130 ZCOLOG Y. 
‘The hair-worms belong to the genera Mermis and Gordtus. 
Ip the former genus the head is beset with papille, and the 
end of the body of the male is undivided, while the oviduct 
of the female opens in the middle of the body. The larva 
is unarmed and hus no metamorphosis. Mermis acuminata. 
Leidy is pale brown and parasitic in the body of the cater- 
pillar of the coddling moth ; another species lives in the. 
bodies of grasshoppers. 
The true hair-worm, Gordius, has no papille on the head, 
and the tail of the male is forked, while the oviduct of the 
female opens at the end of the body. The following account 
of the development of the common Gordius aquaticus Linn. 
which is a parasite of the locust and other insects, and 1s 
common to Europe and this country, is taken from Villot’s 
“‘ Monographie des Dragonneaux.” 
The eggs (Fig. 89, A) are laid in long chains; they are 
white, and excessively numerous. The yolk undergoes total 
segmentation (Fig. 89, B). At the close of this period, 
when the yolk is surrounded by a layer of cells, the germ 
elonyates at what is destined to be the head-end ; this layer 
pushes in, forming a cavity, and in this stage it is called a 
“‘gastrula” (C). By this time the embryo becomes pear- 
shaped (D); then it elongates. Subsequently the internal 
organs of digestion are formed, together with three sets of 
stiff, spine-like appendages to the head, while the body is 
divided by cross-lines into segments. The head lies retracted 
within the body (£). 
In hatching, it pierces the egg membrane by the aid of ita 
cephalic armature, and escapes into the water, where it passes. 
the early part of its life. Fig. 89, /, represents the embryo of 
Gordius aquaticus greatly magnified. It will be seen how 
greatly it differs from the adult hair-worm, having in this. 
stage some resemblance to the Acanthocephalus by its cephalic 
armature, to the Nematoidea or thread-worms by its alimen- 
tary canal, and in the nature of its secretory glands to the 
larve (cercaria) of the Trematodes or filuke-worms, But the 
hair-worm differs from all these worms and even Mermis, a. 
hair-worm much like and easily confounded with Gordius, 
in having a complete metamorphosis after leaving the egg. 
