CHAPTER XV 



NEMATODA 



THE Nematoda, often called Thread-worms or Eound-worms, are a 

 group in which there is wide diversity of habitat and life-history, 

 but most remarkable uniformity in adult structure. The vast 

 majority are parasitic during some period of their existence ; it is 

 indeed by no means certain that even among the so-called free-living 

 Nematodes many have not a parasitic stage in their life -history, 

 because this history is very far from being completely known. 

 Though they infest the most diverse animals and plants, the 

 structure of the young worm, before the genital organs are developed, 

 is remarkably constant throughout the group ; and so is the embry- 

 onic development so far as it is known. Owing to the fact that the 

 eggs and embryos are very minute, the method of sections has been 

 hardly at all applied to the study of the group ; most observations 

 have been made on the living embryo seen through the semi- 

 transparent egg-shell, whilst others have been made on embryos 

 fixed, stained, and mounted whole. 



In spite, therefore, of numerous investigations, much remains 

 obscure in the embryology of these worms. Indeed it must be said 

 that most of these investigations have had for their object the solution 

 of general problems, such as the relative importance of cytoplasm 

 and nucleus in heredity, the relationship of genital and somatic cells, 

 the nature of the polar bodies, etc., rather than the elucidation of 

 the special course which development takes in this group, and the 

 light which it throws on the affinities of the group. These words 

 are not said to disparage the painfully laborious investigations 

 which have already been made, but to indicate that there is still a 

 promising field left for future investigators. 



ASCARIS MEGALOCEPHALA 



The form whose study lias so far yielded the most satisfactory 

 results is Ascaris megalocephala, a large ISTematode, about 9 or 10 

 inches long, which inhabits the intestine of the horse and causes no 



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