NEMATODA 245 



taken into account, are seen to resemble those of the Platyhelminthes. 

 Nearly all the other characters may be called primitive. The sim- 

 plicity of organization, the absence of segmentation at all stages and 

 a vascular system, the diffuse nature of the nervous system and the 

 structure of the muscle cells are all signs of a lowly origin. But it is 

 still maintained by some that these features are not primitive but 

 degenerate and that the origin of the phylum is to be sought in the 

 arthropods, probably in the parasitic forms of that group (the 

 degenerate arachnids called linguatulids). If this view is taken it must 

 be supposed that the parasitic nematodes are the most primitive 

 members of the phylum and that some of their descendants became 

 less and less parasitic, until entirely free-living forms came into 

 existence. This would be an extraordinary reversal of evolution for 

 assuming which at present there are no grounds. 



The view taken in this book is that the free-living nematodes are 

 ancestral to the parasitic forms and that there is no real connection 

 between the arthropods and the nematodes. Not only do the 

 nematodes present no indications of segments or appendages at any 

 point of the life history but also the cuticle is of an entirely different 

 chemical composition in the two phyla, and the loss of cilia most 

 likely a phylogenetically recent phenomenon in the nematodes as in 

 the parasitic platyhelminthes. 



The anatomy of the nematodes is best known from the study of 

 Ascarts which is one of the largest members of the group and the only 

 one adapted for dissection in class. Full accounts of this form are 

 given elsewhere, but the following points must be emphasized. In 

 Ascarts (Fig. 173) there appears to be a wide space between the 

 muscle layer and the endoderm cells, with no epithelial boundary, 

 walls, but on closer examination it is seen to be occupied by a very 

 small number of greatly vacuolated cells, and what appears to be a 

 continuous cavity is really the confluent vacuoles of adjacent cells, 

 and so the term "intracellular" may be applied to it. This arrange- 

 ment has not been verified in many other nematodes but connective 

 tissue cells can usually be demonstrated in the space. They may be 

 phagocytic; the enormous branched cells of Ascarts (Fig. i75)> 

 lying on the lateral lines, take up in their tiny corpuscle-like divisions 

 such substances as carmine and indigo which are injected into the 

 body. 



A striking feature of the histology of Ascarts is the presence of 

 greatly enlarged cells. Not only do the body cavity cells show this, 

 but in the excretory system the greater part of the canal is contained 

 in the body of one cell which divides into two limbs each running the 

 whole length of the body on opposite sides. 



As a simple type of nematode the genus Rhabditis (Fig. 174) will 



