CRAYFISH. 275 



In the anterior (cardiac) region there is a complex mill ; in the 

 posterior (pyloric) region there is a sieve of numerous hairs. The mill 

 is very complex; there are supporting "ossicles" on the walls with 

 external muscles attached to them, and internally projecting teeth which 

 clash together and grind the food. Three of the teeth are conspicuous ; 

 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, hairs, 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. 



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 

 stomach of higher animals. The hind-gut is long and 

 straight. It is lined by a chitinous cuticle, as its origin 

 suggests. There are a few minute glands on its w r alls. 



Body cavity.- -The space between the gut and the body 

 wall is for the most part filled up by the muscles and the 

 organs, but there are interspaces left w r hich contain a fluid 

 with amoeboid cells. These interspaces seem to represent 

 enlarged blood sinuses (a haemocoele), rather than a true body 

 cavity or coelom. One of the spaces forms the blood-con- 

 taining pericardium, or chamber in which the heart lies. 



Vascular system. Within this non-muscular pericardium, 

 and moored to it by thin muscular strands, lies the six-sided 

 heart, which receives pure blood from the gills (via the 

 pericardium) and drives it to the body. 



The arterial system is well developed. Anteriorly, the 

 heart gives off a median (ophthalmic) artery to the eyes and 

 antennules, a pair of (antennary) arteries to the antennae, 

 and a pair to the digestive gland (hepatic). Posteriorly 

 there issues a single vessel, which at once divides into a 



