THE LOBSTER AS A TYPE OF CRUSTACEA 17 



action of muscles attached to certain of these plates 

 the teeth work together so as to divide up the food 

 more finely than had been done by the mandibles 

 and other jaws. The whole apparatus, in fact, serves 

 as a kind of gizzard, and is known as the gastric 

 mill. 



A small part of the intestine at the hinder end is 

 lined, like the stomach, by a continuation of the 

 chitinous covering, which is turned in at the vent. 

 This lining and that of the stomach, with the plates 

 and teeth of the gastric mill, are cast and renewed 

 when the shell is moulted. 



On each side of the food-canal in the thorax lies a 

 .large mass of soft tissue, yellowish-green in colour. 

 This is the digestive gland, or " liver," which secretes 

 the digestive juice, discharging it into the food- 

 canal by a short duct on each side just behind the 

 stomach. 



The heart lies in the middle of the back, just under 

 the hinder part of the carapace, and gives off, in 

 front and behind, a number of arteries which carry 

 the blood to the various organs of the body. From 

 the smaller branches of these arteries the blood 

 passes, not, as in vertebrate animals, into capillaries, 

 but into the spaces lying between the organs of the 

 body, and it finds its way back to the heart, not in 

 definite veins, but by ill-defined venous channels 

 which open into the pericardium, or space surround- 

 ing the heart. From the pericardium the blo<*l 



