214 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



wliicli the cavit}^ of the fore-gut communicates with tliat 

 of the mid-gut (fig. 57, E). Thus a gullet and stomacli, 

 or rather the parts which will eventually give rise to all 

 these, are constituted. And it is important to remark 

 that, in comparison with the mid gut, they are, at first, 

 very small. 



In the same way, the epiblast covering the sternal face 

 of the abdominal papilla undergoes invagination and is 

 converted into a narrow tube which is the origin of the 

 whole hind-gut (fig. 57, C, and fig. 58, E, %). This, like 

 the fore-gut, is at first blind ; but the shut front end soon 

 applying itself to the hinder wall of the archent^ric sac, 

 the two coalesce and open into one another (fig. 57, E). 

 Thus the complete alimentary canal, consisting of a very 

 narrow, tubular, fore- and hind-gut, derived from the 

 epiblast, and a wider and more sac-like mid-gut, formed 

 of the whole hypoblast, is constituted. 



The procephalic lobes become more convex ; while, 

 behind them, the surface of the epiblast rises into six 

 elevations disposed in pairs, one on each side of the 

 median groove. The hindermost of these, which lie at 

 the sides of the mouth, are the rudiments of the 

 mandibles (fig. 58, E and F,4); the other two become 

 the antennae (5) and the antennules (5), while, at a later 

 period, processes of the procephalic lobes give rise to the 

 eyestnlks. 



A short distance behind the abdomen, the epiblast 

 rises into a transverse ridge, which is concave forwards, 



