192 UNSEGMENTED "WORMS" 



living for at least a part of the life-cycle, and feed on putrefying organic 

 matter. Again, although the number of individuals which may infest 

 one host shows how successful the parasitism is, yet Nematodes exhibit 

 few of the ordinary adaptations to a parasitic life, and there is no 

 sharp structural line of demarcation between free and parasitic forms. 

 Among histological peculiarities, the absence of cilia paralleled else- 

 where only among the Arthropods the nature of the muscle cells, the 

 condition of the sub-cuticular layer, are to be noticed. Among the 

 grosser structural peculiarities, the nature of the excretory system, of 

 the body cavity, and of the nervous system, are worthy of special note. 

 Sense organs are never well developed, but in the free-living forms 

 simple eyes may occur. The alimentary canal is usually completely 

 developed, but may, as in Sphicmlaria, be degenerate. Of the rela- 

 tionships nothing is known. 



LIFE HISTORIES. 



1. The embryo grows directly into the adult, and both live in fresh 



or salt water, damp earth, and rotting plants Enoplida?, e.g. 

 Enoplus. 



2. The larvae are free in the earth, the sexual adults are parasitic in 



plants, or in Vertebrate animals, e.g. Tylenchus scandcns, a 

 common parasite on cereals ; Strongyhts and Dochiniits in 

 man. 



3. The sexual adults are free, the larvre are parasitic in insects, 



e.g. Mermis. The fertilised females of Sphcertilaria boinbi 

 pass from the earth into the body cavity of humble-bee and 

 wasp, whence their larvre bore into the intestine and eventually 

 emerge. 



4. The larvae are parastic in one animal, the sexual adults in another 



which feeds on the first. Thus OUnlanus passes from mouse to 

 cat, Cucullamis from Cyclops to perch. 



There are other life histories, and many degrees of parasitism. The 

 most remarkable form is Angiostomum (or Ascaris or Leptodera) 

 nigrovcnosiini. In damp earth males and females occur, the progeny of 

 which pass into the lungs of frogs and toads. There they mature into 

 hermaphrodite animals (the only example among Nematodes), which 

 produce first spermatozoa and then ova. They are self-impregnating, 

 and the young pass out into the earth as males or females. Here there 

 is alternation of generations : and a somewhat similar story might be 

 told of RJiabdo)ieina strongyloides from the intestine of man, and 

 Leptodera appendicnlala from the snail. 



There are several quaint reproductive abnormalities, thus the female 

 Splurnilaria bouibi, which gets into the body cavity of the humble- 

 bee, has a prolapsed uterus, larger than itself; the male of Trichodes 

 crassicatida passes into the uterus of the female. 



