The sexually mature Tænia echinococcus may, for the purposes of diagnosis, be characterised as a remarkably small cestode, seldom reaching the fourth of an inch in length and developing only four segments, including that of the head; cephalic extremity capped by a pointed rostellum, armed with a double crown of comparatively large-rooted hooks, from thirty to forty in number; the four suckers prominent, and succeeded by an elongation of the segment forming the so-called neck; final segment, when sexually mature, equalling in length the three anterior ones; reproductive papilla at the margin of the proglottis rather below the central line; proscolex or embryo giving rise to the formation of large proliferous vesicles, within which the scolices or echinococcus-heads are developed by gemmation.

When an animal is fed with the mature proglottides of Tænia echinococcus the earliest changes that take place are the same as obtain in other cestodes. The segments are digested; the shells of the ova are dissolved; the six-hooked embryos escape. The embryos bore their way into the organs of circulation, and thence they transfer themselves to the different organs of the host; being especially liable to take up their abode in the lungs and liver. Having arrived at this, their resting stage, the embryos are next metamorphosed into hydatids. According to Leuckart’s investigations the juvenile hydatid is spherical at the earliest stages; being surrounded by a capsule of connective tissue formed from the organs of the host. After removal from its capsular covering, the vesicle consists of a thick laminated membrane, forming the so-called cuticular layer, and a central granular mass, which subsequently becomes enveloped by a delicate granular membrane. At the fourth week the echinococcus capsule measures about 1/25″ in diameter, its contained hydatid being little more than half this size. Its future growth is by no means rapid, seeing that at the eighth week the hydatid has attained only the 1/15″ in diameter. At this period the central granular mass develops a number of nucleated cells on the inner surface of the so-called cuticle. These cells, which at first are rounded or oval, become angular or elongated in various directions, and even distinctly stellate; and in this way a new membrane is formed, constituting the so-called inner membrane or granular layer. The intermediate stages between this condition and that of the fully-formed echinococcus hydatid have not been satisfactorily traced in detail; nevertheless, Krabbe and Finsen’s experiment on a lamb showed that within a period of little more than three months well-developed echinococcus-heads may be formed in the interior of the vesicles. It is thus clear that the production of scolices immediately follows the formation of the granular layer, and this is succeeded, though not invariably, by the formation of daughter- and grand-daughter-vesicles, which are sometimes termed “nurses.” These latter may be developed exogenously or endogenously.

Fig. 30.—Ectocyst, endocyst, and brood cap­sule of E­chi­no­coccus. From a Zebra. After Huxley.

The appearance of hydatids varies very much according to their mode of formation, to the kind of host in which they are present, and to the character of the organs in which they happen to take up their residence. The so-called exogenous type occurs sparingly in man, whilst the endogenous type is very abundant. The peculiar form known as the multilocular echinococcus is probably a mere variety of the exogenous type. The exogenous and endogenous hydatids may coexist in the same bearer. In the lower animals we commonly find the organs of the body occupied by numerous lobulated cysts, varying in size from a walnut to a goose’s egg, but sometimes rather larger. They are rarely solitary, being particularly liable to occupy both the liver and lungs in the same animal. The viscera are sometimes crowded with cysts. The hydatids do not usually protrude much beyond the surface of the infested organ, but lie imbedded within its parenchymatous substance.

The multilocular variety was first described by Virchow. In reference to it Leuckart writes as follows:

“Hitherto we know this growth only from the liver, in which it forms a firm, solid, and tolerably rounded mass of the size of the fist or even of a child’s head. At first sight it looks more like a pseudoplasm than a living animal parasite. If you cut through the tumour, you recognise in its interior numerous small caverns, mostly of irregular shape, and separated from one another by bundles of connective tissue, more or less thick, and including a tolerably transparent jelly-like substance. In the intervening stroma a blood-vessel or a collapsed bile-duct runs here and there; but there is nowhere any trace of true liver substance. The outer boundaries of the tumour are in most cases pretty well defined, so that the attempt to cut these growths out is not difficult. In particular spots, especially at the surface, one sometimes sees white, moniliform, jointed lines passing off from the tumour, and even thicker terminations which, perhaps, expand in the neighbouring liver-parenchyme into new (multilocular) groups of different size. In one case, recorded by Virchow, the growth extended, together with Glisson’s capsule, a long way towards the intestine.” To this description it may be added, that the growth on section presents an appearance not altogether unlike alveolar colloid, having, in point of fact, been confounded with that pathological product, with which, however, as stated by Virchow, it has nothing in common. This is proved not only by the occurrence of the pathological features above mentioned, but also, more particularly, by the well-ascertained presence of echinococcus-heads in most of the so-called alveoli. Several hypotheses have been broached with the view of explaining the mode in which these multilocular hydatid growths are formed. Virchow thought that the echinococcus vesicles were primarily formed in the lymphatic vessels, whilst Schröder van der Kolk supposed that they originally took up their abode in the biliary ducts. Although, thanks to the courtesy of Professor Arnold Heller in giving me a specimen, I have been enabled to confirm much that has been written in respect of the morbid appearances, I can add nothing towards the solution of the difficulty in question. Until lately it was supposed that the multilocular variety of hydatids only existed in man, but Professor Böllinger has encountered it in the liver of a calf.

