v CRUSTACEA ENTERIC CANAL 337 



an outer, often chitinous, basal membrane, and secrete, like the 

 hypodermis of the integument, internally, a chitinous cuticle (intima). 

 The fore -gut is embraced by circular muscular hoops, serving to 

 narrow its lumen ; groups of muscles, mostly paired, attached on 

 one side to the intestinal wall, on the other to a portion of the 

 neighbouring integument, effect its expansion and other move- 

 ments. The hypodermis of the oesophagus is often difficult to 

 discover. 



Entomostraca. The end of the oesophagus often projects in the form of a cone or 

 funnel into the first part of the mid-gut, somewhat as does the uterus of a mammal 

 into the vagina. This projection, which in Branchipus is bi-lobed, and set with small 

 cuticular papillse, perhaps .corresponds with the masticatory stomach of the Malaco- 

 straca. 



The intima of the oesophagus is generally longitudinally folded, which permits of 

 its enlargement. 



In the Ostracoda the oesophagus may be enlarged before passing into the mid-gut 

 into a so-called crop. 



In the Leptodora (Daphnidce) two divisions have been distinguished in the fore- 

 gut, an ascending pharynx and a strikingly wide oesophagus running backwards. 

 This latter may perhaps belong to the mid-gut. 



Malacostraca. The possession of a masticatory or fore-stomach is characteristic 

 of the Crustacea belonging to this second principal division. It is found in the 

 Leptostraca, though in a somewhat simpler form than in the other Malacostraca. The 

 masticatory stomach is a spacious sac of varying form. It is chiefly distinguished 

 by the fact that its wall projects into its cavity in the form of definitely arranged 

 folds, ridges, valves, plates, lamellae, and other prominences, on which the chitinous 

 intima is specially strongly developed, so as together to form a very complicated 

 framework. The special form of the masticatory stomach, with its projections, 

 pouches, etc., and its chitinous framework, is extremely important in classification. 

 A more detailed account, however, would take far too much space. The masticatory 

 stomach generally falls into two divisions, an anterior cardiac division, into which 

 the oesophagus enters, and a posterior pyloric division, which opens into the mid-gut. 

 In the anterior division the food which has already been torn in pieces by the 

 oral appendages is still further cut and ground up by the masticatory framework, 

 and the digestion takes place chiefly in the posterior division, into which the 

 secretions of the glands of the mid-gut enter. 1 The special formation of the wall 

 with its chitinous framework, in this second division, both hinders too rapid 

 passage of the food to the mid-gut, and prevents its return into the masticatory 

 stomach. 



The parts of the masticatory framework are moved by suitably arranged muscles, 

 which are attached to the neighbouring integument. 



In many parasitic Isopoda, which suck in food in a fluid condition, the masticatory 

 stomach is much simplified. 



The intestine of the Entoniscidce will be described at the end of this section 

 (p. 341). 



In the Dccapoda, on the anterior wall of the cardiac division of the mas- 

 ticatory stomach, are found two concretions, principally consisting of carbonate 

 and phosphate of lime ; these are the so-called crab's eyes or gastroliths. In 



1 See section on the mid-gut of the Dccapoda (p. 340). 

 VOL. I Z 



