36 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



middle of the back and examine the muscular wall of 

 the body; the tough, transparent cuticle; the alimen- 

 tary tube within with the blood vessel and digestive 

 gland on its upper side ; the partitions connecting the 

 digestive tube with the body wall ; the long series of 

 cavities nearly separated from one another by these par- 

 titions, the whole forming the body cavity ; the continu- 

 ous digestive canal opening at each end to the exterior ; 

 the pair of excretory organs in each separate cavity. 

 Study the digestive tube, consisting of pharynx, esopha- 

 gus, crop, gizzard, and intestine ; note the structure 

 of the wall of the tube in each of these regions ; the 

 supra-esophageal ganglion or "brain" lying above the 

 pharynx ; the nerve cord below the alimentary canal ; 

 the reproductive glands along the anterior part of the 

 canal ; note the bilateral arrangement of all organs ; also 

 that the principal parts of the circulatory system lie 

 above, and of the nervous system below, the digestive 

 system (Fig. 52). 



Draw attention to the economic and geologic impor- 

 tance of the earthworm in overturning the soil as it 

 feeds and constructs its burrows. If cocoons (egg cap- 

 sules) can be found (often attached to straws around 

 manure heaps) examine the various stages of develop- 

 ment of the earthworm. 



The leech and Nereis (Fig. 215) or Arenicola (Fig. 274) 

 may be used for comparison. 



Arthropoda 



i. CRUSTACEA 



Crayfish or Lobster 



Material. Live specimens of the former may be 

 kept indefinitely in aquarium jars containing algae and 

 supplied at intervals with a few crumbs or fragments of 

 beef or fish. 



