202 UNSEGMENTED “WORMS.” 
Class NemMatopa. Thread-worms, Hair-worms, etc. 
The Nematodes are unsegmented, more or less thread-like 
“worms,” some of which are free-living and others parasitic. 
The body is covered by a cuticle, often thick, often subject to 
moulting ; the muscular system consists of elongated muscle- 
cells arranged longitudinally, and usually leaving two free 
“lateral lines.” From a nerve-ring around the gullet, six 
nerves go forwards and six backwards. The alimentary canal 
is usually well developed, has mouth and anus, and is divided 
into three regions. Vascular and respiratory systems are 
unrepresented ; the cavity of the body is not celomic, 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 1s distinct 
sexual dimorphism. The life history ts often intricate. There 
are many remarkable features such as the sluggish ameboid 
spermatozoa, the absence of cilia, 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 ceeca of the grouse, are quite transparent and almost 
invisible when alive. At the anterior end is the mouth, 
furnished with three lips bearing sense papille ; 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 papillz. It is usually about seven inches 
long, while the female may be as much as seventeen. 
(2) Most externally there is a thick chitinoid cuticle, 
perhaps of protective value. With its presence may be 
associated the scarcity of cutaneous glands, and the absence 
of cilia. (4) Beneath this is the sub-cuticula or epidermis, 
thickened along four longitudinal lines—median dorsal, 
