CHAPTER VIII 



THE NEMATODA, NEMATOMORPHA 

 AND ACANTHOCEPHALA 



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 a nerve ; 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 diff^usely 

 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 and bilateral in type, develop- 

 ment direct, larvae only differing slightly from adult. 



The nematodes appear to occupy an isolated position, but many of 

 their characters, though more specialized, resemble those of the 

 Platyhelminthes and Rotifera. They are certainly closely related to the 

 Acanthocephala, Gastrotricha, and the Nematomorpha. One of their 

 peculiar features is certainly secondary, namely the absence of cilia. 

 There are in some nematodes cilium-hke processes to the internal 

 border of the endoderm cells ; in one case active movement has been 

 reported. The excretory canals, when the absence of flame cells is 



