A RRO W WO RMS 3429 



of the body a sort of head can be thrust, which is armed with two circles of small 

 hooks, and tipped with a horny proboscis. With these instruments the creatures, 

 in the first place, bore their way through the eggshell, and, having made their es- 

 cape, lie quietly at the bottom of the water without appearing to wander in search 

 of a host. Insects, however, in the adult and larval stage abound in most fresh 

 waters, and sooner or later the young worms come across them. They then seek 

 out a soft spot, bore a hole by their apparatus of hooks, and by a series of 

 contractions and extensions of the body force an entrance between the muscle fibres 

 of the limb, whence they spread into the body cavity of their host. In the illustra- 

 tion, a and b show two views of the larva with its armature of hooks, and ^ repre- 

 sents two that have effected an entrance into the foot of the larva of a May fly. 

 They also infest in this way water bugs and gnats. All these water insects, how- 

 ever, are liable to be devoured by fresh-water fish, and by this means the young 

 hair worms are set free in the intestines of the fish, where they undergo their 

 metamorphosis, and after five or six months pass into the water in the mature form. 

 Nearly allied is the family Mermithidce , containing the genus Mermis. Like the 

 hair worms, they occur both singly or coiled up and entwined wfth each other. 

 The eggs are curiously constructed, having the form of lenticular capsules, with a 

 pair of tassel-shaped appendages projecting from their flat surfaces. Eggs of M. 

 albicans laid in the summer do not hatch until the spring. After remaining a 

 short time in the earth, the young search for insects and larvae, bore a way into 

 their bodies, where they gradually grow to maturity, and ultimately pass out to lead 

 a free life, when they pair and lay their eggs. They may be found in caterpillars, 

 grasshoppers, and more rarely spiders. 



ARROWWORMS Order CH^TOGNATHA 



The small group of worms included under this heading are of doubtful position. 

 They are glass-like, transparent creatures living in the sea, near *he surface of 

 which they swim in numbers. They are most active and vigorous swimmers, as 

 might be expected from their slender build, and the presence of a large horizontal 



ARROWWORM (25 times natural size). 



fin at the sides of the hinder half of the body, projecting beyond the tail. The 

 shape of the body and the presence of the large tail fin suggested the name Sagitta. 

 The head is bluntly rounded and furnished with a pair of eyes, a pair of feelers and 

 an armature of close-set horny teeth, all of which must be of the greatest service to 

 the animal in its roving, predaceous life. 



