SEVENTH ANNUAL REPORT. 139 



of this invagination of the wall of the vesicle. When these hol- 

 low ingrowths are evaginated so as to form external appendages 

 of the vesicle they present the form and armature of the cestode 

 head, as well as a more or less developed neck, which presents 

 a number of annulated rings, the first traces of future segmen- 

 tation. 



The whole embryo now presents the form of a bladder or 

 vesicle, with which it is connected at one point, a process having 

 all the characters of the head and neck of the mature tapeworm, 

 the head and neck constituting the scolex, while the cyst in which 

 the bladder worm lies is known as an hydatid cyst. 



So long as the scolex remains attached and in the body of its 

 host, it never develops into a sexually mature tapeworm, but 

 must enter the alimentary canal of another animal of a different 

 species. 



All hydatids get their nourishment by inhibition, absorbing it 

 from the surrounding tissues; accidental destruction of vesicle 

 destroys its integrity and the hydatid dies. The shepherds in 

 Europe take advantage of this fact in treating sheep affected with 

 hydatid disease, or sturdy gid, as it is familiarly called, by manip- 

 ulating the head between and behind the ears, where they usually 

 find a softening of the bones of the cranium. This softening is 

 due to absorption of the bone from pressure of the growing 

 hydatid ; but. of course, this sign can only be observed when the 

 hydatid is superficially situated. After location of the hydatid 

 the cyst is punctured, allowing the fluid to escape, and the skin 

 over the opening covered by means of a piece of sticking-plaster. 



DIFFERENTIATION OF HYDATIDS. 



As most hydatids to the naked eye appear very much the same, 

 it is only by careful microscopic examination that the differences 

 are noted and a safe conclusion drawn. As, for instance, the 

 distinguishing features of the Cysticercus hovis, or beef hydatid, 

 are: (i) its habitat, (2) large size of scolex, (3) the considerable 

 pigmentary matter around its suckers, and (4) the absence of 

 booklets on head. For this latter peculiarity it has been named 

 the unarmed hydatid and tapeworm. 



The cysts of Cysticercus ccUuIosa are elliptical, or flask-shaped. 

 These are principally found in the muscular tissue of their host, 

 the scolex being seen as a pearly white spot through the wall of 

 its cyst. The cysts vary in size from a pea to that of a small 



