ANIMAL PARASITISM 



455 



which run the full length of the strobilus, and a transverse excre- 

 tory vessel in each proglottid. As the proglottids become older and 

 are pushed farther away from the scolex, the reproductive organs 

 begin to develop; each proglottid develops a complete set of both 

 male and female reproductive organs; when these become mature 

 and ready to function, the proglottid is a mature proglottid. Each 

 mature proglottid is capable of copulating with itself, by bending 

 the cirrus down into its own vagina, or it may copulate with another 

 mature proglottid of another tapeworm if one is present close by. 



Head 



iHa*.*i:urc seginen-fcs 



vr^«*»*; 



r.*7,ir 



ti"^*' 



M 



■ViV 



^V^ 





Fig. 195. — Common tapeworms, showing different regions of the body. At 

 the left above — scolex of Taenia saginata, beef tapeworm : left below — pro ilottlds of 

 Moniezm, sheep tapeworm ; middle — scolex and proglottids of Taenia solium, pork 

 tapeworm ; right — scolex and proglottids of Dipylidium caninunxt a dog tapeworm. 

 (Courtesy of General Biological Supply House.) 



After copulation the male organs begin to degenerate, the uterus 

 becomes filled with fertilized eggs and takes up more and more 

 space, then the ovaries, vitellaria, and other female organs degen- 

 erate and leave the uterus to occupy nearly the whole proglottid; 

 in this condition the proglottid, now hardly more than a sack of 

 eggs, is called a gravid proglottid. The gravid proglottid at the ex- 

 treme end of the strobilus breaks off and passes out of the host's 



