124 INVERTEBRATE ANIMALS. 
in water or in some moist situation. In form the Gordiacea are 
singularly like hairs, and they often attain a length very many 
times greater than that of the insect in which they live. 
OrpDER VI. Nematopa.—In this order are the ‘ round- 
worms” and “thread-worms,” both of which are parasitic, to- 
gether with a number of worms 
which lead a permanently free exist- 
ence. All the Mematoda (fig. 54) 
are elongated and cylindrical or 
thread-like in shape. They possess 
a distinct mouth, and an alimentary 
canal which is freely suspended in 
an abdominal cavity, and which 
terminates in a distinct anus. They 
possess a system of canals which 
are believed to represent the water- 
vascular system; and the nervous 
system is in the form of a gangli- 
ated cord surrounding the gullet, 
and sending filaments backwards. 
Amongst the best known of the 
parasitic Nematodes are the com- 
mon round-worm (Ascaris lunbrico- 
zdes) and the thread-worm (Oxyurts) 
of the human subject, both of which 
inhabit the alimentary canal; and 
the guinea-worm (/7/aria), which 
spends a portion of its existence in 
the cellular tissue of man, especially 
of the legs, and which attains a 
Fig. 54.—Nematoda. A, Vinegar- length of several feet. More dan- 
eel (Anguiliula aceti). B,Do- gerous than any of these is the 
rylaimus stagnalis, a free tows : : . 
Nematode, living in stagnant, 27@¢A@na, which spends its imma- 
water. ture stages encysted in the muscles 
of some such animal as the pig, and 
only attains maturity and becomes capable of producing eggs 
when introduced into the alimentary canal of some other warm- 
blooded vertebrate animal. When this takes place, a train of 
symptoms are originated which somewhat resemble rheumatic 
fever, and appear to be very generally fatal. 
A 
