PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 



141 



body of the snail, swim about in the water 

 for a short time, and then encyst on a leaf 

 or blade of grass. The encysted cercaria is 

 called a metacercaria. If the leaf or grass is 

 eaten by a sheep, the metacercariae escape 

 from their cyst wall and make their way 

 from the sheep's digestive tract to the bile 

 ducts, where they develop into mature flukes 

 in about 6 weeks. 



The great number of eggs produced by a 

 single fluke is necessary because many eggs 

 do not reach water; the majority of the 

 larvae do not find the particular kind of 

 snail necessary for their further develop- 

 ment; and the metacercariae to which the 

 successful lar\'ae give rise have little chance 

 of being devoured by a sheep. The genera- 

 tions within the snail, of course, increase 

 greatly the number of larvae which may de- 

 velop from a single egg. This complicated 

 life history should also be looked upon as 

 enabling the fluke to gain access to new 

 hosts. The liver fluke is not so prevalent in 

 the sheep of this countr}' as in those of 

 Europe. 



Clonorchis sinensis is an important hu- 

 man parasite, especially in certain parts of 

 Japan and China. An illustration (Fig. 74) 

 is included here, since in some ways this ani- 

 mal is easier to study than Fasciola hepatica, 

 and specimens may be obtained from biolog- 

 ical supply houses. This species lives in the 

 bile ducts of man, cats, dogs, and other 

 mammals. The eggs are passed in the feces; 

 the early larvae live in snails; the cercariae 

 enter various species of fresh-water fish, 

 where they become metacercariae which are 

 infective to man; man and other animals are 

 infected by eating uncooked, parasitized fish. 



Pork tapeworm (Taenia solium) 



The pork tapeworm lives as an adult in 

 the digestive tract of man. A nearly related 

 species, the beef tapeworm, is also a parasite 

 of man. The pork tapeworm is long and 

 consists of a knoblike "head," the scolex 

 (Fig. 76), and a great number of similar 



parts, the proglottids, arranged in a linear 

 series. The animal clings to the inner wall 

 of the intestine by means of hooks and 

 suckers located on the scolex. No hooks, 

 however, are present on the scolex of the 

 beef tapeworm. Behind the scolex of the 

 pork tapeworm is a short neck followed by a 

 string of proglottids which gradually in- 

 crease in size from the anterior to the 

 posterior end. The worm may reach a length 

 of 10 feet and contain 800 or 900 proglot- 

 tids. Since the proglottids are budded off 

 from the neck, those at the posterior end 

 are the oldest. 



No digestive tract is present, the digested 

 food in the intestine of the host being ab- 

 sorbed through the body wall. The ner\'ous 

 system (Fig. 76) is similar to that of the 

 planarians and the liver fluke, but not so 

 well developed. Longitudinal excretory 

 canals (Fig. 76), which have branches end- 

 ing in flame cells, open at the posterior end 

 of the worm and carrv metabolic waste out 

 of the body. 



A mature proglottid is almost completely 

 filled with reproductive organs (Fig. 76). 

 The eggs develop into 6-hooked embryos 

 (Fig. 76) while still within the proglottid. 

 If they are then eaten by a pig, they escape 

 from their envelopes and bore their way 

 through the wall of the intestine into the 

 blood or lymph vessels to be carried even- 

 tually to the voluntar)' muscles, brain, or 

 eyes, where they form cysts. A scolex is de- 

 veloped from the cyst wall (Fig. 76). The 

 larva is known as a bladder worm or cysti- 

 cercus at this stage (Fig. 76). If insuffi- 

 ciently cooked pork containing cysticerci is 

 eaten by man, the bladder is thrown off, and 

 the scolex, which develops, becomes fas- 

 tened to the wall of the human intestine, 

 and a series of proglottids is developed. Man 

 can also serve as the intermediate host if ova 

 are ingested or enter the stomach as a re- 

 sult of reverse peristalsis. Since the cysti- 

 cercus may be located in the brain or eyes, 

 infection with this parasite may be a seriou§ 

 matter. 



