312 ' ZOOLOGY sect. 



would shortly begin to migrate into the muscles, and produce the 

 various symptoms of " trichiniasis." 



It will be noted that in this case the parasite is able to exist in 

 various hosts, and that both sexual and asexual stages are passed 

 through in the same host, dispersal of the species taking place by 

 the flesh of an infected animal being eaten by another, either of 

 the same or of a different species. 



The female Guinea-worm (Filaria medinensis) attains a length 

 of 30-200 cm. (l-6ft.), and lives in the subcutaneous connective- 

 tissue of Man. The eggs develop in the uterus, and the new-born 

 young pass out of the body of the host through abscesses caused 

 by the presence of the parasite. If, as must often be the case, 

 they escape into water, they make their way into the body of a 

 Water- flea (Cyclops), which is the intermediate host, and in this 

 condition probably reach their human host once more in his 

 unfiltered drinking-water. Filaria bancrofti and other species are, 

 in the larval condition, parasites in the blood of man. The adult 

 females of F. lancrofti live normally in the lymphatic vessels. 

 They are viviparous, and the young when they escape reach the 

 blood and are thus distributed. Normally they are to be found in 

 the peripheral vessels only at night-time, when the superficial 

 vessels are more dilated and thus permit of their passage. They 

 are transmitted from one human host to another by the agency 

 of mosquitoes, which act as intermediate hosts. F. hancrofti is 

 very widely distributed in tropical countries, and is the cause of a 

 disease called filariasis, with a variety of symptoms — such as 

 anaemia, lymphatic tumours, elephantiasis. 



CLASS II,— ACANTHOCEPHALA. 



This class contains a number of genera of parasitic worms, of which 

 Echinorhynch-ws is the chief. The present section will be devoted to this genus, 

 a not uncommon parasite in the intestine of Mammals, Birds, Reptiles, 

 Amphibians, and Fishes. The largest species, H. (Gigantorhyiidius) giffccs, is 

 found in the Pig (Fig. 250, A), and has once been recorded in the human 

 subject : it may attain, in the female, a length of 50 cm., or more than half a 

 yard. Most species are small, not exceeding 1 cm. in length, 



Externa] Characters. — The body is cylindrical, and ends in front in a 

 protrusible portion, the proboscis (A, p., B, pr.), which is cylindrical and is 

 covered with many rows of recurved ohitinoid hooks. The worm lies with the 

 proboscis sunk in the wall of the intestine of its host, which is sometimes riddled 

 with holes formed in this way. In some species there is a distinct neck (B, n.) 

 between the proboscis and the trunk, and there may be a globular dilatation at 

 the anterior end of the neck. At the hinder end of the body is a single 

 aperture, the gonopore or reproductive aperture {f/np. ) : connected witli this, in 

 the male, is a protrusible, bell-like structure, the bursa (6.), which acts as a 

 copulatory organ, like the somewhat similar organ in certain Nematoda. There 

 is no trace of mouth, anus, or excretory pore. 



The body-wall is covered with a stout cuticle, beneath which is a striated 

 protoplasmic layer, probably representing the ectoderm. Then comes a layer of 



