PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 



139 



IS a saclike intestine with a single opening, 

 which serves both as mouth and anus. In 

 the simplest forms, the intestine is un- 

 branched; but in others, branches occur that 

 may penetrate to all parts of the body, thus 

 rendering a circulatory system unnecessary. 

 However, in certain trematode families, 

 channels filled with fluid occupy a consider- 

 able part of the body. The fluid surging back 

 and forth as a result of muscular contrac- 

 tions may in effect serve as a transport sys- 

 tem. The Cestoda have lost the intestine 

 and absorb nutriment through the general 

 surface of the body. An excretory system oc- 

 curs in almost all of the flatworms; and in 

 some, it is very complicated. The most char- 

 acteristic feature of this system is the flame 

 cell. The nervous system consists of a net- 

 work with a concentration of nervous tissue 

 at the anterior end, the brain, and several 

 longitudinal nerve cords. Flatworms are 

 characterized by a complex reproductive 

 system. 



Fasciola hepatica— a 

 parasitic trematode 



Fasciola is known as the sheep liver fluke. 

 It lives as an adult in the bile ducts of the 

 livers of sheep, cows, pigs, and many other 

 herbivores. Olsen has estimated that on the 

 Gulf coast alone there is a yearly loss of 44 

 tons of condemned livers and 58 tons of 

 other meat, not to mention the mortality of 

 livestock, especially among calves, reduction 

 in milk production, and lessened breeding. 

 Human infections of the liver fluke are 

 relatively rare, but this is probably due to 

 infrequent exposure to the parasite rather 

 than to its failure to develop in man. Water 

 cress is one of the commonest means of 

 human infection. In Cuba human infections 

 are reported common, and in some years 

 reach epidemic proportions. 



The mouth of Fasciola lies in the middle 

 of a muscular disk, the oral sucker (Fig. 73). 

 The ventral sucker serves as an organ of at- 

 tachment. Between the mouth and the ven- 



tral sucker is the genital pore through which 

 the eggs pass to the exterior. The excretory 

 pore (Fig. 73) lies at the extreme posterior 

 end of the body. 



The digestive system consists of a mouth, 

 phar) nx, short esophagus, and intestine with 

 two main branches (Fig. 73). The excretory 

 system is similar to that of planarians, but 

 only one main tube and one exterior open- 

 ing are present. The nervous system con- 

 sists of a small ganglion at the anterior end 

 of the body which gives off a few longitudi- 

 nal nerves. Sense organs are almost lacking. 

 Complex muscle layers lie just beneath the 

 cuticle. 



The body of the liver fluke is covered with 

 a thick, heavy, elastic cuticle. The paren- 

 chyma is a loose tissue lying between the 

 body wall and the digestive tract; within it 

 are embedded the various internal organs 

 described above, as well as the reproductive 

 system. 



Except in the schistosomes and one other 

 group, both male and female reproductive 

 organs are present in every adult fluke (Fig. 

 75 ) ; they are extremely well developed, and, 

 as in planarians, quite complex. One liver 

 fluke may produce as many as 500,000 eggs; 

 and, since the bile ducts in the liver of a 

 single sheep may contain more than 200 

 adult flukes, there may be 100 million eggs 

 formed in one parasitized animal. The eggs 

 segment in the uterus of the fluke, then 

 pass through the bile ducts of the sheep into 

 its intestine, and finally are carried out of 

 the sheep's body with the feces. Those eggs 

 (Fig. 73) that encounter water produce 

 ciliated larvae (miracidia) that swim about 

 until they encounter a certain fresh-water 

 snail, into which they burrow. Here, in 

 about two weeks, they change into saclike 

 sporocysts containing germ balls or em- 

 br}Os. Each germ ball within the sporocyst 

 develops into a second kind of larva (redia). 

 These usually give rise to one or more gen- 

 erations of daughter rediae, after which they 

 produce a third kind of larva with a long tail, 

 known as a cercaria. The cercariae leave the 



