EXCRETION 
63 
45. Digestive System. — The mouth is just back of the 
teeth, and connects with the stomach by a short esophagus. 
The stomach is divided into front and back parts. The 
front part possesses a grinding structure known as the 
gastric mill, which serves to shred and crush the food and 
make it ready for digestion in the back part. The liver, 
or digestive gland, pours a fluid into the stomach, which 
Figure 51. — Crayfish, Showing How the Eggs Are Carried. 
This is the position that the animal takes when the eggs are being aerated; 
the rest of the time the abdomen is flexed as in Figure 48. 
prepares the food for absorption by the walls of the stom¬ 
ach and intestines. The intestine begins at the back end 
of the stomach and extends to the last segment. 
46. Respiration. — Crayfish obtain oxygen from the water 
by means of gills which are well covered by the overhanging 
skeleton of the head-thorax region, but are really outside of 
the body. Most of the gills are plume-like in shape and are 
attached to the appendages, but some of them are attached 
to the thorax. Water is made to circulate through the gill 
chamber by means of the gill scoop or bailer. The finely 
branched gill affords a large amount of surface for the ab¬ 
sorption of oxygen. 
47. Excretion. — The organs for excretion of waste are 
the green glands that are found at the base of the antennae. 
Blood going to these glands loses some of the waste which 
