336 COMPARATIVE ANATOMY CHAP. 



IV. The Enteric Canal. 



The intestine of the Crustacea has a simple straight course through 

 the body. The mouth lies on the ventral side of the head, bordered 

 and protected by an upper and an under lip (paragnatha) ; it is 

 surrounded by appendages which serve for the taking in of food 

 (mandibles, maxillae, and maxillipedes). The anus is found in the 

 terminal segment of the body. We must distinguish, according to 

 their ontogenetic origin and structure, 3 sections in the intestine : the 

 fore-gut, the hind-gut, and the connecting mid-gut. The fore- and hind- 

 guts, proceeding respectively from the ectodermal stomodaeum and 

 proctodseum of the larva or embryo, are lined internally by a chitinous 

 cuticle (intima). This cuticle, secreted by the hypodermis, passes at 

 the mouth and anus into the chitinous exoskeleton. Only the epithelium 

 of the mid-gut, which proceeds from the mesenteron, is of endodermal 

 origin. The mid -gut in almost all Crustaceans is distinguished by 

 the possession of diverticula which play the part of a hepato- 

 panereas. 



As in other divisions of the animal kingdom, so also in many Crustaceans, pro- 

 nounced parasitism has brought about a degeneration of the enteric canal. In the 

 parasitic Cirripedia we find various stages of degeneration leading to the condition of 

 the Rhizocephala, in which an enteric canal is wanting not only in the adult animals, 

 but also in the free-swimming larvse. In the parasitic Isopoda the hind-gut with the 

 anus, and sometimes a large part of the mid-gut as well, may entirely disappear. 



A. The Fore-gut. 



There is a characteristic difference between the condition of the 

 fore-gut in Entomostraca and Malacostraca. In the former it is a simple 

 alimentary tube (oesophagus), which, passing between the cesophageal 

 commissures, runs dorsally, to pass later into the mid-gut which runs 

 backwards. In the Malacostraca, on the contrary, it falls into at least 

 two divisions, that which follows the buccal cavity, the ascending 

 narrower oesophagus, and the broader masticatory or fore-stomach 

 which lies in the head, and which leads to the mid-gut. Special 

 salivary glands entering the oesophagus seem generally to be wanting 

 in the Crustacea (such glands have only been observed in Cray- 

 fish), but glands emerging on the upper lip, in the buccal cavity, and 

 on the maxillae, are common; these are usually called salivary glands. 

 They probably belong to the category of the leg- and the other dermal 

 glands. 



For the present we set aside the special modifications which 

 the fore-gut may undergo ; its structure is essentially the following. 

 Its wall consists of a layer of hypodermis cells, which rest upon 



