ASTACUS FLUVIATILIS. 269 



podite (1) ; the next, small and conical, is the basipodite (2) ; 

 the third, cylindrical, short, and marked by an annular con- 

 striction, is the ischiopodite (3) ; next comes a long joint, 

 the meropodite (4) ; then the carpopodite (5) and p)Top}odite 

 (6) ; and, finally, the terminal dactylopodite (7)/ 



The next four somites, proceeding anteriorly, have a sim- 

 ilar general character to that which has just been described, 

 but they cease to be movable upon one another, partly by 

 reason of the calcification of the interepimeral and inter- 

 sternal membranes, partly on account of the development of 

 these membranes by a folding inward, or involution, into 

 processes, the apodemes, which project inward and unite 

 with one another in the cavity of the thorax. In an Astactis 

 which has been macerated — or, better, boiled in caustic alkali 

 — the floor of the thoracic cavity is seen to be divided into a 

 number of incomplete cells, or chambers, by these apodemal 

 partitions, which will be observed, on careful examination, 

 to arise partly from the intersternal, partly from the inter- 

 epicneral, membrane connecting every pair of somites. The 

 former portion of each apodeme is the endosternite^ the latter 

 the endopleurite ^ of Milne-Edwards. As a general rule, each 

 endosternite is distinguishable into three apophyses: the 

 arthrodial,, which passes outward and unites with the de- 

 scending division of the endopleurite to form one boundary of 

 an articular cavity for a limb ; the mesopthragmcd^ which is 

 directed inward, uniting with its fellow, and forming an arch 

 over the passage left in the middle line between eacli pair of 

 endosternites — the so-called sternal canal ; lastty, the X)ara- 

 phragmal division is a small process, which passes forward, 

 upward, and outward, and unites w^ith the anterior division 

 of its own endopleurite, and with the posterior division of the 

 endopleurite in front of it. 



The endopleurite likewise divides into three apophyses, 

 one descending or arthrodial, and two which pass nearly 

 horizontally inward : the anterior horizontal apophysis unit- 

 ing with its own paraphragmal apophysis, the posterior with 

 the paraphragmal of the antecedent endosternite. The pos- 

 terior horizontal apophysis, therefore, crosses the space be- 

 tween every pair of apodemes diagonally, whence the ap- 

 pearance of a double row of longitudinal cells opening above, 

 on each side of the sternal canal. It will be understood, 



1 Probably the coxo- and basipoditc together answer to the protopodite of 

 the abdominal appendages, the remaining joints representing the endopodite. 



