i88 



ELEMENTARY ZOOLOGY. 



Different species have become adapted to different hosts, but the 

 life of the liver-fluke of the sheep (Distomum hepaticum) is best 

 known. It may be given briefly as follows (compare at each step 

 with Fig. 54) : 



The adult fluke is found in the bile duct of the liver of the sheep, 

 where it produces such disturbance as to cause disease and death. 



FIG. S3- Diagram of a planarian, showing the distribution of the digestive tract 

 and the nervous system, b, brain with the pigmented "eyespot"; c, sensory cilia; d, 

 a main division of the digestive tract; d', the minuter divisions of the tract; l.n., lateral 

 nerve, from which fibres pass to the tissues; m, mouth; r, outlet of the reproductive 

 bodies. The chief structures not shown are excretory system, reproductive organs, 

 and the general body tissues. 



Questions on the Figure. How does this form compare in shape 

 and symmetry with the jelly-fish and sea-anemone? What is the 

 meaning to the animal of the much-branched digestive tract? Is 

 there a body cavity? A circulatory system? (Consult references.) 



Each worm may lay hundreds of thousands of eggs; but these eggs 

 do not further infect the sheep. After fertilization and early 

 development they pass out the bile duct into the intestine and thence 

 to the outer world. If these embyros fall into water or get carried 

 into water before they die, each develops into a free-swimming 

 larva (Fig. 54, c). This animal can sustain itself only a short time 



