180 



SYSTEMATIC ZOOLOGY. 



ORDER I.--TURBELLARIA. 



These are small free-living forms which occur in fresh 

 (fig. 24) or salt water, and occasionally in moist earth. 

 They are common in our ponds and streams, crawling 

 slowly over the bottoms or upon submerged sticks and 

 stones. They have a mouth and digestive tract, the 

 latter rod-like, three or many branched. 



ORDER H.--TREMATODA. 



Like the last, these have mouth and digestive tract, but 

 they differ in being parasitic on or in other animals, and in 

 having sucking-discs (from one to many) developed upon 

 the body. Some of them become serious pests. One 

 form, the liver-fluke, produces the disease known as 

 'liver-rot' in sheep. Other forms occur in man, espe- 

 cially in the tropics, being introduced in drinking-water. 



They cause serious sickness. In the case 

 of many trematodes the parasite must pass 

 into two animals in order to complete its 

 life-history, the process being frequently 

 complicated by an alternation of genera- 

 tions. Thus in the case of the liver- 

 fluke, just mentioned, the eggs are laid 

 by the fluke while still in the liver; 

 they pass out through the bile-duct and 

 intestine to the exterior, where they 

 hatch a peculiar embryo on the grass. 

 This bores into a snail, and by budding 

 gives rise to other kinds of parasites, so 

 that from a single egg a large number 

 Fl sta g f onive e r-fluke of individuals may be produced. The 

 ThomaS) P fafter nna ^ stage in the snail is a tailed form, 

 the cercaria, fig. 25, which escapes and 

 getting on the grass is eaten by shoop, when it drops its 



