CHAPTER VIII 



THE PHYLUM NEMATODA 



Unsegmented worms, with an elongated body pointed at both ends; 

 ectoderm represented by a thin sheet of non-cellular hypodermis, con- 

 centrated to form two lateral lines and to a less degree dorsal and ventral 

 midlines, secreting an elastic cuticle, made of protein, not chitin, 

 usually moulted four times in the life of the individual ; cilia absent 

 from both external and internal surfaces; a single layer of muscle 

 cells underneath the hypodermis, divided into four quadrants, each 

 muscle cell being elongated in the same direction as the body and 

 composed of a peripheral portion of contractile protoplasm and a 

 larger internal core of unmodified protoplasm which sends a process 

 to the nerves ; the space between the body wall and the gut sometimes 

 filled by a small number of highly vacuolated cells, the vacuoles 

 joining together and simulating a perivisceral cavity; excretory 

 system consisting of two intracellular tubes running in the lateral 

 lines; nervous system made up of a number of nerve cells rather 

 diffusely arranged but forming a circumpharyngeal ring and a number 

 of longitudinal cords of which the mid-dorsal and mid-ventral are the 

 most important; sense organs of the simplest type; sexes usually 

 separate, gonads tubular, continuous with ducts, the female organs 

 usually paired, uniting to open to the exterior by a ventral vulva, the 

 male organ single, opening into the hind gut, thus forming a cloaca, 

 in a diverticulum of which lie the copulatory spicules ; spermatozoa 

 rounded and amoeboid, fertilization internal; alimentary canal 

 straight and composed of two ectodermal parts, the suctorial fore gut 

 and the hind gut and an endodermal mid gut without glands or 

 muscles; segmentation of egg complete, development direct, larvae 

 only differing slightly from adult. 



The nematodes occupy an isolated position, and in the opinion of 

 the writer are best considered as occupying a position like that of the 

 rotifers, not far removed from the Platyhelminthes, and representing 

 a precoelomate stage of organization. One of their peculiar features 

 is certainly secondary, namely the absence of cilia. There are in some 

 nematodes cilium-like processes to the internal border of the endo- 

 derm cells; in one case active movement has been reported. Nearly 

 all the other characters may be called primitive. The simplicity of 

 organization, the absence of segmentation 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 



