SCOLECIDA. 123 
and united by a cord. There are generally rudimentary eyes 
or pigment-spots, which vary in number from two to sixteen. 
The remaining members of the Zurbellaria are known as 
ribbon-worms (Vemertd@), and are not uncommonly found on 
the sea-shore. They differ from the Plaxarida in being worm- 
like in shape, by the fact that the alimentary canal is furnished 
with a distinct anus, and by the absence of an external opening 
to the water-vascular system of the adult, in some cases at any 
rate. Their development sometimes shows phenomena very 
similar to what occurs in the Echinodermata, the larva (fig. 52, 
4) being a free-swimming ciliated organism, of which only a 
portion is employed in producing the adult animal, the remain- 
der being cast off as useless. 
ORDER IV, ACANTHOCEPHALA.—The “ thorn-headed worms” 
included in this order are all internal parasites. They are 
worm-like in shape, marked with trans- | 
verse wrinkles, and destitute of any mouth 
or alimentary canal. The anterior ex- 
tremity of the body (fig. 53) forms a kind 
of proboscis or snout, which is armed 
with recurved hooks, and has placed at its 
base a single nervous ganglion. Beneath 
the skin is a network of canals, containing 
a clear fluid, and believed to represent 
the water-vascular system. The thorn- 
headed worms include some of the most 
formidable parasites with which we are 
as yet acquainted, the best known being 
the various forms of Zchinorhynchus, 
which are found inhabiting the aliment- 
ary canal in many mammals, birds, and Fig. 53.—Acanthocephala. 
fishes, but not as yet in man. a Lichinorhynchus gigas, 
nat, size; a’ head of the 
same, magnified. 
ORDER V. GorDIACEA.—The Gordiacea, 
or ‘“‘hair-worms,” are thread-like parasites which in the earlier 
stages of their existence inhabit the bodies of various insects, 
chiefly beetles and grasshoppers. They possess a mouth and 
alimentary canal, but have no anus. The sexes are distinct, 
and they leave the bodies of the insects which they infest to 
breed, subsequently depositing their eggs in long chains either 
