GENERAL ZOOLOGY 



which enable each zygote, through multipHcative larval stages, to develop 

 into numerous adults. A step in this direction, however, is seen in the life 

 cycle of Echinococcus granulosus, which is found as a minute adult in dogs, and 

 as a larva in cattle, sheep, swine, and occasionally man. The hexacanth 

 larvae of this worm, once they have gained access to the tissues of an inter- 

 mediate host, transform into bladder worms. The originally single inverted 

 scolex in each of these cysticerci proliferates asexually to produce thousands 

 of daughter cysts, attached to the inner wall of the parent bladder or floating 

 free in its fluid contents (Fig. 11.20). The cyst slowly eMWges, growing in 

 the course of years to the size of an orange or larger. It may, if it lies in the 

 brain, for example, bring about serious consequences to the intermediate host. 

 From the standpoint of the parasite the advantage lies in the fact that the 

 final host, feeding upon the flesh of an intermediate host containing one of 

 these huge cysts, will receive a massive infestation of potential adult tape- 

 worms. 



The taenioid tapeworms which have been used as examples above represent 

 only one, and probably the most highly specialized, of several orders of 

 Eucestoda. Parasites dangerous to man are also found in other orders, in 

 which the life cycles and morphology of the tapeworms differ from those of 

 the taenioids. An example is Diphyllobothnum latum, the broad tapeworm of 

 man and other carnivores. This worm possesses in each proglottid a uterine 

 pore in addition to the genital pore. Through this uterine opening eggs are 

 emitted singly as they mature, instead of reaching the outside through the 

 shedding of entire proglottids. The life cycle of Diphyllobothnum (Fig. 11.21) 

 requires for its completion a small crustacean, such as Cyclops, and a fish, as 

 intermediate hosts. The stages infective for the final host are plerocercoids, 

 encysted in the flesh, and particularly in the livers, of infected fish. A bear, 

 a fox, a dog, or a man may become infested by eating raw fish. Records of 

 massive infestations with tapeworms of this species have been obtained among 

 fisherfolk in the Baltic area, who are accustomed to eating raw fish liver 

 spread on bread. 



Tapeworm infestations are now comparatively rare among adult human 

 beings in communities where sanitary precautions are practiced. Meat 

 inspection, cold storage, proper cookery, and widespread knowledge regarding 

 modes of infection have almost eliminated Taema solium and T. saginata as 

 serious problems for the United States and Western Europe. This is in con- 

 trast to conditions prevailing before about 1850, when the life cycles of these 

 parasites, and hence the means of preventing infestation, became known. 



The Phylum Nemertinea 



Members of the phylum Nemertinea are sometimes called "ribbon worms" 

 because the bodies of many of them are greatly elongated and flattened. 

 Most species are marine, although a few fresh-water and terrestrial nemer- 



340 



