Ill 



of cysts, varying in size from a child's head downwards. The liver 

 was taken out of the body on the day succeeding the animal's death* 

 — and on the 27th I proceeded to examine the contents of one of the 

 largest cysts (with a portion of its wall) and one of the smaller cysts. 



It was at once obvious that the cysts contained the Echinococcus 

 veterinorum, and I may here mention that the Echinococci were in 

 full life, and remained so for three days, until, in fact, the fluid in 

 which they were contained had become slightly oifensive. 



It will conduce to clearness perhaps, if I state in successive order 

 I. What I saw myself. II. The theory of the formation of the 

 Eckinococc2is-cjsts, and of their relation to other forms of Entozoa, 

 which I have to offer. III. What has been done hitherto. 



I. The cysts are nearly spherical vesicles having a very elastic 

 proper wall ; so elastic, in fact, as to exercise a continual tension 

 upon the contained fluid, which, if the cyst be pierced, spurts out in 

 a jet, for some time. 



The outermost layer of the cyst is an adventitious membrane, 

 formed by the infested animal around the Echinococcus-cyst, as it 

 would be developed round any other foreign body ; with this I have 

 nothing to do. Within this, and in nowise adherent to it, follows 

 the proper wall of the Echinococcus-cy&t, which must be carefully 

 distinguished into two portions. The outer is thick, yellowish and 

 constituted by a great number of delicate, structureless laminae com- 

 posed of a substance closely resembling Chitin. It is to this laminated 

 membrane that the elasticity of the cysts is due — and it must be 

 regarded as precisely analogous to those structureless cysts which 

 surround the pupa forms of Distomata imbedded in the body of 

 snails, or to those similarly structureless cysts which enclose the 

 encysted Tetrarhynchi, and which Van Beneden saw in course of 

 formation by a process of exudation, around the Scolex form of those 

 worms. The innermost layer of this, which, for distinction's sake, I 

 will call the Ectocyst, is whiter and softer than the others, and appears 

 to be in course of formation. 



The inner portion of the wall of the Echinococcus-cyst is closely 

 adherent to the last described layer of the ectocyst, but may, with 

 great care, be separated from it, when it is at once evident that 

 there is no organic connexion between the two ; this layer may be 

 very conveniently termed the endocyst — it is the only active living 

 part of the whole wall of the cyst, and rejiresents the proper body- 

 wall of the animal. It is very pale and delicate, and not more than 

 ■^^-^ of an inch thick (PI. XXVIII. fig. 5). It is composed of very 

 delicate cells Woo— sttoo- of ^n inch in diameter, without obvious 

 nuclei, but often containing clear, strongly refracting corpuscles, gene- 

 rally a single one only, in a cell. These corpuscles appear to be solid, 

 but by the action of dilute acetic acid, the interior generally clears up 

 very rapidly, and a hollow vesicle is left of the same size as the original 



* I beg here to express my obligations to the Secretary of the Zoological 

 Society, without whose kind recollection of a wish to examine fresh Entozoa, 

 which I had expressed, I should not have had the opportunity of making the ob- 

 servations contained in the present paper. 



