The parasitic Platyhelmia—the Tapeworms (_Cestoda_) and the Flukes (_Trematoda_)—occupy Case I.; the life history of the common Tapeworm (_Tænia solium_) is shown by the aid of models and figures. A model of the anterior end of the common Tapeworm shows the four suckers and the crown of hooks; the unjointed neck is followed by the joints (_proglottids_), which increase in size the farther they are from the neck. Several entire specimens of _Tænia_ follow, showing the size of the whole worm and the form of its joints. The structure of the body is shown in the models of two joints. The growth and development of the _Tapeworm_ is dependent on a migration or a change of the hosts which it inhabits in the various stages of its life; and although the different kinds of Tapeworm differ from each other somewhat in certain details of their migration and development, their life history exhibits, on the whole, the same events which we find in _Tænia solium_, a common Tapeworm of man in Northern Europe. This worm is matured in the intestines of man; its final joints consist merely of fertilized ova which have already passed through the earlier stages of development; when the joints are detached and discharged, their contents escape in the form of embryos contained in a thick chitinous shell. If these are now swallowed by a pig, the shell is digested by the gastric juices of the new host, and a rounded embryo, which is provided with three pairs of hooks, is set free; by means of these hooks the guest makes its way through the wall of the stomach or intestine, and finally settles down in the muscles of its host. The embryo now loses its hooks, and gradually acquires a bladder-like form, the central cavity of which is filled with fluid. This bladder-worm (_Cysticercus_) has its outer wall pushed inwards at the anterior end, and on this hooks and suckers become developed. We have now a narrow head and neck with an attached bladder, the head being at this time hollow. If during the long time that these bladder-worms remain alive, the pig is killed for food, its flesh is found to be “measly”; if it is afterwards insufficiently cooked and eaten, the worms are conveyed into the human stomach. Here the bladder-like termination becomes absorbed, and, the neck beginning to grow, we have the commencement of the form from which we started, and the completion of that “vicious circle” which is so curious a characteristic of many forms of parasitic life. [Illustration: Fig. 8. _Tænia solium_: showing the head (_h_) with its suckers (_s′_) and crown of hooks (_s_), the unjointed neck (_n_), and a few of the succeeding joints (_j_). ] In other Tapeworms the cyst may be more complicated than that in the pig, as, for example, the form found in the sheep’s brain or the liver of the horse.