NEMATODA. 53 



usually from a few lines to two or three inches in length ; how- 

 ever, there are some exceptionally large individuals whose lon- 

 gitudinal admeasurement extends to several feet. Cogent examples 

 of this kind may be instanced in the great, red-coloured, kidney- 

 worm (Eiostrongylus gigas) , in the large round- worm of the horse 

 (Ascaris megalocephala) , and in the guinea- worm (Dracunculus 

 medinensis) of tropical countries. 



Habits. — Most of the Nematoda are parasitic only during a 

 portion of their life-time, though it must be allowed, as a rule, 

 that the period of their non-parasitic existence is comparatively 

 limited. Some few are altogether non-parasitic. In the case 

 of those round-worms which expel their ova in a very imperfectly 

 developed condition, a long immersion of the eggs in water 

 appears to be necessary for the development of the embryo. In 

 such cases the time of their non-parasitic condition will vary from 

 six to twelve months, or even more. After this period, if they 

 are not soon passively transferred to their proper hosts, they will, 

 in aU likelihood, rapidly perish. Even in the case of the armed 

 embryos of Mermis, the continuance of life is probably dependant on 

 the success which attends their efforts to bore their way into the 

 bodies of the hosts — grubs and caterpillars — which are suitable 

 to them. In the Nematodes, therefore, larval migration is necessary 

 for the welfare and continuation of the species, and this " wander- 

 iQg," as it is called, may be accomphshed either in an active or 

 in a passive manner. 



Distribution.— 'Nematodes infest abundantly the human body; 

 they are scarcely less numerous in the higher apes and monkeys, 

 being also tolerably frequent in the bats. Almost always present m 

 the carnivora, we find them especially common in cats, dogs, and 

 weasels. No less than six species, referable to four distinct 

 genera, are described as infesting the hedgehog, not a few also 

 occurring in the allied moles and shrews. In rats, mice, squirrels, 

 rabbits, hares, and other rodents, they are equally abundant. Some 

 few species have been found in the edentates, but they are 



