xii.] THE FRESH- WATER CRAYFISH. 151 



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, basipodite; next, cylin- 

 drical and marked by an annular constriction, 

 ischiopodite ; the next, longer, meropodite\ then 

 successively, the carpopodite, propodite, and 

 dactylopodite. Probably the coxo- and basi- 

 podite together represent the protopodite of 

 the abdominal appendages : the remaining 

 joints the endopodite : the exo- and epipodite 

 are wanting. 



ft. The next ambulatory leg: generally similar 

 to the preceding, but possessing, attached to 

 the coxopodite, a long membranous flattened 

 appendage (epipodite} which ascends into the 

 gill-chamber: it bears a gill. 



y. 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). 



