v CRUSTACEA ENTERIG CANAL 339 



costraca (at least in the Decapoda, Isopoda, and probably also in the 

 Anisopoda) almost entirely disappeared as a special division of the 

 enteric tube. It is here used up in the formation of its strongly devel- 

 oped glandular diverticula, the hepatopancreatic tubes. In these cases 

 the hind-gut which proceeds from the ectodermal proctodseum represents 

 by far the largest portion of the enteric tube, running through the 

 body from the masticatory stomach to its hindermost end. 



The fact that in the Isopoda and the Decapoda the whole enteric tube with the 

 exception of the point of entrance of the hepatopancreas, proceeds from the embry- 

 onic or larval stomodseum and proctodseum is ontogenetically established ; in the other 

 Malacostraca ontogenetic investigations as to the limit between mid- and hind-guts 

 have yet to be made. Up to the present time conclusions as to the extent of the 

 hind-gut have been based exclusively upon that of the chitinous intima. 



The walls of the mid -gut and of its diverticula show the same 

 general structure. The distinct epithelium is placed on a basal mem- 

 brane and sometimes shows on the surface turned towards the lumen of 

 the intestine a (non-chitinous) cuticular limiting membrane. On the 

 outer side of the basal membrane the mid-gut and its diverticula are 

 encircled by hoop-like, regularly repeated, circular muscles, which are 

 seldom transversely striated. Longitudinal muscles are more rare, and 

 where they occur are not numerous. They lie on the inner side of the 

 circular muscles. In life we can observe, not only in the mid-gut but 

 also in its diverticula, rhythmical waves of contraction, which are often 

 very strong ; these also bring about, especially in the smaller Entomo- 

 straca without hearts, a sort of circulation of hsemolymph in the lacunar 

 system of the body. 



The Mid-gut of the Entomostraea. 



This generally falls into an anterior widened division (stomach, 

 chyle stomach, stomach-intestine) and a posterior narrower division 

 which we might designate the small intestine. The diverticula of the 

 mid-gut, present usually in a single pair, open into the former division. 



The details of the arrangement of the mid-gut diverticula of the Entomostraea 

 are very varied. The two diverticula of the Branchiopoda are themselves sub- 

 divided. They vary greatly in size. In Apus they have lateral branches beset with 

 numerous glandular lobes. In the Cladocera (Fig. 192, p. 289) two short horn-like 

 diverticula are generally found, which are directed forwards. In the Ostracoda the 

 two diverticula are so long that they often project on both sides into the shell fold. 

 Diverticula of the mid-gut are wanting in a good many Copepoda ; in others they are 

 present singly or in pairs, simple, or else complicated by the formation of accessory 

 coaca. The arrangement of the two diverticula of the disc-shaped flattened Branchiura 

 (Argulus) (Fig. 195, p. 291) recalls that in many Branchiopoda. Each of the two 

 diverticula divides first into an anterior and a posterior branch, each of which again 

 branches, and the branches penetrate as far as the lateral edges of the cephalo-thorax. 

 In the generally longitudinally folded stomach of the non-parasitic Cirripedes diver- 

 ticula also not unfrequently enter ; Balanus has 8 diverticula, which may be branched 

 (B. pcrforatus}. 



