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Rh that a condition is brought about in which, after many generations, the host becomes “tolerant” of the parasite, and the parasite is not lethal to the host, though perhaps capable of setting up considerable disturbance in its vital functions. Many animals are found to contain almost constantly certain internal parasites without being, apparently, in the least affected by them; and it should be borne in mind that in most cases it is not to the interest of the parasite to destroy the host or to overtax its resources. But when the parasite is transferred naturally or artificially to a species or race of host which does not ordinarily harbour it, and which therefore has not acquired powers of resisting its attacks, the parasites may be most deadly in their effects. Thus the white traveller in the tropics is exposed to far greater dangers from the indigenous disease-producing organisms than are the natives of those climes.

In some cases two organisms have become mutually adapted to each other as host and parasite to such an extent that the parasite is not capable of flourishing in any other host. An instance of this is Trypanosoma lewisi of the rat, which cannot live in any other species of animal but a rat, and which is not as a rule lethal to a rat, at least not to one otherwise healthy. Contrasting in an instructive manner with this species is Trypanosoma brucii, which occurs as a natural parasite of buffaloes and other big game in Africa, and is, apparently, harmless to them, but which is capable of being transferred to other animals by inoculation. The transference may take place naturally, by the bite of a tsetse-fly, or may be effected artificially; in either case T. brucii is extremely lethal to certain animals, such as imported cattle, horses and dogs, or to rats and guinea-pigs. Other animals, however, may be quite “repellent” to this parasite, that is to say, if it be inoculated into their blood it dies out without producing ill effects, just as T. lewisi does when injected into an animal other than a rat. Thus it is seen that T. brucii, when introduced into the blood of an animal which is specifically or racially distinct from its natural hosts in the region where it is indigenous, is either unable to maintain itself in its new host, or flourishes in it to such an extent as to be the cause of its death.

We may assume, therefore, at least as a working hypothesis, that a lethal parasite is one that is new to its host, and that a harmless parasite is one long established. Since all parasites must have been new to their proper hosts at some period, recent or remote, in the history of the species, it would follow that the first commencement of parasitism would be in almost all cases a life and death struggle, as it were, between the two organisms concerned, and it is quite conceivable that the host might succumb in the struggle and so be exterminated. Ray Lankester has suggested that the extinction of many species of animals in the past may have been due, in some cases, to their having been attacked by a species of parasite to which they did not succeed in becoming adapted, and by which they became, in consequence, exterminated entirely.

Organization of the Protozoa.—The body-form may be constant or inconstant in the Protozoa, according as the body-substance is or is not limited at the surface by a firm envelope or cuticle. When the surface of the protoplasm is naked, as in the common amoeba and allied organisms, the movements of the animal bring about continual changes of form. The protoplasm flows out at any point into processes termed pseudopodia, which are being continually retracted and formed anew. Such movements are known as amoeboid, and may be seen in the cells of Metazoa as well as in Protozoa. The pseudopodia serve both for locomotion and for the capture of food. If equally developed on all sides of the body, the animal as a whole remains stationary, but if formed more on one side than the other, the mass of the body shifts its position in that direction, but the movement of translation is generally slow. If the animal remains perfectly quiescent and inactive, the laws of surface-tension acting upon the semi-fluid protoplasmic body cause it to assume a simple spherical

form, which is also the type of body-form generally characteristic of Protozoa of floating habit (Radiolaria, Heliozoa, &c.).

In the majority of Protozoa, however, the protoplasm is limited at the surface by a firm membrane or cuticle, and in consequence the body has a definite form, which varies greatly in different species, according to the habit of life. As a general rule those forms that are fixed and sedentary in habit tend towards a radially symmetrical structure; those that are free-swimming approach to an ovoid form, with the longest axis of the body placed in the direction of movement; and those that creep upon a firm substratum have the lower side of the body flattened, so that dorsal and ventral surfaces can be distinguished; it is very rare, however, to find a bilaterally symmetrical type of body-structure amongst these organisms. In some cases the cuticle may be too thin to check completely the changes of form due to the movements of the underlying protoplasm; instances of this are seen amongst the so-called “metabolic” Flagellata, in which the body exhibits continually changes of form, termed by Lankester “euglenoid” movements, due to the activity of the superficial contractile layer of the body manifesting itself in ring-like contractions passing down the body in a manner similar to the peristaltic movements of the intestine.

The body-substance of the Protozoa is protoplasm, or, as it was originally termed by Dujardin, sarcode, which is finely alveolar in structure, the diameter of the alveoli varying generally between ½ and 1 μ. At the surface of the body the alveoli may take on a definite honeycomb-like arrangement, forming a special “alveolar layer” which in optical section appears radially striated. Besides the minute protoplasmic alveoli, the protoplasm often shows a coarse vacuolation throughout the whole or a part of its substance, giving the body a frothy structure. When such vacuoles are present they must be carefully distinguished from the contractile vacuoles and food-vacuoles described below; from the former they differ by their non-contractile nature, and from the latter by not containing food-substances.

In many Protozoa and especially in those forms in which there is no cuticle, the body may be supported by a skeleton. The material of the skeleton differs greatly in different cases, and may be wholly of an organic nature, or may be impregnated with, or almost entirely composed of, inorganic mineral salts, in which case the skeletal substance is usually either silica or carbonate of lime. From the morphological point of view the skeletons of Protozoa may be divided into two principal classes, according as they are formed internal to, or external to, the body in each case. Instances of internal skeletons are best seen in the spherical floating forms comprised in the orders Radiolaria and Heliozoa; such skeletons usually take the form of spicules, radiating from the centre to the circumference, and often further strengthened by the formation of tangential bars, producing by their union a lattice-work, which in species of relatively large size may be formed periodically at the surface as the animal grows so that the entire skeleton takes the form of concentric hollow spheres held together by radiating beams. The architectural types of these skeletons show, however, an almost infinite diversity, and cannot be summarized briefly. External skeletons have usually the form of a shell or house, into which the body can be retracted for protection, and from which the protoplasm can issue forth during the animal's phases of activity. Shells of this kind, which must be carefully distinguished from cuticles or other membranes that invest the body closely, are well seen in the order Foraminifera; in the simplest cases they are monaxon in architecture, that is to say, with one principal axis round which the shell is radially symmetrical, and at one pole is a large aperture through which the protoplasm can creep out. In addition to the principal aperture, the shell may or may not be pierced all over by numerous fine pores, through which also the protoplasm can pass out. For further details concerning these shells and their very numerous varieties of structure the reader is referred to the article.