PARASITIC ADAPTATION Morphology.—An organism adapted to the parasitic mode of life often undergoes such a great modification that it becomes difficult to recognize the group to which it belongs. A number of forms such as Sacculina, Peltogaster, Portunion or Xenocoeloma (in their parasitic stages of development) could hardly be recog nized as belonging to various families of Crustacea, if it were not for the free-living larval stages showing the essential characters of the corresponding families. The same applies to various marine snails (Gastrosiphon, Entoconcha and Enterexenos) living as parasites in starfishes (echinoderms). Parasites have the tendency to lose their organs of locomotion, such as wings, legs and other appendages. The skin, in endoparasitic forms, being already pro tected by the tissues of the host, becomes thinner and the articula tions between the segments gradually disappear. Sensory organs become atrophied. The alimentary canal in some cases becomes atrophied (Guinea worm), or it may disappear completely (tape worm), and the parasite then feeds through the whole surface of its skin (tapeworm) or by means of absorbing appendages (Mon strillidae), or a ramified root system (Sacculina).
Hermaphroditism.—This is also a condition frequently met with in parasites. This may be explained partly by the more or less fixed position of the parasite, which greatly reduces the chances of meeting between individuals of different sexes. On the other hand, when the sexes are separate the sexual dimorphism is extremely marked, the female being usually much more developed than the male (except in the Bilharzial parasite), and both sexes show a tendency to live in close association. Thus, among the parasitic Crustacea the males are attached to the females. In a parasitic worm (Syngamus trachealis) which causes gapes in fowl, the body of the male is permanently fixed and almost fused to that of the female. In Schistosomes, which cause human Bil harziasis, the female is carried in a tube formed by the lateral inrolling of the body of the male.
Fertility.—The life of a parasite with a more or less restricted host specificity, and which passes during its development through one or two intermediate hosts, is dominated by one main factor, namely the great losses of its progeny. In fact, an enormous percentage of its eggs, scattered haphazard, never find the right conditions for development ; the greater proportion of the larvae which hatch never reach the host, and die in various stages of development ; and among those which reach the right host, many are lost during the internal migration and never reach maturity.
Of a parasite, we can say that it is not death, but life, which is a marvellous accident of nature. The enormous losses sustained by parasites during the successive stages of their development are compensated by various means. One of them is their greatly in creased fertility. While various internal organs such as the ali mentary canal, nervous system and circulatory system, are more or less atrophied, the genital organs, on the contrary, and especially those of the female, increase in size, proliferate and become very active. It was calculated that the round worm (Ascaris lumbri coides), parasitic in man, produces 64 million eggs a year; while in a tapeworm (Taenia solium) 8o million eggs are formed every year. The genital organs in some parasites fill the whole of the body cavity, and in a small round worm (Sphaerularia bombi) parasitic in the bumble bee, the gravid uterus and the vagina gradually protrude outside the body of the parasite, increase in size, and finally form an enormous sac to which the shrivelled body of the female is attached as a hardly-perceptible appendage.
Asexual Reproduction.—Asexual multiplication, which takes the form of fission, budding, or parthenogenesis, occurs usually in the young or larval stages of the parasite, especially when it infects the secondary or intermediate host. This kind of multiplication occurs in a great number, but not in all parasites. It is very com mon in Protozoa, Orthonectids, tapeworm and flukes; and it occurs also in parasitic Crustacea (Thompsonia). In insects the asexual multiplication assumes the form of polyembryony, and occurs in a few species of Hymenoptera parasitic in other insects. Here the female of a parasitic Hymenopteron (Encyrtus) lays only one egg inside the egg of the apple moth (Hyponomeuta), but during the development of the latter, the egg of the parasite divides into many cells which give rise to several embryos.