passively by the second, or definitive, devouring the first, or intermediate host. Both stages of the parasite exhibit extreme modification of structure, the second being characterized by an enormous development of the hermaphroditic gonads and of the alimentary surface, which is merely the integument, and is therefore in immediate contact with the food supply. To this type of parasitism we may also refer the Dicyemids, and many of the flukes and round-worms. In many of these cases the association with the host may be effected without any effort on the part of the parasite, and the small size, the enormous number and the method of distribution of its eggs are properly interpreted as so many direct and necessary adaptations to chance.
The Rhizocephalous crustacean Sacculina, which may serve as a paradigm of the second type, or that of temporary parasitism with free early ontogenetic stages, also produces an enormous number of minute eggs. These, however, develop into free-swimming Nauplii, which in turn become Cypris larvæ and as such seek out their Decapod or Isopod hosts. Owing to the activity and comparatively high organization of these larvæ, the element of chance in bringing about the host association, though still considerable, is not as great as it is in the tape-worm. When it has joined its host, the Cypris larva, through one of the most remarkable methods of development known to exist among animals, proceeds to undergo structural modifications so extreme that, without a knowledge of the earlier stages, the crustacean affinities of the organism would never be suspected. "In the adult state the body consists of two portions: a soft bag-like structure, external to the host, carrying the reproductive, nervous and muscular organs and attached to some part of the host's abdomen by means of a chitinous ring; and a system of branching roots inside the host's body, which spring from the ring of attachment and supply the external body with nutriment."[1] In Sacculina, as in the tape-worm, the gonads are hermaphroditic and reproduction takes place by a continual round of self-fertilization. To this type of temporary parasitism with free larval stage we may also refer the myzostomes and other parasitic annelids and the parasitic mollusks. In all these cases metamorphosis supervenes while the animal is still very small and hence precedes growth and the incidence of the modifications produced by the parasitic habit.
As an example of the third type or that of temporary parasitism with free adult stage, we may select the Ichneumonid Hymenopteron. The eggs are few in number and rather large and are deposited by the mother directly in or on the host, which is the larva of some other insect. The sluggish, bag-shaped parasitic larva, on hatching from the egg, feeds for some time on the blood-tissues and fat-body of the host, but is careful not to prevent the latter from moving about, procuring its food and growing to maturity. When it has reached this stage, how-
- ↑ Geoffrey Smith in the Cambridge Natural History, Vol. IV., p. 95.
