CKUSTACEA 103 



pyloric region, with fine hair-like setae, projecting from pouches 

 and depressions, and these act as a sieve. The chitinous 

 cuticle of the stomodeum ends posteriorly in a series of 

 processes, and it is shed and renewed at every ecdysis. 



Glands are situated in the walls of the oesophagus, and 

 their ducts are continued through the chitinous layer. They 

 have been called salivary glands. 



The mid-gut, or enteron, is the very short region suc- 

 ceeding the stomach. It is free from a chitinous lining, 

 receives on each side the duct from the digestive gland or 

 liver which is a paired expansion of the enteron, and its roof 

 is produced into a small caecum. The digestive gland or liver 

 is very large ; it lies on each side of the stomach and extends 

 as far back as the beginning of the abdomen. It is made 

 up of a mass of small glandular tubes, which open ultimately 

 by a single duct on each side into the mid-gut. The mid-gut 

 and the digestive gland together form the digestive and absorp- 

 tive region of the alimentary canal. The gland is provided 

 with special large vacuolated cells which furnish the digestive 

 ferments, and fat cells for the storage of glycogen and fat. 

 The products of digestion are also absorbed by the digestive 

 gland, but the fats may be absorbed to some extent by the 

 mid-gut. The gland appears to be able, moreover, to arrest 

 poisonous substances, and to share with the excretory organs 

 the regulation of the quantity of the blood plasma. It has an 

 excretory function as well, yielding yellow, brown, or green 

 pigment. 



The hind-gut, or proctodeum, is a straight tube lined 

 throughout by a thin chitinous cuticle and opening on the 

 ventral side of the telson. Intestinal glands occur with ducts 

 leading through the cuticle, and are very similar to the so- 

 called salivary glands. This chitinous cuticle of the hind-gut 

 is also shed at each ecdysis. 



Circulatory System. The blood consists of a plasma 

 containing a pigment, haemocyanin (a proteid united with 

 copper), and corpuscles of two kinds, both colourless, (1) leuco- 

 cytes and (2) thrombocytes. The former are like the white 

 corpuscles of the blood of vertebrates. The latter are usually 

 spindle-shaped as in other animals, and have the peculiar 



