ; next, cylin- 

 drical and marked by an annular constriction, 

 . ischiopodite ; the next, longer, meropodite ; then 

 successively, the carpopodite, propodite, and 

 dactylopodite. 



/3. The next ambulatory leg: generally similar 

 to the preceding, but possessing, attached to 

 the coxopodite, a long membranous flattened 

 appendage {epvpodite) which ascends into the 

 gill-chamber : it bears a gill. 



7. The next anterior ambulatory leg: differing 

 from the last only in having its propodite 

 prolonged so as to be opposable to the dac- 

 tylopodite and form a pair of forceps (chelce). 



B. The most anterior ambulatory leg: resembling 

 ; 7. closely and, like it, bearing a gill. 



M. 10 



