NEMATODA 215 



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 which at present there are no 

 grounds for assuming. 



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 

 Ascaris 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 

 Ascaris (Fig. 166) 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 Ascaris (Fig. 168), 

 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 Ascaris 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. 167) 

 will be described, as it is seen alive as a transparent object under the 

 microscope. Most species are free-living. They are obtained by 

 allowing small pieces of meat to decay in moist earth. The larvae 

 which exist in an "encysted" condition in the soil are attracted by 

 the products of decay, and in a few days become sexually mature 



