V CRUSTACEA— ENTEEIC CANAL 341 



the dorsal enteric wall a longitudinal channel, which, at the end of the mid-gut, is 

 continued into a csecum, reaching into the anal segment posteriorly, and ending in 

 two lateral projections. 



C. The Hind-Gut. 



The hind-gut in Crustaceans is as a rule short and limited to the 

 last segment or segments of the body. Its epithelium is lined with a 

 frequently very strong chitinous intima. Its wall is almost always 

 provided with well-developed hoop- like circular muscles. Special 

 muscles or groups of muscles (dilators) are stretched between the 

 hind-gut and the neighbouring integument (and widen the former by 

 their contraction). In those Isopoda, Anisopoda, and Decapoda in which 

 the hind-gut is very long, taking the place of the small intestine of 

 other Crustacea, we find these dilators only at the posterior differen- 

 tiated division of the hind-gut, called the rectum. 



Among the AmpMpoda, Orchestia has a hind-gut which is strikingly 

 long for this group, reaching as far forward as into the 7th thoracic 

 segment. 



The anus lies in the terminal segment — dorsally in the Entomo- 

 straca, ventrally in the Malacostraca. 



Special glands or diverticula entering the hind-gut are wanting in 

 the Crustacea. 



In the Lynceidm among the Cladoccra, however, a glandular contractile CEecum is 

 found ventrally in the hind-gut, which in Pleuraxus is prolonged into a long vermi- 

 form appendage wound round the gut. 



In the Stomatopoda it is said that 2 glandular saccules enter the anal gut. As 

 other excretory organs are wanting in the adult animals these are supposed to have 

 excretory functions. 



In the meantime it is not certainly proved that these glands of the Lynceida: and 

 Stomatopoda really belong to the hind-gut and not rather to the terminal division of 

 the mid-gut. 



The widening of the hind-gut on the contraction of the dilators draws in water 

 through the anus, and subsequent contraction of the lumen of the gut expells it again 

 with ffecal particles. It has been stated without sufficient foundation that these 

 sucking movements, at least in certain Entomostraca (Phyllopoda), imply a respiratory 

 function in the hind-gut. 



The chitinous intima of the hind-gut is ejected through the anus when the 

 integument is shed. 



The peculiar modification which the enteric canal undergoes in the females of 

 many parasitic Isopoda can here only briefly be described, taking as an example 

 Fortunion mcenadis (Sntoniscidce). The narrow oesophagus leads into a large 

 sac composed of two lateral sacs. The inner wall of the sac projects into its 

 lumen in the shape of numerous long processes covered with chitinous intima. In 

 this division of the gut, which has been called the cephalogaster, the absorption 

 of the food takes place. The cephalogaster is continued posteriorly into a second 

 division, the typhlosolis, whose wall, much thickened dorsally, projects into the 

 lumen in such a way that the latter in a transverse section is crescent-shaped, 

 the concave side being directed upwards. A strong cuticle lines the typhlosolis 

 and carries closely - placed long and stiff hairs which, projecting from the 



