172 MEDICAL ZOOLOGY. 



DEVELOPMENT OF CESTODA. 



Cestode worms without exception exhibit alternate generation. 



The following terms will be used in its description : 



Proglottis. The mature segment, containing the organs of generation and 

 the impregnated eggs. 



Embryo. The first larval stage. 



Cysticercus. The second larval stage. The head-like process to Cysti- 

 cercus armed with suckers is known as the scolex. It is persistent in the 

 strobilic form. 



Strobile. The segmented individual. The term is commonly applied to 

 the scolex and immature segments. 



In T. solium the proglottis escapes from the strobile and is voided from 

 the intestine, the eggs soon afterward being discharged into the feces. In 

 this position segmentation of the ova occurs ; within each an embryo, having 

 a subrotund form and furnished at one extremity with a circle of hooks re- 

 sulting. The ova in this condition are taken up by the hog with its food 

 and conveyed into the intestine. Here the embryo escapes, perforates the 

 intestinal walls by means of its hooks to gain convenient positions within 

 the various muscles or viscera of the body. Favorable conditions being 

 afforded, the second larval form, or that of the Cysticercus, is assumed. 

 The primal hooks are now lost, the larva being distinguished by the scolex, 

 with its suckers and peculiar crown of hooks. In this stage it becomes in- 

 active, while the integumental covering becomes rigid from the presence of a 

 quantity of calcareous matter. The presence of large quantities of such cap- 

 sules in the flesh of the butchered hog, where they appear as minute spots 

 visible to the naked eye, constitutes ' measly' pork. Such meat, when eaten 

 raw or imperfectly cooked, carries into the system numbers of infecting larvae, 

 each of which, when the essentials of development are presented to it, es- 

 capes from its encasement, becomes attached by its hooks to the mucous 

 membrane, and, losing its bladder-like appendage, produces, by a process of 

 gemmation, the features characterizing the mature worm. The terminal 

 segments in succession produce the organs of generation in time to be con- 

 verted into proglottides, subsequently to escape with their eggs. 



In T. mediocanellata the development is the same, the sites selected 

 affording the chief points of variance. The freed ' embryo' is swallowed by 

 some ruminating animal, as the cow, and is received living within the 

 alimentary canal of man, through the consumption of raw or imperfectly 

 cooked beef. 



In T. echinococcus the larva is most commonly taken up by the cow or 

 sheep, and the larval form (hydatid) lodged within its tissues. The third 

 stage of development is attained by the dog devouring the infected beef, 

 when the mature form is evolved within the alimentary canal. Or the larva 



