Do the cetacea generally live on fish, and do they become the prey of some aquatic carnivora? We have reason to think so, from the presence of certain agamous cestodes, which have been frequently found in too great number to allow us to suppose that they have lost their way in these aquatic mammals. There have been seen in the substance of the muscles of many species, or rather in the layer of blubber which covers the skin, agamous cestode worms of the genus _Phyllobothrium_, which can only accomplish their evolution in some large _squalus_. There must then be contests between dolphins and sharks, contests in which the dolphins are worsted, in spite of their superiority. These Phyllobothria have been found in the _Delphinus delphis_, the 208 _Tursio_, and the _Ziphius_. As the _Orca_ attacks the whale, and feeds upon its flesh, there would be nothing surprising in our finding in these large cetacea, some agamous cestode destined to pass through the last phase of its evolution in this terrible carnivorous animal. The cestode can scarcely be called a parasite under the first vesicular form. It is sufficient for it to pass through its first transformation in the midst of the tissues, and it will remain weeks, months, even years, without undergoing any change; it asks for nothing but an hospitable roof; and this mysterious being, that had often come they knew not whence, encamping rather than lodging, always without progeny, was long since cited by the naturalists of a former age in favour of the old hypothesis of spontaneous generation. It is not the same with the second form. Here the worm, always lodged in the intestines, grows with extraordinary rapidity, and fulfils all the conditions of a true parasite. In a fertile soil it extends itself and produces young as long as it has any life, and in no group of the animal kingdom do we find any fecundity to be compared to that of this worm. Boerhaave described a broad tape-worm, three hundred ells in length. Eschricht estimates the number of the segments of this worm as ten thousand; and if we consider that each segment, or, we should rather say, each complete worm, may perhaps enclose thousands of eggs, we may form some idea of the profusion of germs which can be scattered by each individual.