58 



BEGINNERS' ZOOLOGY 



pair of legs in the female, and the fifth 

 pair of legs in the male (Fig. 88). The 

 eggs are carried on the swimmerets. 



Internal Structure. — Suggestions. If 

 studied by dissection, it will be necessary 

 to have several crayfish for each pupil, one 

 for gaining general knowledge, and others for 

 studying the systems in detail. Specimens 

 should have lain in alcohol for several days. 



The Food Tube. — Is the stomach in the 

 head portion of the cephalothorax or in the 

 thoracic portion? ( Figs. 88, 89). Is the stomach 

 large or small? What is its general shape? 

 Does the gullet lead upward or backward? 

 Is it long or short? (Fig. 88.) The mid tube, 

 which is the next portion of the food tube, is 

 smaller than the stomach. On each side of 

 it are openings from the bile ducts which 

 bring the secretion from the digestive gland, 

 sometimes called the liver. Does this gland 

 extend the whole length of the thorax? Is 

 it near the floor or the top of the cavitv? 

 The third and last portion of the food tube 

 is the intestine. It extends from the thorax 



to the vent. Is it large 



or small? Straight or 



curved? The powerful 



flexor muscles of the tail 



lie in the abdomen below 



the intestines. Compare 



the size of these muscles 



with the extensor muscle 



above the intestine (Fig. 



90). Why this difference? 



Does the food tube ex- 

 tend into the telson? Lo- 



Hg. 90. — Section of ( raykish showing 



cate the vent (Fig. 90). stomach s, liver it, and vent a. 



Fig. 89. — Level length- 

 wise section showing 



h, heart. 

 d, green gland. 

 ie , liver. 

 kie, gills. 

 kh, gill cavity. 

 ma, stomach. 



(After Huxley.) 



