50 TROPICAL MEDICINE AND HYGIENE 



disappears when the cyst is formed, and does not become 

 the scolex of the adult. 



The structure of the Cestodes resembles in many 

 points that of the Trematodes. The adult Cestode 

 consists of a head scolex which is armed with fixative 

 structures by which it is attached to the intestinal wall of 

 its host. This scolex contains the ganglionic nerve-mass 

 and the commencement of the water vascular system. 

 The fixative apparatus consists of two long, slit-like 

 suckers in the Dibothriocephaloidea, or of four round or 

 oval suckers in the Taeniidae, and in these there is also a 

 terminal conical retractile mass rostellum. 



DlBOTHRIOCEPHALOIDEA. 



The Dibothriocephaloidea. The scolex is long and 

 narrow, there is no rostellum, and no other armature but 

 the elongated suckers, or bothridia. The genital pore 

 opens on the flat surface of the proglottis. The uterus 

 is much coiled, forming a kind of rosette in the middle of 

 the worm, and has an external opening posterior to the 

 genital pore. The testicles are numerous and arranged 

 laterally, anterior to the vitellaria yolk-glands which 

 are also lateral. 



The eggs are enclosed in a shell which, like the eggs 

 of the Fasciolidae, has an operculum. When the eggs are 

 passed the ovum is immature and surrounded by yolk. 

 An embryo with six hooks forms and round it a capsule 

 embryonic capsule which is ciliated. The embryo and 

 capsule together form the oncosphere (fig. 21, 6). When 

 this development is complete the ciliated embryo escapes, 

 and after a short free life enters an intermediate host, 

 usually a fish. In these features the bothriocephalus 

 resembles a trematode, but differs from it in that in the 

 interior of the ovum there is a formed embryo, the head 

 of which forms the head of the mature worm. The outer 

 portion carrying the cilia is quite separate. 



Entering the intermediate host, this outer envelope 

 disappears, and the embryo is set free, passing through 



