WORMS 179 



Roundworms (Nematoda) 



The roundwoi-ms, as we have ahxady noted, are placed in a separate 

 phylum Nemathelminthes. Although in the popular sense they are 

 quite obviously "worms" a man has more in common with a snake than 

 a roundworm has with a flatworm. Large numbers of nematodes are 

 free-living and are to be found teeming in the soil and water. Their 

 morphology is generalised and rather unspecialised — a fact which has 

 puzzled a great many biologists. Some have concluded that all free- 

 living nematodes are derived from parasitic forms, while others see in 

 the relative simplicity of their anatomy a pre-adaptation to the parasitic 

 mode of life. A great deal of confusion exists, however, in the minds of 

 various writers on the definition of adaptation and modification, as the 

 two following quotations, taken from the works of two leading authori- 

 ties and both published in 1946, will show. One writes : " The majority 

 of the parasitic forms are relatively giants and are often much modified 

 by their parasitic life." The other writes : " The nematodes, on 

 account of their simplified anatomy, appear to have escaped the effects 

 of parasitism." 



Roundworms are cylindrical, generally tapering to a point at both 

 ends. They have a well-developed intestine, a body cavity and— with 

 few exceptions— the sexes are separate. The females are generally 

 larger than the males and the latter have differently formed tails, often 

 with a saucy curl at the tip. Occasionally, there is a marked sexual 

 dimorphism. The females of the blood red spirurid Tetrameres found in 

 the proventriculus of many wild birds are almost globular, whereas the 

 males retain the typical cylindrical shape. One male nematode 

 {Trichosomoides) which is parasitic in the urinary bladder of rats lives 

 a life of ease inside the vagina or uterus of its own female. The cuticle, 

 although transparent, is tough and apparently impermeable— in many 

 cases reminiscent of the cuticle of arthropods, although it is not 

 chitinous. This cuticle is sometimes expanded into fin-shaped flaps, 

 which are useful for purposes of classification. 



The life-cycle of the nematode is simple compared with that of the 

 fluke or tapeworm. Although, between the egg and the adult worm, 

 there are four moults and the successive larval stages may diff'er in 

 minor structural details, there is no alteration of distinctive larval 

 generations or asexual multiplications either by budding or poly- 

 embryony. Their tgg production is, however, higher than many of the 



FFC— N 



