Zoological Sociei^i^^^^ 379 PROCEEDINGS OF LEARNED SOCIETIES. ZOOLOGICAL SOCIETY. On the Anatomy and Development of Echinococcus VETERINORUM. By ThOMAS HuXLEY, F.R.S. [With a Plate.] On the 25th of November, 1852, a fine female Zebra, whilst at play within its paddock, accidentally broke its neck. The animal had always appeared to be quite healthy, and it was in perfectly good condition — but, upon examination, its liver was found to be one mass 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 veteriiiorum ; 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 offensive. 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 Echinococcus-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-cyst, 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 chitine. 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 * I beg here to express my obligations to the Secretary of the Zoological Society, without whoso 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.