214 THE MORPHOLOGY OF THE COMMON CRAYFISH, 



which the cavity of the fore-gut communicates with that 

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

 or rather the parts which will eyentually 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 Avay, the epiblast covering the stenial face 

 of the abdominal paj^illa 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, hg). This, like 

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

 applying itself to the hinder wall of the ai'chenteric sac, 

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

 Thus the complete ahmentary 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 {2), while, at a later 

 period, processes of the procephalic lobes give rise to the 

 eyestalks. 



A short distance behind the abdomen, the epiblast 

 rises into a transverse ridge, which is concave forwards. 



