54 ENTOZOA. 



much more prevalent in swine and their allies. In herbivorous 

 animals, generally, they are common, being remarkably numerous 

 in the horse, ass, ox, deer, and sheep. One or two species are 

 constantly present in the seal and porpoise respectively, but we 

 are very imperfectly informed as to their presence or absence in 

 the large cetacea. The Nematodes, as a group, are particularly 

 hable to mvade all kinds of birds, especially game and poultry. In 

 reptiles they are certainly less numerous ; but in fishes they 

 are almost as prevalent as in the feathered tribe. On the whole, 

 therefore, it may be affirmed that, in so far as reference is made 

 to the vertebrate host, the nematode species are more generally, 

 numerously, and uniformly distributed than the individual members 

 of other helminthic orders. 



Number and Arrangement. — It is more difficult to arrive at an 

 approximatively true estimate of the total number of nematode 

 species in existence, than it is to form a similar estimate in regard 

 to the other kinds of helminths. This arises first fi:'om the cir- 

 cumstance that the forms are undoubtedly more numerous and 

 more widely distributed ; secondly, because a multitude of juvenile 

 forms have been taken for adults ; and thirdly, because the re- 

 putedly distinct forms are less markedly definable than are those 

 belonging to other orders of parasites. Altogether, nearly 800 

 species have been either described or indicated, but at the very 

 lowest estimate I should think that not less than 250 of these so- 

 called species might be legitimately cast aside as spurious. This very 

 necessary deletion permits us to assume that the order of Nema- 

 todes, as at present known, comprises about 550 species. The 

 various forms, naturally grouped, may be classed in the following 

 manner : — 



AnguilluUdce (Family I.) 

 Gordiidce (Family II.) 

 Oxyuridce (Family III.) 

 Filaridce (Family IV.) 

 Nkmatoda. \ Cheiracanthidie (Family V.) 

 Ascuridcc (Family VI.) 

 Stronyylidcc (F'amily VII.) 

 C'liadlunidcc (Family VIII.) 



