Restricting ourselves here to the consideration of such as are peculiar to man, we would preface our special description of them with the following general remarks:

(a.) Intestinal worms, in their consummated development, are all provided with organs of generation. Those in which the latter have not been demonstrated, are propagated by buds or by offshoots, if they be not imperfect, that is, either larvae or diseased animals. As opposed to the doctrine of equivocal generation, these facts are important, if we consider:

(b.) The migrations and the attendant metamorphoses of the helminthes.

The migrations of the helminthes consist, first in the search for a suitable animal to inhabit, and in introducing themselves into it, when found, through channels formerly unthought of. Secondly, they consist in abandoning the animal dwelt in, for the purpose of casting their ova under favorable conditions, then in passing through one of their metamorphoses, and lastly in searching for another animal for their habitation. They pass, under various phases of development, for the most part through natural orifices of the body, more especially into and out of the intestinal canal. Their occurrence, however, even in the paren-chymata, is intelligible upon grounds of direct experience. As illustrative of this, the larvae of cercarioid trematoda, and of the tetrar-hynchi, have been observed to migrate through the parenchymata of mollusca and fishes. It is also deserving of notice, in this place, that helminthes may reach, and settle in any parenchyma through the circulating channels, probably by boring for themselves a passage into the bloodvessels of the intestinal canal. This applies to the nematoid, thread-like animals found by Valentin, Vogt, Gruby, Ecker, and others, in the blood of frogs, dogs, and ravens, and probably representing the embryones of helminthes.

This migration of the helminthes may involve frequent aberrations, and these in their turn many phenomena, which an extended inquiry will perhaps correctly set down to a morbid condition. We refer more particularly to the encysting, the atrophy, and the deformity of certain helminthes; amongst others, of the trichina spiralis, and the taenioid cystica.

(c.) The metamorphoses of the helminthes, coincident with their migrations, are of the greatest interest. They constitute a circle of generations, which Steenstrupp, following up the investigations of other naturalists, has pointed out in the trematoda (as in the medusae, bulb-polypi, and salpae). A parent animal produces a brood altogether dissimilar to itself, nor identified with it until after three or four generations. These intervening generations of larvae - these pro-nutrices and nutrices - form without sexual mediation, and are the source of the numerous fallacies taught by the older helminthologists.

(d.) All this accords perfectly well with the strict limitation of certain worms to particular countries. The most striking example is afforded in those two riband-worms, the botryocephalus latus, of Russia, Poland, Prussia up to the Vistula, and Switzerland; and the taenia solium of the remainder of Europe.

(e.) On the other side, the doctrine of the origin of the helminthes out of intestinal mucus and the like, has not a single point of real evidence in its favor. A disposition to worms exists only in so far as an organism abnormally nourished offers to helminthes, introduced into it from without, a nidus well adapted for their development. In mankind the following helminthes occur: -