Zoological Society. 385 



set free. The pedicle contracts upon itself so as to have a rounded 

 form, but it very often betrays its previous adherence by the ragged 

 fragments of the endocyst, which it carries with it. 



Whether this is properly a normal process in the Echinococcus it 

 is difficult to say, but as Dr. Guido Wagner and Van Beneden have 

 shown, it occurs normally in the Tetrarhynchidce, and it exactly re- 

 sembles that detachment of the "tail" from the Cercaria, which 

 takes place in the Distomata. 



As little is it known whether the Echiaococci undergo any further 

 development. The suggestion first made by Delle Chiaje, that they 

 may dilate into cysts and develope young Echinococci within them- 

 selves, appears to me highly improbable ; and it is an hypothesis 

 which is not needed to account for the secondary cysts. 



b. Fixed Secondary Cysts. — The development of these indeed 

 takes place in such a manner as to preserve the homological rela- 

 tions of the Echinococci to the exterior of the parent. The secondary 

 cysts, in fact, are thus formed : Echinococci are developed not only 

 from the inner surface of the endocyst, but from its outer surface. 

 Their growth is probably accompanied by that of the endocyst itself, 

 which thus becomes raised up from the ectocyst and projects into the 

 general cavity. Of course any internal Echinococci which happen to 

 be attached to this part of the endocyst are raised up with it : they 

 may be fewer or more according to circumstances. The neck of attach- 

 ment of the secondary cysts gradually narrows, and at last the secondary 

 cyst, whose size depends entirely upon the number of Echinococci 

 developed under the endocyst at one spot, is detached and falls into 

 the cavity. So long as the secondary cyst remains attached, its 

 external Echinococci have the normal clear appearance, and are in 

 full health ; but when once it is separated, they appear rapidly to 

 wither away and become yellow, losing their hooks and their cor- 

 puscles, and eventually disappearing. The original point of attach- 

 ment of the sac remains as an obtuse cicatrix. 



Von Siebold, who has beautifully described the development of the 

 secondary cysts, has, I think (vide infra), mistaken the one mode of 

 development of the Echinococci outside the endocyst for the only 

 mode. He appears to have seen the endocyst, when he describes 

 the " delicate membrane in which the young Echinococcus-heads are 

 enclosed," and to assume merely, that this membrane bursts and sets 

 the Echinococcus free upon the inner surface of the parent cyst. Un- 

 derstanding the mode of development to be as stated, above, it is easy 

 to comprehend how it is, that the Echinococci are so nearly at the 

 same stage of development in all the secondary cysts ; and that this 

 stage has no relation to the size of the cysts. The existence of the 

 external Echinococci upon the secondary vesicles in this way also, 

 becomes not only intelligible, but almost necessary. 



II. The theory which I have to offer of the nature of the Echino- 

 coccus, is based upon three facts which are now well established. 

 1st. That young Cestoid Worms, which, from some cause or other, 

 have passed into any other part of the organism of the animal upon 

 which they are parasitic, than the intestine, become abnormally 

 Ann. $ Mag. N. Hist. Ser. 2. Vol. xiv. 25 



