PARASITIC ANIMALS 



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' To this category belong almost exclusively ecto- 

 parasites, which differ from tlieir free-living rela- 

 tions only in their diet.' (2) Ectoparasitism an 

 established and invariable habit during a prolonged 

 period or during the whole of the parasite's life. 

 This is called ' stationary ' ectoparasitism in the 

 translation of Leuckart's great work, but the term 

 hardly suggests the idea. ' These parasites either 

 pass through all their developmental stages on the 

 host, or at first lead an independent existence under 

 a form more or less different from that of the adult.' 

 (3) Endoparagittsm, in which the parasites are dur- 

 ing a great part of their life, and almost invari- 

 ably during tlieir maturity, ' boarders ' within the 

 body of their host. Moreover, all the developmental 

 stages are almost never passed through within one 

 host, a transference from one kind to another being 

 necessary for the completion of the life-cycle. But 

 of endoparasitism there are many varieties. 



Tlie Hosts of Parasites, Probably no animals, 

 except some of the simplest, are free from the 

 attacks of parasites. Yet some are more liable 

 than others e.g. because they oiler greater in- 

 ducements to those parasitically inclined, because 

 they are more accessible, or because they 

 eat infected food. Thus, vertebrates are more 

 abundantly infested than invertebrates. ' Man lias 

 more than fifty distinct species of parasites, the 

 dog and the ox some two dozen each, the frog 

 perhaps twenty." Some species of parasite are 

 limited to one kind of host; thus, the adults of 

 Bothriocephalus latus and Oxyuris vermicularis are 

 not known except in man, while Trichina spiralis 

 i- found in man, pig, rat, dog, cat, ox, &c., and 

 Distomitm hepatintm in many ruminants and other 

 ungulates, in rodents, in the kangaroo, anil in man. 

 The systems most infested are the most accessible 

 viz. the skin and the alimentary canal, by ecto- 

 and endo-parasites respectively ; but there are no 

 organs in which parasites may not be harboured. 



Origin of the Parasitic Habit, It is probable 

 that most cases of parasitism began gradually. 

 Animals found temporary shelter on or in others, 

 and the habit grew upon them. In some cases 

 it might begin fortuitously e.g. as the embryos or 

 adults wandered or were swallowed ; or it might 

 be a shift saving those which adopted it from 

 some presumed keenness in the struggle for exist- 

 ence ; or it might simply express a sluggish con- 

 stitution. In many cases, however, we can hardly 

 doubt that the habit began with the naturally 

 more sluggish females, prompted not by hunger, 

 but by the impulse to seek some conveniently shel- 

 tered place for the birth of the young. In fact, 

 there are not a few parasitic female Crustaceans 

 whose mates live freely. Of the evolution of 

 parasites from free-living ancestors the free stages 

 still included in the life-history of most, the close 

 relationships between some free and some parasitic 

 members of the same class e.g. Crustaceans and 

 Nematodes and the frequent occurrence of tem- 

 porary parasitism afford sufficient evidence. It is 

 also instructive to consider the three classes of 

 Plathelminthes Turbellarians ( Planarians, c.), 

 Trematodes (Flukes, &c.), and Cestodes (Tape- 

 worms, &c.) of whose genetic relationship there 

 seems little doubt; the Turbellarians are almost all 

 free-living ; the Trematodes are mostly external, 

 1. 111 sometimes internal parasites ; the Cestodes are 

 all endoparasitic. 



Life-history of Endoparasites. Most endopara- 

 sitic animals have an eventful life-history. They 

 are not always parasites, or they are not always 

 parasite:) within the same kind of host. Most of 

 them are at some time free ; many of them have 

 some sort of metamorphosis. Hut, as their life- 

 histories are very various, they do not readily admit 

 of being summed up in general statements. 



Let us begin, however, with the adult sexual 

 animals. In this state they are always almost para- 

 sitic, partly because rich copious diet, warmth, and 

 relative quiescence favour reproductive maturity ; 

 partly because many probably began their parasitic 

 career at the reproductive period, when shelter and 

 readily attained food were specially advantageous ; 

 partly because it is not likely that animals which 

 had become parasitic would relinquish this habit 

 in adult and mature life. In fact, with the excep- 

 tion of some thread- worms (Gordius, Mermis, &c. ) 

 and some few insects (ichneumon-flies, gad-flies, 

 &c. ) which are parasitic in their youth and free as 

 adults, it is generally true of parasitic animals 

 that the eggs are produced, fertilised, and deposited 

 in the parasitic stage. In regard to the repro- 

 duction it should be noted ( 1 ) that the fertility is 

 often enormous, for a tapeworm may produce 

 42,000,000 eggs, and a female thread- worm 64,000,000 

 in a year ; (2) that in those cases where the female 

 alone is parasitic fertilisation may take place 

 before parasitism has begun ; that otherwise it 

 occurs within the body of the host ; that Trema- 

 todes and Cestodes are hermaphrodite and some- 

 times fertilise themselves; (3) that in tapeworms 

 the fixed ' head ' buds off a long chain of joints, 

 each of which is sexually complete, becomes even- 

 tually distended with eggs and embryos, and is 

 liberated singly or along with others from the 

 intestine of the host. 



The eggs or embryos of the parent endoparasite 

 usually pass from the host along with the excreta, 

 and ' there are no intestinal worms, at least among 

 the typical and constant parasites, whose embryos 

 come to maturity near the parent ; or, in oilier 

 words, there are none which pass their whole life- 

 cycle in one locality.' Some of the embryos are 

 locomotor e.g. those of the liver-fluke and of 

 Hothriocephalus latus, which are active migra- 

 tory ; others are passively carried along with food 

 and drink into new hosts. There the embryos 

 rarely become or remain quiescent, but wander 

 from the food-canal through the tissues and organs 

 of the host until a fit resting-place is found. But, 

 to state another of Leuckart's general conclusions, 

 ' the quiescent stage following upon the wandering 

 embryonic stage does not conclude the life-history 

 of the parasite, which requires, in order to complete 

 its development, a radical change in its environ- 

 ment in other words, a second migration.' 



But before leaving the so-called intermediate 

 host which is different from that of the parent or 

 that of the adult we should notice that within it 

 asexual multiplication may occur. Thus, several 

 asexual generations characterise that part of a 

 liver-fluke's life during which it sojourns in a 

 water-snail (Limna>us) prior to reaching its final 

 or ' definitive ' host in the sheep. In other cases, 

 the asexual multiplication within the intermediate 

 host is of a simpler kind, being restricted to bud- 

 ding, as when the bladder-worm or proscolex of 

 T<rnia echinococcus within ox or man develops 

 many ' heads ' or scolices, each of which on being 

 transferred to dog or wolf will grow into a tape- 

 worm. Or there may be no true multiplication 

 e.g. in the numerous bladder- worms which form only 

 one head, and remain quiescent until the host 

 happens to be devoured by another, within which 

 the ' head ' of the bladder-worm may bud off an 

 adult tapeworm chain. 



Connected with this change of host there are 

 two main problems : ( 1 ) How is the change effected? 

 (2) how did this extension of the life-history to two 

 distinct hosts arise? In regard to the modes of 

 transference it will be enough to give two illustra- 

 tions. The young liver-fluke actively migrates 

 from a water-snail and from the water, encysts 

 on stems of grass, and is then eaten by a sheep. 



