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complete without taking into account the occurrence of hydatids. These structures, often spoken of as bladderworms, form, as is now well known, the scolex condition of a minute tapeworm (Tænia Echinococcus) which lives in the dog. From a sanitary and professional point of view this parasite is of more importance than all the others put together, but it must be obvious that it would be out of place here to more than glance at its strictly zoological position. Every experienced Surgeon has to deal with instances of its occurrence in important organs, and probably not less than four hundred persons perish in the United Kingdom every year from this worm. In Australia and in Iceland the echinococcus disease is excessively fatal to man. The parasite is also scarcely less frequent amongst animals, although in these bearers its presence is only rarely attended with fatal consequences. Zoologically and morphologically the common hydatid is of great interest. Whilst the sexually mature worm supplies us with a form of human tapeworm altogether unique, (both as regards its size and the small number of its proglottides,) the larva, in the character of an hydatid, presents us with a type of polycephalous bladderworm which, so far as I am aware, has no parallel. The hydatid furnishes us also with a curious illustration of the extreme possibilities of tapeworm multiplication from a single germ. Starting with the postulate that the sum total of the products of a single impregnated germ or ovum fairly represents the "individual," (zoologically, so to say,) we find that whilst, on the one hand, the egg of any ordinary tapeworm begets only one Tænia, the egg of the hydatid-tapeworm is capable of producing, under favourable circumstances, several thousand tapeworms. To appreciate this truth, it is only necessary to observe that the six-hooked embryo becomes one hydatid. The maternal bladderworm may by proliferation beget daughter and grand-daughter hydatids, all of which in their turn may give rise to the formation of echinococcus heads in their interior, Separately these so-called heads represent as many tapeworms, and collectively they amount to many thousands. Thus, when a dog or wolf swallows the polycephalous hydatid and its offspring, all the loads of the colony of larvæ or scolices will becomes connected into sexually mature tapeworms in the intestine of the new host. The zoological individual, therefore, will comprise not merely one tapeworm, "-but a multitude of tapeworms. In other words, whilst the egg of an ordinary tapeworm like Tænia mediocanellata supplies a single colony or strobile of 1,200 joints, (proglottides or zooids,) the egg of the little Tænia Echinococcus supplies several thousands of colonies or strobiles, each of which is made up of three segments, without reckoning the head, this singular mode of tapeworm multiplication is also witnessed, though in a much less degree, in certain other forms of polycephalous bladderworms.

2h—Echinococcus hominis, Rudolphi.