STRUCTURE OF CRAYFISH 



329 



external muscles attached to them, and internally projecting teeth 

 which clash together and grind the food. Three of the teeth are con- 

 spicuous ; a median dorsal tooth is brought into contact with two large 

 laterals. On each side of the anterior part of the gizzard there are two 

 limy discs or gastroliths, which are broken up before moulting, and 

 though quite inadequate to supply sufficient carbonate of lime for the 

 new skeleton, seem to have some relation to this process. The 

 occurrence of chitinous cuticle, setae, teeth, and gastroliths in the 

 gizzard is intelligible when the origin of the fore-gut is remembered, 

 and so is the dismantled state of this region when moulting occurs. 



AO 



Fig. 175. — Longitudinal section of lobster, showing some 



of the organs. 



H., Heart; A.O., ophthalmic artery; a.a., antennary artery; a.h., 

 hepatic artery; ST., sternal artery; S.A., superior abdominal 

 artery ; M.G., mid-gut ; D.G., digestive gland ; H.G., hmd-gut ; 

 Ex., extensor muscles of the tail ; Fl., flexor muscles of the tail ; 

 I. A., inferior abdominal artery ; G., gizzard ; C, cerebral gangha ; 

 P., pericardium ; T., testes. 



The mid-gut is very short, but outgrowths from it form 

 the large and complex digestive gland. The mid-gut, here 

 as always, is the digestive and absorptive region, but both 

 processes are carried on to a large extent in the digestive 

 gland, which communicates with the mid-gut by two wide 

 ducts. It is roughly three-lobed at both sides, and consists 

 of an aggregated mass of caeca, closely compacted together. 

 The gland is more than a " liver," more even than a 

 " hepatopancreas." It absorbs peptones and sugar ; like 

 the Vertebrate liver, it makes glycogen ; its digestive 

 juices are comparable to those of the pancreas and the 



