372 ZOOLOGY. 



8. Development of the Cysticercus. The hexacanth 

 embryo is incapable of further development except in the 

 body of the first host, which in the case we are considering 

 is usually a pig. By means of their six hooks, aided by 

 the peristaltic movements of the host's intestine, the 

 embryos make their way through the intestinal wall, and 

 reach some factor of the portal vein, in which they are 

 carried along till they reach a capillary of smaller diameter 

 than themselves. 



In the species we are considering the further develop- 

 ment usually takes place in the muscular tissue, giving 

 rise to what is called " measly pork." 



The embryo is nourished by absorption from the host's 

 tissues ; it developes a central cavity and becomes an 

 ellipsoidal vesicle, in the walls of which excretory tubules 

 with flame-cells are developed. At one point the wall of the 

 vesicle is invaginated and at the base of the invagination 

 are developed the four suckers and double circlet of 

 hooks which characterise the " head " of the mature tape- 

 worm. 



These structures are on the inside of the invagination, 

 which of course corresponds to and is continuous with the 

 external surface of the vesicle. By pressure the invagina- 

 tion can be everted, and then the suckers and hooks are on 

 the exterior of the extremity of a structure similar in all 

 respects to the head of the adult Taenia, which in fact it 

 becomes. This head is often called the " scolex." The 

 whole embryo at this stage is known by the English term 

 " bladder- worm," or the technical term cysticercus, meaning 

 " bladder- tail," because the larva is like a small tape- worm 

 with a bladder at the tail end. 



The cysticercus undergoes no further development in the 

 body of the pig : it may live for a long time in this con- 

 dition without change, but if not transferred to another 

 host it dies and shrinks to a small mass which is finally 

 absorbed. In the muscle tissue of the pig the cysticercus 

 is enclosed in a "cyst" formed round it by the host's 

 tissue in response to the irritation caused by the presence 

 of the parasite. 



