(368). The Echinococcus veterinorum, long considered as a distinct Entozoon,is in reality merely a hydatid cyst filled with the larvae (Scoleces) of Taenioid worms; it occurs in the liver, the cavity of the abdomen, the heart, the voluntary muscles, and the ventricles of the brain of man, - in the liver, lungs, etc. of the ox, sheep, goat, ape, pig, etc. The walls of the true cysts consist of numerous concentric layers or plates, resembling those of colloid cells. The liquid existing within them is yellowish or reddish, and albuminous. The larvae appear to the naked eye as minute white opake specks, varying in size from about 1/300" to 1/100" in length; they also vary greatly in form: when the head is retracted, they appear more rounded than when it is protruded. The hooks surrounding the anterior end of the body consist of a basal portion, an internal transverse blunt tooth, and a curved terminal portion or claw; they are about 1/1500" to 1/1000 in length. In some of the larvae a kind of pedicle exists, by which they are attached to the walls of the cyst. In a quite recent state the larvae have been seen swimming actively in the liquid of the cyst by means of cilia upon the surface of the body.

They appear usually to be developed from the interior of the cyst; but, as Kuhn long since showed, they are sometimes produced by external gemmation: the contents produce a slight protrusion of a part of the wall of the cyst; the protruded portion enlarges, afterwards becoming constricted at the base, and at last probably separates from the parent, to become itself a parent in a similar manner. Hence it appears that the larvae cannot be regarded as the parasites of the cyst, but must be viewed as arising from a partial segmentation of the contents of the parent. The Echinococci do not acquire their full development into Tamice unless they reach the alimentary canal. The cysts and their contents, including the Echinococci, undergo a kind of degeneration, becoming partially converted into fatty or calcareous matter, or the entire contents become amorphous and granular, the hooks remaining longest unaltered, but finally disappearing also*.

* Vide De Cysticercorum in Taenias Metamorphosi pascendo, experimenta in Institute Physiologico Vratislaviensi administrata. Auctor G. Lewald: Berolini, 1852.

(369). Trematoda

In the fluke, Distoma (Easciola, Linn.) hepati-cum, we have an Entozoon of more complex and perfect structure - one of those forms, continually met with, which make the transition from one class of animals to another so insensible that the naturalist hesitates with which to associate it.

(370). The Distoma is commonly found in the liver and biliary ducts of sheep and other ruminants, deriving nourishment from the fluids in which it is immersed. The body of the creature, which is not quite an inch in length, is flattened, and resembles in some degree a minute sole or flat-fish. At its anterior extremity is a circular sucker or disk of attachment, by which it fastens itself to the walls of the cavity in which it dwells, as well as by means of a second sucker of similar form, placed upon the ventral surface of the body. In the annexed diagram (fig. 71) the posterior sucker has been removed in order more distinctly to exhibit the internal structure of the animal. The name which this Entozoon bears seems to have been given to it from a supposition that it possessed two mouths, one in each sucker, whereas the anterior or terminal disk (a) only is perforated, the other being merely an instrument of adhesion. The alimentary canal (b) takes its origin from the mouth as a single tube, but soon divides into two large branches, from which ramifications arise that are dispersed through the body, each terminating in a blind clavate extremity. These tubes, from being generally filled with dark bilious matter, are readily traced, even without preparation, or they may be injected with mercury introduced through the mouth.

* Kuhn, Ann. Sc. Nat. 1 ser. xxix. p. 273; Siebold, Wiegmann's Archiy, 1845, and Siebold and Kolliker's Zeitschr. iv.; G-luge, Ann. Sc. Nat. 2 s6r. viii. p. 314; Owen, Hunterian Lectures, i. p. 46; Dujardin, Helminthes, p. 635; Huxley, Ann. Nat. Hist. 2 ser. xiv. p. 379.

(371). Through the walls of the ventral surface of the body two nervous filaments (c) are discoverable, which, crossing over the root of the anterior sucker or acetabulum, and gradually diverging, may be observed to run in a serpentine course towards the caudal extremity, where they are lost: it would even seem that on either side of the oesophagus there is a very slight ganglion, from which other nervous filaments arise to supply the suckers and the anterior part of the body.

(372). The organs of generation in the fluke are very voluminous, occupying, with the ramifications of the alimentary tubes, the whole of the interior of the animal: in the diagram they are not represented on the right side, in order that the distribution of the intestine may be better seen; and on the left side the alimentary vessels are omitted, to allow the general arrangement of the sexual system to be more clearly intelligible.

(373). These animals are completely hermaphrodite, not only possessing distinct ovigerous and seminiferous canals, which open separately at the surface of the body, but even provided with external organs of impregnation, so that most probably the cooperation of two individuals is requisite for mutual fecundity.

(374). To commence with the female generative system, we find the ovaria (h) occupying the whole circumference of the body. When distended with ova, the ovigerous organ is of a yellow colour; and when attentively examined under the microscope, it is seen to be made up of delicate branches of vesicles united by minute filaments, so as to have a racemose appearance. From these clusters of ova arise the ovigerous canals, which, uniting on each side of the body into two princijml trunks, discharge their contents into the large oviducts (g.) The oviducts terminate in a capacious receptacle (e), usually called the uterus; and from this a slender and convoluted tube leads to the external orifice, into which a hair (d) has been inserted. On each side of the uterus we find a large ramified organ, made up of caecal tubes (f), which opens into the uterine cavity, and no doubt furnishes some accessory secretion needful for the completion of the ova.