Fig. 31.—Group of Echinococcus-heads, from an hydatid found in the liver of a sheep. Magnified about 25 diameters. From a drawing by Professor Busk.
Fig. 32.—Three brood-capsules, containing Echinococcus-heads. Magnified 76 diameters. After Professor Erasmus Wilson.

Selecting any ordinary fresh example of the exogenous kind, and laying the tumour open with a scalpel, we notice in the first instance an escape of a clear transparent, amber-coloured fluid. This previously caused the distension of the sac. If the tumour is large, this escape will probably be followed by a falling in, as it were, of the gelatiniform hydatid membrane, in which case the inner wall of the external adventitious investment or true fibrous cyst will be laid bare. If the hydatid be next withdrawn from the cyst, it will be seen to display a peculiar tremulous motion, at the same time coiling upon itself wherever there is a free-cut margin. Further examination of portions of the hydatid will show that we have two distinct membranes; an outer, thick, laminated, homogeneous elastic layer (the ectocyst of Huxley), and an internal, thin, soft, granulated, comparatively inelastic layer—the endocyst of the same author. The terms are convenient. The ectocyst is structureless, consisting of a substance closely allied to chitine. For this and other reasons it has been called the cuticular layer, but the endocyst is the essential vital part of the animal, representing a huge compound caudal vesicle. In an hydatid from the zebra, Huxley found that the endocyst was “not more than 1/2000th of an inch in thickness, being composed of very delicate cells of 1/2000″ to 1/5000″ in diameter, without obvious nuclei; but often containing clear, strongly refracting corpuscles, generally a single one only in a cell.” Prof. Huxley adds: “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 corpuscle. No gas is developed during this process, and sometimes the corpuscles are not acted upon at all by the acid, appearing then to be of a fatty nature. A strong solution of caustic ammonia produces a concentrically laminated or fissured appearance in them. Under pressure and with commencing putrefaction a number of them sometimes flow together into an irregular or rounded mass.”

The precise mode of development of the echinococcus-heads or scolices has been a subject of lengthened discussion between Leuckart and Naunyn. According to Leuckart the earliest indication of the scolex consists of a slight papillary eminence on the inner surface of the granular endocyst. After a short period this prominence displays in its interior a vacuole-like cavity, the latter being occupied, however, with a clear limpid fluid. Its margins become more and more clearly defined, until the cavity is by and by seen to be lined with a distinct cuticular membrane. The papilla increasing in size, becomes at first elongated or oval, eventually scoleciform, or even, perhaps, a true echinococcus-head. Thus far the description bears out, in a measure, the theoretical notions entertained by the older authors; but the developmental process does not stop here. The scolex-development has now to sacrifice itself by developing in its interior a brood of scolices or echinococcus-heads. In other words, it becomes transformed into the so-called brood-capsules of Leuckart and other authors. These structures were previously well known to Professors Erasmus Wilson and George Busk. Mr Wilson spoke of the capsule as “a delicately thin proper membrane, by which the Echinococci are connected with the internal membrane of the acephalocyst” (‘Med.-Chir. Trans.,’ 1845, vol. xxviii, p. 21). Mr Busk described the echinococcus-heads as “attached to a common central mass by short pedicles, which appear to be composed of a substance more coarsely granular, by far, than that of which the laminæ of the cyst are formed. This granular matter is prolonged beyond the mass of Echinococci into a short pedicle common to the whole, and by which the granulation is attached to the interior of the hydatid cyst.” What Mr Busk here describes as a granulation can only be equivalent to the brood-capsule and its entire contents, but he elsewhere speaks of the capsule itself as a “delicate membranous envelope.” It should be borne in mind that Busk’s paper was communicated to the Microscopical Society so early as the 13th Nov., 1844; being published in the ‘Transactions’ for that year.