158 COMPARATIVE ANATOMY 



crayfish, is remarkable for the great extent of the stomodaeum 

 and proctodaeum and the small extent of the mid-gut. The 

 mouth opens into a buccal cavity, whose posterior wall is 

 raised into a conical fleshy projection, the lingua. The 

 lingua is covered in behind by the labium, and in the angle 

 between the two open the ducts of a pair of salivary glands. 

 In front of the lingua the buccal cavity is continued into the 

 oesophagus, which is very narrow and laterally compressed in 

 the neck, but in the thorax gradually expands into a very long 

 and spacious thin-walled crop extending far back into the 

 abdomen. The crop is succeeded by a short conical gizzard 

 or proventriculus with thick muscular walls. The inner 

 lining of the proventriculus is raised up into six prominent 

 longitudinal folds covered with hard chitinous plates which, 

 by the contraction of the muscular wall, can be approximated 

 in the middle line so as to form a very efficient grinding and 

 straining apparatus. Behind the tooth-like plates are six 

 cushion-like swellings covered with setae. The oesophagus, 

 crop, and proventriculus are lined by a chitinous membrane 

 continuous at the lips of the mouth with the external integu- 

 ment, and together they constitute the foregut or stomodaeum. 

 The chitinous lining of the proventriculus projects backwards 

 in spout-like form into the mid-gut, and the anterior of the 

 spout is longitudinally folded so that its cavity is reduced to 

 a narrow star-shaped passage. 



The mid-gut is a short tube lined by columnar epithelial 

 cells arranged in clusters, which have a fan-shaped figure in 

 transverse section. The anterior end. of the mid-gut is pro- 

 duced into eight, or sometimes fewer, short digitiform diverti- 

 cula known as the hepatic caeca. The walls of these caeca are 

 lined by an epithelium similar to that of mid-gut itself, and it 

 has been demonstrated that they secrete a digestive fluid 

 which has a solvent action on proteids and starches. 



The hind-gut, which follows directly on the mid-gut, is 

 divisible into three portions. First a short and narrow tube, 

 the so-called ileum ; following upon this a wider tube nearly 

 an inch in length called the colon, and terminally a short 

 pyriform sac with longitudinally folded walls, the rectum, 

 which opens to the exterior by the anus between the podical 

 plates. The whole of the hind-gut is a proctodaeum lined by 

 a chitinous cuticle which is covered with setae and thrown into 



