308 THE INVERTEBRATA 



one or more pairs of diverticula ("hepatic coeca"), which serve for 

 secretion and absorption and may branch to form a "Hver". This 

 gland, however, unlike the liver of vertebrates, forms all the enzymes 

 necessary for the digestion of the food and absorbs from its lumen the 

 products of digestion. It stores the reserves in the form of glycogen 

 and fat. Occasionally there is an anterior median dorsal coecum. Coeca 

 are also sometimes found at the hinder end of the mid gut : these 

 are more often median. In a few cases the hind gut is absent and the 

 mesenteron ends blindly. In the Rhizocephala and the monstrillid 



OSS.1 



OSS. 5 



hri. \ 



^''V- u:tth. 



Fig. 217. The left-hand side of the fore gut and mid gut of Astacus, viewed 

 from within, bri. bristles which form part of the filtering apparatus; cd. car- 

 diac chamber of the "stomach"; cm. median dorsal coecum of the mid gut; 

 la.p. lateral, finely-filtering pouch of the pyloric chamber ; la.tth. lateral teeth ; 

 m.', 771.", left anterior and posterior gastric muscles; mdg. mid gut; me.tth. 

 median tooth ; oe. oesophagus (not cut open) ; oss. i-oss. 6, cardiac, urocar- 

 diac, prepyloric, pyloric, pterocardiac, and zygocardiac ossicles — calcifications 

 of the cuticle of the stomach which constitute the mechanism of the gastric 

 mill : when the cardiac and pyloric ossicles are pulled in opposite directions by 

 the contraction of the muscles m.' and rn." attached to them, the teeth are 

 brought together; py. pyloric chamber; vlv. median-dorsal and lateral valves 

 projecting from the pyloric chamber into the mid gut. The opening of the 

 left liver duct is seen as a dark spot behind the valves. 



copepods the alimentary canal is absent throughout life, for these 

 animals take in by absorption through the skin during the parasitic 

 period enough nutriment to last through an entire life history. 



Digestion is extracellular. The fore gut is frequently the seat of 

 mechanical processes, and sometimes of chemical action by juices 

 secreted by the mid gut diverticula, but never of absorption. The 

 latter process as well as most of the chemical work is performed by the 

 mid gut, including the hepatic diverticula. In the hind gut the faeces 

 are passed to the anus, being in some entomostraca sheathed in a 



