68o 



VERMES CLASS NEMA TOHELMINTHES. 



tissue of man in the tropical countries of the Old World, measuring, when 

 adult, many inches in length. It is usually extracted by being carefully 

 wound round a small roller, an operation which often occupies several days, 

 owing to the precautions that are taken not to fracture the animal. But if 

 allowed to remain beneath the skin, the worm effects its escape to the outer 

 world through the abscess to which its presence gives rise. Then the young 

 with which the females are laden make their way into fresh water streams or 

 lakes, and falling in with minute water-fleas of the family Cyclopidye, bore 

 their way into the interior of these crustaceans, which are swallowed by 

 mankind in connection with drinking-water ; and so, by means of an inter- 

 mediate host, the young Guinea-worms are introduced into their final place of 

 abode. A similar transference of parasitic worms from one host to another is 

 of common occurrence in this, as in other, groups of entozoic vermes, and we 

 meet with it again in another thread-worm ( FUaria sanguinis-hominis) which, 

 in various parts of the Old World, infests the blood system of man, giving 

 rise, by the stoppage of the circulation in the small vessels, to the disease 

 known as elephantiasis. In this case the embryos are sucked from the 

 vascular system by mosquitoes or other suctorial diptera, and so become 

 scattered abroad, making their way in all probability back again into the 

 human body along with drinking water. 



To the genera Gordius and Mermis belong the horse- hair worms, so named 

 on account of their length, slenderness, and colour, which recall the form of 

 a long hair from a horse's mane or tail. In Gordius, which is found in fresh 

 water streams and ponds, the alimentary canal is functionless and the mouth 



closed. The eggs, deposited in clusters 

 and glued together by an adhesive sub- 

 stance, sink to the bottom of the water, 

 and there remain until the hatching of 

 the young. These are armed in front with 

 a sharp proboscis and circles of hooks, by 

 means of which they bore their way into 

 the bodies of aquatic insect larvee, such 

 as gnats and mayflies, and there remain 

 encysted until the host is devoured by a 

 fish or carnivorous insect. Thus intro- 

 duced into the alimentary canal of a 

 fresh host, the young Gordius subse- 

 quently penetrate into the body-cavity, 

 and undergo the semi-final stages of their 

 development, acquire the form charac- 

 teristic of the adult, and then making 

 their escape into the water become fully 

 adult (see Fig. 8, 6). The other hair-worm 

 known as Mermis lives in damp earth. 

 Here the eggs are laid, and the young 

 penetrate the skin and take up their 

 abode in the bodies of caterpillars or other insects, whence, after a prolonged 

 residence, they escape again to the ground, become mature, and lay their 

 eggs. 



The descriptions that have hitherto been given of the life-histories of the 

 parasitic thread-worms deal with species which lead a free existence, at all 

 events during a part of life. But the last of the species that we have 



Fig. 8. THREAD-WORMS. 



a, Trichinosis-worm (Trichina spiralis) 



encysted in muscle. 



b, Horse-hair worm (Gordius) escaping 



from an insect (Mantis). 



