to an animal which, like a tapeworm (Fig. 10), obtains its food ready-made in the very kitchen, so to speak, of its host. Hence the lack of a digestive apparatus follows the finding of a free commissariat by the parasite. Organs of sense are not necessary for an attached and rooted animal; these latter, therefore, go by the board, and the nervous system itself becomes modified and altered. Degradation, wholesale and complete, is the penalty the parasite has to pay for its free board and lodging; and in this fashion Nature may be said to revenge the host for the pains and troubles wherewith, like the just of old, he may be tormented. Numerous life-histories testify clearly enough to the correctness of the foregoing observations. Take, as an example, the history of Sacculina (Fig. 11, a), which exists as a bag-like growth attached to the bodies of hermit-crabs, and sends root-like processes into the liver of its host. No sign of life exists in a
Fig. 11.—Sacculina and Young.
sacculina beyond mere pulsation of the sac-like body, into and from which water flows by an aperture. Lay open this sac, and we shall find the animal to be a bag of eggs and nothing more. But trace the development of a single egg, and one may derive therefrom lessons concerning living beings at large, and open out issues which spread and extend far afield from sacculina and its kin. Each egg of the sac-like organism develops into a little active creature, possessing three pairs of legs, generally a single eye, but exhibiting no mouth or digestive system—parasitism having affected the larva as well as the adult. Sooner or later, this larva—known as the nauplius (b)—will develop a kind of bivalve shell; the two hinder pairs of limbs are cast off and replaced by six pairs of short swimming-feet; while the front pair of limbs develops to form two elongated organs whereby the young sacculina will shortly attach itself to a crab "host." When the latter event happens, the six pairs of swimming-feet are cast off, the body assumes its sac-like appearance, and the sacculina sinks into its adult stage—a pure example of degradation by habit, use, and wont. So also with certain near neighbors of these crab-parasites, such as the Lerneans, which adhere to the gills of fishes. Beginning life as a three-legged "nauplius," the lernean retrogresses and de-
