_Echinococcus_ is most frequent in Iceland, where it affects 2 to 3 per cent of the population, and a still larger proportion of sheep; while in Copenhagen, Northern Germany, some districts of Switzerland, and Victoria it is not uncommon, but is frequently found during _post-mortem_ examinations when no definite symptoms of its presence had been previously noticed. _Echinococcus_[102] varies greatly in size, form, and mode of growth, but is distinguished in the formation not of one scolex only, as in the _Cysticercus_, but in the production of a number of vesicles, usually from the inner wall. Within these, large numbers of scolices may be developed. The whole organism continues to swell by the formation of a watery liquid within it, and if its growth be rapid the fluid tension may cause the rupture of the enclosing connective-tissue capsule formed around the parasite, at the expense of the host, and the protrusion of the daughter vesicles. It is the consequent injury to the surrounding organs of the host, at this critical stage, often only reached after the lapse of several years, that occasions serious or even fatal results. Zoologically, _Taenia echinococcus_ and _T. coenurus_ are interesting, since they exhibit an {81}indubitable alternation of asexual generations in the larval state, with a sexual adult stage. _Bothriocephalus latus_ Brems., the broad tape-worm, which attains a length of 20-30 feet, or even more, occurs in man endemically in the eastern Baltic provinces, certain parts of Switzerland, generally throughout Russia (especially near Kasan), in North America, and commonly in Japan,—that is, in districts where the population partake largely of pike or other fish in a raw or partially-cooked state. Elsewhere it occurs sporadically, and in Munich, where it was unknown before 1880, its presence has been traced to emigrants from infected districts, who settled on the shores of the Starenberger Lake, from which Munich was supplied with fish. How the pike, the usual but not invariable intermediate host, becomes infested (and its musculature is frequently riddled with the larvae) we do not accurately know, but some Invertebrate, the prey of the pike, is probably the first host into which the free-swimming ciliated larva (Fig. 42) finds its way. In Greenland, _B. cordatus_ is very common in the dog, and probably also in man, though few cases have been recorded. _B. mansoni_ Cobb. (= _B. liguloides_ Leuck.) was, till recently, known only in the larval state from China and Japan. Iijima, however, has found older specimens in the latter country. _B. cristatus_ Dav. is a species founded somewhat doubtfully on two fragments found, one in a child, the other in a man, in France.