54 VETERINARY PARASITOLOGY 
They are divided into two chief orders : 
1. Nematoidea (the Nematode worms). 
2. Acanthocephala (the thorn-headed worms). 
1. THE NEMATODES 
have a transversely striated skin. They possess a 
digestive canal running the whole length of the body, 
but no circulatory apparatus. 
They may be found in any tissue of the body, except 
the osseous skeleton. Their colour may be white, red, or 
brownish. The head may be armed, but definite hooks, 
such as are seen in the Teeniz, are absent. In many 
the anterior end of the body carries just behind the head 
a pair of small membranous wings. Often there is a 
caudal pouch into which the genital duct and intestinal 
canal open by means ofacloaca. In the males chitinous 
spicules are often present, and are used in the process of 
copulation as “‘ claspers.”’ The Nematodes are oviparous 
or viviparous, and the females have a vulva on the floor 
of the body. Into this the uterus opens. This organ 
is provided with one or two ovarian tubes. In the 
majority the life-history has not been thoroughly worked 
out, but it is evident that some members complete their 
life-cycle in one host, while others require an inter- 
mediate host. The females are by far the commoner, 
and are larger. The males often exhibit spiral coiling 
of the posterior extremity. 
The Nematoidea are divided into : 
(x) Ascaridze. 
(2) Oxyuride. 
(3) Filariide. 
(4) Trichotrachelide. 
(5) Strongylide. 
(1) Ascaride.—The body tapers towards each ex- 
tremity. The mouth has three lips, and between these 
