236 unsegmented worms 



Phylum Nematohelminthes 



Class Nematoda, e.g. Ascaridae. 



Class Nematomorpha, Gordiidae. 



Class Acanthocephala, e.g. Echinorhynchus 



Class Nematoda. Thread-worms, Hair-worms, etc. 



The Nematodes are unsegmented, more or less thread-like 

 " worms,"" some free-living and others parasitic. The body is 

 covered by a cuticle, often thick, usually striate, often subject to 

 moulting ; the muscular system consists of elongated muscle- 

 cells arranged longitudinally, and usually leaving two free 

 " lateral lilies." From a nerve-ring around the gullet, six 

 or so nerves go forwards and also backwards. The gut is 

 usually well developed, with mouth and anus, and is divided 

 into three regions. Vascular and respiratory systems are 

 unrepresented ; the cavity of the body is not coelomic ; the 

 remarkable excretory system consists of two lateral canals 

 opening anteriorly by a single pore. The sexes are usually 

 separate and the reproductive organs simple ; there is distinct 

 sexual dimorphism . The males have usually copulatory spicules, 

 and sometimes a membranous bursa. The vulva may be any- 

 where on the ventral surface, often well forward. The life-history 

 is often intricate. There are many remarkable features such as 

 the sluggish amoeboid spermatozoa, the almost complete absence 

 of cilia and flagella, and the absence of migratory phagocytes. 



Type, Ascaris megalocephala, the Round- worm of the horse 



This round-worm occurs in the small intestine of the 

 horse, while other species similarly infest man, ox, pig, 

 etc. The body is cylindrical in cross-section and tapering 

 at each end. The colour is dead-white, the absence of 

 pigment being very characteristic of Nematodes. Some of 

 the small thread-worms, e.g. Trichostrongylus pergracilis in 

 the caeca of the grouse, are quite transparent- and almost 

 invisible when alive. At the anterior end is the mouth, 

 furnished with three lips bearing sense papillae ; the anus is 

 posterior and ventral. The male is smaller than the female, 

 and has a recurved tail furnished with two horny spines and 

 numerous sense papillae. It is usually about seven inches 

 long, while the female may be as much as seventeen. 



