ANIMAL.] PARASITISM 261 associations. Of these there are many familiar instances, such as the hermit crabs bearing Sagartia parasitica (fig. 1), or having their shell-mouth enveloped by Adamsia. One of the quaintest instances is a lately described species of crab which wields an anemone firmly grasped in either claw. In such cases the association is obviously useful : the crab is protected from the octopus and other enemies by the nematocysts of its comrade, which also aid in holding the prey, while the Actinia too gains its share of the food, and vicariously acquires means of locomotion. To such cases where two animals are associated together for mutual sup port and advantage the term " Commensalism" is applied. In the struggle for existence increased complexity of needs, and difficulty in satisfying them, evokes in the individual organism a certain specialization of function and conse quent differentiation of structure. Similar causes result not so much in the differentiation of each individual of a species as in the specialization of certain individuals for certain specific functions, resulting again in that specialization of structure which is called polymorphism. Thus in a Hydmctinia or Siphonophore colony many different individuals of the same species have been specia lized in each to perform a certain function. The same purpose is served by those associations, not of individuals of the same species, but of two individuals of different species, united as we have just seen for mutual advantage, and each working out some definite part of the common life-problem. Just as polymorphism in the same species is physiologically equivalent to differentiation in the indi vidual organism, so is commensalism between different species the physiological equivalent of polymorphism in a single species. But cases of co-operation on equal terms are rare ; size constitutes the most frequent disparity, and the smaller tends to become first wholly dependent upon the other for support, then for concealment, and finally perhaps for sustenance. The reverse may occasionally occur, the weaker being utilized for the purposes of the stronger ; thus a species of Dromia adapts a colony of sponge or ascidian as a removable upper garment for concealment. 1 Parasitism within the same Species. In some cases even within the morphological unity of the species a physio logical relation is established analogous to commensalism if not to parasitism. Thus in BoneUia the diminutive and degenerate male lives in the uterus of the female, in Trichosomum crassicauda of the rat three or four male are found within the spermatheca of the female, while in Bilharzia the incipient reciprocal of these cases is found, the male being host. Many of the most remarkable cases are also afforded by the Cirripedia, in which a female may bear males in various states of dependence and degenera tion. In viviparous animals a certain absorption by the young from the tissues of the parent can hardly avoid taking place ; this is therefore so far an analogy to endo- parasitism. This advantage is clearly retained and deve loped if absorption take place by an organ specialized for the purpose. Thus in the well-known shark Mustelus lijevis the young are attached to the oviduct by a placenta developed on the yolk sac ; and the like arrangement, though morphologically different, is physiologically the same among the Mammalia. Hyperparasitism. Not only are very few animals alto gether free from parasites, but even parasites themselves find their nemesis in being themselves infested by lesser parasites, though not "ad injinitum." Thus Leuckart mentions that water-lice and thread-worms are found on parasitic crustaceans, and the endoparasitic larvae of some Hymenoptera are themselves preyed upon by other larvae 1 For an account of many cases of commensnlisin, sou V. Beneden. Animal I nrasitfs, and Semper, Animal Life, both in International Science Series. (Pteromalinx). Nematodes are found in Nicothoe, and associated with Sacculina are frequently found two other crustacean parasites, one of which, after destroying the greater part of its host, continues to subsist upon the nourishment afforded by its root-like processes which sur vive the operation. Classification. Some classification of these various parasitic forms is necessary. Van Beneden introduced the useful term commensals or messmates, under which he includes (1) oikosites or fixed and (2) coinosites or free partners. These he distinguishes not only from parasites but from mutualists where two species are associated, but neither share a common food nor does one prey on the other. Parasites he divides according to the duration of their state of attachment to a host, distinguishing (1) those which are free all their life (leeches, bugs, fleas, &c. ); (2) those free as adults but parasitic when young (Ichneumon, Mermis, &c.) ; (3) those free only in youth, and attaining their adult form either directly in the first host entered, or only after a migration from one host to another (most parasitic worms) ; (4) those which pass all phases of their life on or in their host, e.g., Slrepsiplcra, Tristomum, &c. In this classification there is no attempt to define the degree of dependence or the closeness of the association, except in the general distinction between parasites and commensals ; the group of mutualists is entirely superfluous and confused, no clear definition being given, and in the examples of the various groups the limits of his own defini tions are not adhered to. Leuckart distinguishes parasites as ecto- and endo-parasitic, and divides the former into temporary and permanent. Endoparasites he divides according to the nature and duration of their strictly parasitic life. (1) Some have free-living and self-supporting embryos which become sexually mature either in their freedom or only after assuming the parasitic habit. (2) Others have embryos which, without having a strictly free life, yet pass through a period of active or passive wandering, living for a while in an intermediate host. They may either (a) escape to pass their adult life in freedom (Archigctes and Aspidogaster}, or (ft) they may become sexual, or (r) they may bore their way to another part of the body ( Trichina), or (d) most frequently they pass to their final host either directly when their intermediate host is devoured as food, or indirectly seeking for themselves another intermediate host, or producing asexual forms which do so (Trematodes and Cestoids), (3) Others again have no free-living or even migratory embryonic stage, but pass through their complete life-cycle in one host (Trichocephalus, Oxyuris, &c. ). This somewhat detailed classification has at least the advantage of clearness, and of showing to some extent the various degrees of parasitism. Kossmann has proposed a more physiological classification dealing with the organization and habit of the parasite. This he has applied to the Crustacea : I. Biosnwlici, or vegetative without independent digestive organs, e.g., Rhizoccphala ; II. Digcstorii, with independent digestive system, and including (1) Scdentarii, Copepoda atcktmcta, Sopyridse, Euboniscidee, Crypton iscidas; (2) Vagantes, Copepoda holotmcta, Branchiura, Cymothoidte. The great variety of details, however, makes it almost impossible to establish any logically accurate division. Any strict classification of such a variety of organisms having only in common the physiological corre spondence of their mode of life is almost impossible, and the most that can be done is to point out the existence of series of adapta tions varying with the intimacy and constancy of the association and the degree of dependence. Origin of Parasitism and Transmission of Parasites. With the dismissal of the theory of generatio squivoca, the question of the origin of parasites is limited to the discus sion of the causes which might induce such a change of habit and environment. There are obviously many oppor tunities for one animal either in adult or larval state being swallowed by another in food or drink, in which case, if the environment were not too utterly different from that previously enjoyed, parasitism might arise in a purely unconscious way. It is again easily conceivable that animals which have sought a host for temporary protection from climate or enemies, or for safety and seclusion in the bearing and breeding of the young, might, finding the environment congenial and a supply of food at hand, remain there during a large portion of their life. It is worth noticing, as corroboratory of the idea that the host was in many cases resorted to primarily as a sort of mater nity asylum, that we find many parasitic females with free males, e.g., Nicothoe. Given an animal with a carnivorous habit, it is intelligible enough that during a period of
