There are three genera and numerous species of Entozoa. Every animal has one or more species peculiar to itself; fourteen infest the human race. They have a soft, absorbent body of a white or whitish colour, in consequence of being excluded from light, and living as they do by absorbing the vitalized juices of the animals they infest. Their nutritive system is in the lowest state of development; yet there are some of a higher grade. All are remarkable for their vast productiveness. The Tænioïdæ, which belong to the inferior group, are intestinal, many-jointed worms, which have neither mouth nor digestive organs; and what is called the head has only hooks and suckers to fasten it to the internal membrane of the animal at whose expense it lives. The common Tænia, or Tape-worm, sometimes ten feet long, which is the type of the order, has four suckers and a circle of hooklets round a terminal proboscis to attach it to its victim. Though destitute of the organs of nutrition it is extremely prolific, for each segment of its long flat body is a reproductive monœcious zooid, which forms and lays its own eggs exactly as if it were a single independent animal, thus furnishing a very remarkable instance of the law of irrelative repetition, which is a series of organs performing the same functions independently of one another. Two pairs of canals containing a clear colourless liquid extend throughout the body of the worm. Bags, or vesicles called cysts, had been found in the glands and muscles of various animals, afterwards discovered to contain young worms, which attain their perfect development within such creatures as eat the flesh containing the cysts. Under circumstances so unprecedented, it required no small skill and patience to determine the life-history of these singular creatures. The cysts differ in size and form according to the genera, and are embedded singly or in groups in the flesh of their victim, on whose ready prepared juices they live. The greater number of the Tænia genus begin their lives as sexless cysted larvæ, and on entering their final abode, segments are successively added till the worm has arrived at its adult state. The tape-worm of the cat has its origin in the encysted larvæ found in the livers of the mouse and rat. One species of Entozoa, while in its primary state, inhabits the stomach of the stickleback, and only comes to perfection within the aquatic birds that feed on this fish. Another species infests the livers of the salmon tribe, and gets its perfect form in the pike and perch.