Flukes are never found in the _arteries_ of the liver, as has been erroneously stated by some writers, their abode being limited to the gall bladder and its ducts. In these they are often present in such numbers as to cause great distention of the sac and tubes, and in some instances the irritation produced by them leads to the thickening of the walls of the gall bladder, and to a deposition of calcareous matter between its coats; frequently also to complete obliteration of portions of the ducts. Hence the crackling sound sometimes perceived when handling the liver of a rotten sheep. (163.) _The Hydatid or Blob_ (_Cysticercus tumicollis_, _Fig. 1._ _Plate_ VII.) so frequently found in sheep, is in form one of the simplest of the entozoa (literally dwellers within), being little more than a bag containing a quantity of fluid. As relates to outline, this hydatid bears no small resemblance to a Florence flask. It is said to have a head, _h._--a neck, _n._--a body, _b._-- and a posterior or caudal vesicle, _c.v._ Its claims to the title of an animal have been much disputed, but as it has been seen to move spontaneously, and as the contained fluid is always essentially different from that by which the hydatid is surrounded, the question may be looked upon as set at rest.[34] [34] The property of acting on organized matter, so as to convert it into substances similar to those which constitute the agent, is characteristic of a vital power. The method of their reproduction is in unison with their structure, simple in the extreme. Nothing, however, very precise is known about the process. The vesicle which acts as heart and stomach serves also as the reproductive cavity, but how or by what means it is fecundated would be difficult to determine. The young hydatids are found adhering to the inner surface of the parent cavity. When they have attained maturity, the parent dies and shrivels, and the young ones begin to eliminate their nourishment from the juices of the quadruped which they infest. These entozoa are found in general on the surface of the intestines, between them and their outer membrane (_peritoneum_), and on the exterior of the lungs and liver. They are always included in a cyst, to the inner surface of which they adhere by means of two hook-shaped processes projecting from the head. These cysts are always on the surface of the different viscera of the sheep, and in this way may be distinguished from another sacular animal, or rather supposed animal, termed acephalocyst or headless bag, which is sometimes found in clusters in the substance of the lungs, liver, &c. and is often confounded with the true hydatid.