120 PARASITES OF MAN. 



completo 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 (Tcenia 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 do 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 loss 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 supphes us with a fonn 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 multiplica- 

 tion 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 tapewornx begets only one Ttenia, 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. This 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 tapewoi-ms, and 

 collectively they amount to many thousands. Thus, when a dog or wolf 

 swallows the polycephalous hydatid and its offspring, all the heads of 

 the colony of lai'va3 or scolices will become connected into sexually 

 mature tapeworms in the intestine of the new host. The zoological 

 individual, thei-efore, will comprise not merely one tapeworm, "but a 

 multitude of tapeworms. In other words, whilst the egg of 

 an ordinary tapewonn like Tctnia mediocancUata supplies a single 

 colony or strobile of 1,200 joints, (proglottides or zooids,) the egg of the 

 little Tfcnia Echinococcus supphes several thousands of colonics 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. 



24. — Echinococcus hominis, Rudolphi. 



Syn. — E. reterinorum, Bremser, Gurlt, &c. ; E. scoUcijmriens and 

 E. altricijMriens, Kiichcnmeister ; E. 2^ohjmorphus, Diesing; 

 Acephalociistis, Laenuec, John Hunter, Owen, &c. ; PolyccphaJus, 

 Goeze ; Hijdatis, Liidcrsen ; Uijdatigena, i3atsch ; Vcsicaria, 

 Schrauk. 



