648 THE AMERICAN NATURALIST.  [VoL. XXXIII. 
cannot be considered proved until the structure of the genital 
organs of Nectonema is better known, and on this account I 
do not now include Nectonema among the Gordiacea. 
The hair worms are in the adult stage fresh-water forms, very 
long and slender, with a thick cuticle on the surface of the 
body, without external segmentation or appendages. They are 
cylindrical in form, but become flattened after the discharge 
of the genital products. The head end usually tapers to a 
point, while the form of the posterior end differs in the two 
sexes. There are certain genera of nematodes (such as Mer- 
mis and Filaria) which bear a close external resemblance to 
them, but a mature hair worm may be recognized by the follow- 
ing peculiarities: The greater portion of the body is of the 
same diameter, the posterior end is never sharply pointed, and 
the surface of the cuticle is marked by elevated areoles or 
papilla (the latter often complex in structure), or, by intersect- 
ing layers, is divided into rhombs. 
The eggs are laid in the water in the form of long strings. 
From the egg develops a minute larva, characterized by a 
proboscis armed with hooks, which swims in the water for a 
short while, then enters into an aquatic insect, usually the larva 
of a may-fly. The larval stage ends at this point, and the 
embryonal commences. After a period the still immature and 
very small worm passes into a new host, most frequently the 
imago of a coleopterous insect, in the body cavity of which 
it may complete its development. The mode by which the 
worm passes into the second host seems to be dependent 
entirely upon environmental circumstances. In such a case 
where the first host is the larva of a may-fly, and the second 
a carnivorous beetle, it has been suggested that in a season of 
drought, when the pools become dried up, the may-fly is left 
stranded, and so becomes the easy prey of a beetle, the latter 
in eating the may-fly larva thus swallowing the young hair 
worm. But very frequently immature and nearly mature hair 
worms occur in the body cavity of orthopterous insects, such 
as grasshoppers and crickets, which are purely herbivorous, and 
which would probably never consume may-fly larve. For these 
cases two explanations occur to me by which to explain the 
