CHAPTER VIII 



PHYLUM NEMATODA 



THE Nematoda include a very great number of species commonly 

 termed Thread-Worms, or from the shape of their cross section 

 Hound-Worms. Certain species attain a great length, as long as 

 a man, but more commonly they are small and insignificant and 

 often microscopic. The general shape of the body is cylindrical, 

 usually pointed slightly at each end; the surface is smooth with at 

 most a few bristles, so that they easily insinuate themselves into 

 the cracks in the damp earth, or between the tissues of the animals 

 and plants in which for the most part they live. 



As a rule, like most animals which pass their time in the dark, 

 Nematodes are white or whitish-yellow in colour. The body is 

 glistening and smooth, but not slimy. It is ensheathed in a thick 

 cuticle, which is in some cases ringed. No locomotor organs exist, 

 and the animals progress by wriggling, bending first to one side 

 and then to the other. The cuticle, which is moulted about four 

 times during the life of the animal, is secreted by an underlying 

 layer called the ectoderm, in which no trace of cell limits can be 

 detected, but nuclei are scattered in it. Along the middle dorsal 

 and middle ventral lines and along each side, this layer is thickened 

 and projects inward. The median ridges support two nerves, the 

 lateral ridges two canals, which are believed to be excretory. 



By these ectodermal thickenings the body- wall is divided into 

 quadrants. In each quadrant there is a layer of longitudinal 

 muscle-fibres of very peculiar appearance. In all muscle-fibres 

 there is a patch of unmodified protoplasm termed sarcoplasm sur- 

 rounding the nucleus, the rest being modified into those fibrils which 

 are the visible sign of a heightened contractile power. In the 

 Nematoda, in which all metabolism is at a low ebb, only the 



