=Life history.= Starting with the adult tapeworm (Fig. 124) in the small intestine of the dog or wolf, the eggs are scattered over the ground and are swallowed by the intermediate host with the fodder or water. Upon arriving in the stomach the egg-shell is destroyed, and the six-hooked embryo, which is thus freed, bores its way through the intestinal wall, and wanders, actively or passively (that is, carried along by the blood), to various organs of the body—liver, lungs, ovaries, bones, skull, etc.—where it develops first into an _acephalocyst_, which may develop further, as shown by the accompanying illustrations. The heads which are formed, upon being devoured by a dog or wolf, then develop into adult tapeworms. Young animals are most exposed to this disease; in adults or aged animals the migration and development of the embryo are more difficult. [Illustration: $1] These embryos perforate the tissue of the liver, become fixed in it, and derive from it the nourishment necessary for their conversion into cystic bladders of varying size, either sterile or fertile. The number of vesicles is rarely large, and when only one or two are present they seldom produce sufficient disturbance to attract attention. On the other hand, when numerous they deform the liver, produce glandular atrophy, increase the total size of the organ, and lead to the appearance of clearly marked symptoms. [Illustration: $1] The cystic vesicles contain a clear, limpid, transparent fluid, in which float secondary, daughter, or granddaughter vesicles. [Illustration: $1] =Symptoms.= Echinococcosis of the liver has no well-marked symptoms, and is therefore difficult to diagnose in animals whose liver is deeply seated, and therefore beyond palpation. The signs which may characterise the period of penetration of the embryos through the intestine and into the depths of the liver, and which are probably represented by slight colic, vague pain and diarrhœa, usually pass unnoticed. But later, when the liver is extensively invaded appetite becomes irregular without apparent cause, animals show intractable diarrhœa, general feebleness, dulness, and wasting. [Illustration: $1] [Illustration: $1]