ALLIES OF THE EARTHWORM 



229 



The class to which the tapeworms belong is called Ccsto'da 

 (Gr. kestos, girdle; eidos, form). 



Definition of Platyhelminthes (Gr. platys, flat ; helmins, 

 worm). The three classes, Turbellaria, Trematoda, and Ces- 

 toda, constitute the phylum Platyhel- 

 min'thes. The members of this phylum 

 are worms with flattened bodies, that 

 are not divided into somites. All the 

 species are hermaphroditic. 



Trichina. Trichi'na spira'lis (Fig. 

 113, 1) is one of the most dangerous 

 of parasites. Like the tapeworm it 

 requires two hosts to complete its de- 

 velopment. The adult Trichina may 

 be found in the intestine of a pig, rat, 

 or of man. The female Trichina, about 

 3 mm. (| in.) long, brings forth its young 

 alive ; the young ones bore their way 

 through the intestine and along the thin 



connective tissue of the trunk or legs. 



rr(1 ,, , . FIG. 113. Trichina. Much 



Ihere they come to rest, and inclose enlarged. (After 



themselves in a thick, tough membrane 

 or cyst (Fig. 113, 2), remaining encysted 

 until the muscle is eaten by another 

 animal. Then the cyst dissolves, per- 

 mitting the young and still undeveloped Trichina to grow and 

 reach maturity in the intestine of the second host. If the 

 first host is the pig, the second host may be the human being. 

 The danger to the human being comes while the young 

 are making their way into the muscular tissue. The boring 

 of thousands and sometimes millions of these larval parasites 

 in the muscles disintegrates the tissue and causes a fever, 

 which is very frequently fatal. A simple preventive remedy, 

 which every one should apply, is to see that all pork eaten 



Leuckart) 



1, parasite; 2, membrane 

 of cyst; 3, muscle-fiber 

 of pig 



