262 



CIVIC BIOLOGY 



The eo-o-s pass out tlirougli the bile ducts and hatch into minute, free- 

 swimming embryos (the miracidia), which bore into fresh-water snails. 

 Witliin the snail the parasite develops into a sporocyst, which produces 

 still another form of the worm, known as the redia. The redise, in turn, 

 produce asexually other redi?e or still another form (the cercaria). The 



cercarise are tadpole-shaped, 

 and, passing out of the snail, 

 swim about in the water 

 until the tail drops off and 

 they encyst upon the leaves 

 of plants. Here they are 

 eaten by animals and make 

 their way up the bile ducts, 

 and so the life cycle is re- 

 peated. The adults in the 

 liver are hermaphroditic. A 

 large 'Species (DLstomum mag- 

 nurn), jjrobably imported 

 from Italy, may become a 

 serious obstacle, especially 

 to sheep-grazing, in portions 

 of the West. The main rem- 

 edy is avoidance of low pas- 

 tures during wet seasons. 



Tapeworms — cestodes {ces- 

 tos, "a girdle")- ^^ idea of 

 the general form of a com- 

 mon tapeworm, adult and 

 bladder stages (^cysticercus^, 

 is given in Fig. 111. In the 

 adult the head is a small 

 knob provided with four suckers and a circlet of booklets. This head 

 has no mouth or sense organs, but serves merely to anchor the worm 

 to the wall of the intestine. The neck is the short, unsegmented por- 

 tion close to the head, and behind this the characteristic segments 

 begin to form. These grow by absorption of the digested food through 

 the skin; hence there is no need of digestive organs, the entire con- 

 tents developing practically into reproductive cells, eggs, and sperm; 

 and finally the joints {proglottides) break off and pass out, containing 

 each its many thousands of minute fertilized eggs. In 1861 Leuckart 



Fig. 114. Tapeworms 

 After Leuckart 



