CESTODA. 785 



To describe minutely all the changes through which these and 

 other tape-worms allied to them pass, would require a separate 

 treatise. We are concerned chiefly to present a general view of 

 the subject ; and, perhaps, in further illustrating the phenomena 

 of tape-worm life, we cannot do better than select that particular 

 species which, in its larval state, gives rise to the familiar 

 disease, variously termed vertigo, gid, staggers, and sturdy. 



Practical veterinarians have repeatedly asked in what manner 

 sheep become affected with gid. For them it is not sufficient 

 to be told that the sheep obtains the worms from the dog 

 on the one hand, and that the dog in its turn obtains tape- 

 worm from the sheep. There are, as we have seen, several tape- 

 worms liable to reside in the dog, and only one of these 

 cestodes is concerned in the matter at issue, and that is the 

 Tcenia cxnurus. 



If the head of a yearling affected with gid be opened, one or 

 several hydatids will be found in the brain. These hydatids 

 must not be confounded with the common and often much 

 larger hydatids found in the viscera of various animals, nor with 

 the slender-necked hydatids liable to infest the abdomen of the 

 sheep. The brain-hydatids in question are ccenuri, being readily 

 recognised by their polycephalous character ; that is to say, they 

 are furnished with numerous processes termed heads. The full- 

 grown gid hydatid is always lodged within a sheath or cyst, and 

 when removed, whether by operation or post mortem, invariably 

 displays these heads at the surface. To the naked eye, indeed, 

 the heads may merely exhibit the appearance of minute whitish 

 granulations, especially if they happen to be inverted and 

 retracted within the walls of the hydatid. The coenurus other- 

 wise presents the general appearance of an ordinary bladder- 

 worm, containing in its interior a clear, amber- coloured, watery 

 fluid. 



On subjecting the so-called heads to microscopic examination, 

 they will be found to display a double crown of minute hooks 

 in front, besides four sucking discs, such as are commonly seen on 

 the head of an ordinary tape-worm. A single large coenurus may 

 support several hundreds of these heads ; each head in reality 

 representing a young tape-worm. This relationship has been 

 proved by experiment. If, for example, a fresh coenurus be 

 given to a dog, each of the hundred or more heads becomes cou- 



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