PARASITIC ANIMALS 



757 



Here, and in some other cases, the migration is in 

 part active. On the other hand, the bladder-worm 

 of the pig lies quite passive in the muscles or con- 

 nective tissue of that animal, and cannot reach its 

 final host unless ' measly ' pork be eaten by man. 

 Here, and in most other cases, the migration is 

 passive. The second problem is very difficult. Is 

 the host in which the adult is found the primitive 

 host, and has that of the immature stages been 

 intercalated ? or is the intermediate host really the 

 primitive one in which the animals used to become 

 mature, while the final host represents a secondary 

 prolongation of the life-history? Leuckart ex- 

 presses himself unconditionally in favour of the 

 second theory that ' the intermediate hosts were 

 originally the true definitive carriers, which for- 

 merly brought their intestinal worms to sexual 

 maturity, but have since become merely interme- 

 diate, because the development of the parasites 

 has extended itself over a greater number of stages 

 in the course of further differentiation. ' 



The Environment of Parasites. It is at present 

 debated (see HEREDITY) whether the precise in- 

 fluences exercised on parasites by their hosts are 

 transmissible or not. But it can hardly be doubted 

 that the habits and surroundings of parasites have 

 been somehow influential in their evolution. It is 

 certain that individual parasites may vary in 

 different parts of the body and in different hosts, 

 and it is admitted by all that parasites exhibit 

 ' adaptations ' to their life and surroundings. It is 

 therefore important to take account of the precise 

 relations between host and parasite. Ectoparasites 

 will experience mechanical influences due to the 

 movements of their bearers, they will often be 

 carried from one locality to another, they will 

 sometimes share in the warmth of their hosts, they 

 usually find abundant food, and they are often not 

 only sheltered but sedentary. Endoparasites will 

 experience pressure from the tissues in which they 

 lie, or from the peristaltic movements of the food- 

 canal in which they are lodged ; their immediate 

 environment usually involves confined space, scant 

 oxygen, considerable warmth, and total darkness ; 

 they will be affected by abundant and rich nutri- 

 tion, by surrounding gases and juices, and by their 

 frequently sedentary life. Now it is at least a 

 plausible theory that the usual alienee of sense- 

 organs in endoparasites is due to the unstimulating 

 character of the environment, which has caused 

 them to degenerate, and this view is partly con- 

 firmed by the occasional occurrence of sense-organs 

 in the larvre alone, and by the facts that locomotor 

 appendages are absent or much reduced in the 

 adults of many fixed ectoparasitic crustaceans, 

 because they have gone out of use ; that a food- 

 canal is absent in many endoparasites, partly 

 because the superficial absorption of complex sur- 

 rounding juices left it functionless ; that the pas- 

 sivity o? many is increased by living in surround- 

 ings in which the respiratory processes must be 

 very sluggish ; or that the prolific reproduction 

 especially perhaps the budding growth of tape- 

 worms is in part due to the abundant and yet 

 stimulating nutrition. 



E/eetf of Parasites on their Hosts. In the 17th 

 and 18th centuries the injurious effects of parasites 

 were much exaggerated. All sorts of diseases, 

 including many which we now know to be asso- 

 ciated with Bacteria, were said to be due to 'worms,' 

 and physicians gravely discussed 'An mors natii- 

 ralu tit stihstantia verminosa ? ' As accurate diag- 

 nosis began to be less unusual, a strange reaction 

 in favour of parasites found many supporters. In- 

 testinal worms were called ' the good angels and 

 unfailing helpers of children," and were said to aid 

 digestion and even development. But since the 

 middle of the 19th century, when the experimental 



study of parasites began in earnest, a knowledge of 

 the various injuries which parasites may do, to 

 man and to domesticated animals at least, has 

 become more and more precise and complete. Only 

 a few illustrations need be given. Numerous large 

 parasites will certainly diminish the nutritive 

 supplies of their host ; large bladder- worms and the 

 like press upon adjacent organs, cause obstructions, 

 and give rise to many troubles ; the movements 

 and migrations of parasites within the body of their 

 host produce pain and inflammation, and may even 

 result in the perforation and destruction of im- 

 portant organs. Even external parasites may do 

 considerable damage ; witness those crustaceans 

 which occur beneath the tails of crabs, and some- 

 times effect the virtual castration of their hosts. 

 On the other hand, there are many less important 

 parasites whose effects are very slightly if at all 

 injurious. It is a question of much practical im- 

 portance how the endoparasites which infest man 

 find their way to their host, but as details will 

 be given in such articles as TAPEWORM and 

 TRICHINA, it is enough here to say that food in 

 which parasites are known to lurk should be in- 

 spected, cleaned, and sufficiently cooked. 



Historical. Most of the ancient and mediaeval 

 naturalists and physicians who expressed any 

 opinion on such matters believed that parasites 

 were spontaneously generated within the bodies 

 of their hosts. It was not till the 17th century, 

 when Swammerdam and Kedi showed how maggots, 

 lice, &c. developed from eggs, that the belief in 



?eneratio cequivoca began to be seriously disputed, 

 t was gradually replaced by the theory that para- 

 sites came from without, that, ceasing to be free- 

 living, they entered the bodies of other animals and 

 were there modified. But this conclusion was; 

 too hastily leaped at, and no care was taken to. 

 prove that the free-living forms in question didi 

 really develop into parasites. In many cases,, 

 indeed, it was soon shown that they did not, and' 

 this disappointing result helped Pallas and others- 

 in the latter part of the 18th century to recognise- 

 rightly that parasites were propagated like othen 

 animals by means of eggs. They concluded,, 

 however, that these eggs were more or less< 

 directly carried from one host to another, there 

 to develop into the original form, while we know 

 that the life-history of parasites is rarely so simple ; 

 nor was there more than a slight warrant for 

 another favourite idea that young animals inherited 

 parasites from their mothers. At the beginning of 

 the 19th century the helminthologists, such as 

 liudolphi and Bremser, were very active and greatly 

 extended the list of known parasites, but the life- 

 histories remained a puzzle, and many naturalists 

 relapsed into a belief in spontaneous generation. 

 The increasing use of the microscope led to most 

 important results: in 1831 Mehlis discovered the 

 Inrusorian-like embryo of certain flukes; Von 

 Siebold (1832) detected the six-hooked embryo 

 within the still unliberated ova of the tapeworm ; 

 Eschricht (1841) compared the life-history of in- 

 ternal parasites to that of ichneumon-flies and bot- 

 flies ; Steenstrup ( 1842) published his famous essay 

 on alternation of generations; Von Siebold (1843- 

 50) and Van Beneden (1849-50) worked out the 

 metamorphoses of several parasitic worms; Kiichen- 

 meister ( 1853), Leuckart ( 1856), and others showed 

 experimentally how infection with larval stages 

 resulted in the development of adult parasites. 

 The foundations of modern helminthology were 

 thus laid, and we have now a vastly increased 

 knowledge of the number of parasites, a precise 

 acquaintance with the life-history and migrations 

 of some of the most important, a scientific system 

 of medical diagnosis and treatment, and some 

 realisation of the general biology of parasitism. 



