173 ZCOLOGT. 



'f he hair-worms belong to the genera Mermis and Gordius. 

 Id the former genus the head is beset with papillae, 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 has 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 papilla 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 develoiDment of the common Gordius aquaticus Linn, 

 which is a parasite of the locust and other insects, and is 

 common to Europe and this country, is taken from Villot's 

 "Monographie des Dragonneaux." 



The eggs (Fig. 119, A) are laid in long chains ; they are 

 white, and excessively numerous. The yolk undergoes total 

 segmentation (Fig. 119, B). At the close of this period, 

 when the yolk is surrounded by a layer of cells, the germ 

 elongates 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 (i>) ; 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 {E). 



In hatching, it pierces the egg membrane by the aid of its 

 cephalic armature, and escapes into the water, where it passes 

 the early part of its life. Fig. 119, F, represents the embryo of 

 Gordius aquaticus greatly magnified. It will be seen how 

 gi-eatly it differs from the adult hair-worm, having m this 

 stage some resemblance to the Acanthocephdlus by its cephalic 

 armature, to the Nematoidea or thread-worms by its alimen- 

 tai7 canal, and in the nature of its secretory glands to the 

 larvffi (cercaria) of the Trematodes or fluke-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. 



