96 



BRANCH ARTHROPODA 



z->-— ;- 



are three " teeth " or hard processes which are controlled 

 by muscles attached to them and to the carapace. By the 

 action of these muscles the food is ground between these teeth, 



which are sometimes called the 

 " gastric mill." In the poste- 

 rior part of the stomach there 

 is a series of filaments or stiff 

 hairs which prevent any coarse 

 or unground food from passing 

 into the intestine. So the 

 stomach is a masticating rather 

 than a digestive organ. When 

 the food is ground fine it passes 

 into the intestine, a straight tube 

 extending from the stomach to 

 the vent . The food is acted upon 

 by the digestive fluids from the 

 glands which lie on each side 

 of the stomach and whose ducts 

 enter just back of the stomach. 

 Digestion and absorption take 

 place in the intestine. 



Circulation.— V^hen the heart 

 (Fig. 71) contracts the blood 

 flows both forward and back- 

 ward. Five tubes, or " arteries," 



Fig. 70. — Astacus fluvialilis. A 

 male specimen, with the roof of the 

 carapace and the terga of the ab- 

 dominal somites removed to show the 



^ ,, , viscera (nat. size): aa, Antennary 



^'''^' "''^" artery; ag, anterior gastric muscles; 



amm, adductor muscles of the mandibles; cs, cardiac portiori of the stom- 

 ach; gg, green glands; h, heart; hg, hind gut, or large intestine; Lr, liver; 

 oa, ophthalmic artery; pg, posterior gastric muscles; saa, superior abdominal 

 artery; t, testis; rd, vas deferens. (Huxley.) 



carry it forward, and two, backward. These " arteries " keep 

 dividing until they form minute capillaries with open ends. 

 The blood runs into the irregular body spaces, or sinuses, and 



