10 



FLATWORMS 



Many of the lower animals are popularly known as ' worms '. 

 They have little in common save that their bodies are longer 

 than they are broad and have bilateral symmetry, and that their 

 organisation is rather simple. The lowliest of such creatures are 

 known as the tlatworms or Platyhelminthes. As their name 

 implies the bodies of these worms are flat. They have no anus 

 and no blood vessels or body cavity, being constructed internally 

 of a spongy mass of tissue (parenchyma, p. 119), containing 

 muscle fibres, and, embedded in this, a gut (except in the tape- 

 worms), a nervous system with a rudimentary brain, an excretory 

 system formed of branched tubes ending internally in cihated 

 ' flame cells ' (p. 121), and a complicated, nearly always hermaph- 

 rodite, generative system. The gut has only one opening. The most 

 characteristic flatworms are the relatively simple free-living 

 Turbellaria (p. 131), but more important to man are the two 

 classes that are parasitic — Trematoda or flukes, and Cestoda or 

 tapeworms. These are more complicated in structure, and 

 especially in their reproductive organs and life-histories, than 

 the others. 



CLASS TREMATODA 



FASCIOLA 



Sheep which are fed in damp meadows are liable to a serious 



and usually fatal disease known as ' liver rot ', in which the wool 



falls oft", dropsical swellings appear, and the animal wastes away. 



This has been found to be caused by a parasite known as the 



liver fluke, Fasciola hepatica [=Distomuni hepatictim), which 



lives in the bile ducts of the sheep and sometimes of other animals, 



including cattle, and more rarely a wide variety of mammals 



from kangaroos to man. It is a flat, brownish worm (Fig. 83), 



about one inch long by half an inch broad, shaped like a leaf 



with a blunt triangular projection at the broader end. At the 



tip of this projection lies the mouth, in the midst of an anterior 



sucker, and just behind the projection an imperforate posterior 



or ventral sucker is placed in the middle of the ventral side ; 



when the worm is kept in the laboratory it moves by looping, 



with alternate attachments of the suckers. Nearly midway 



118 



