SCOLECIDA. 



115 



and the nervous system is in the form of a gangliated cord 

 surrounding the gullet, and sending filaments backward. 

 Among the best known of the parasitic Nematodes are the 

 common round-worm {Ascaris lumbricoides) and the thread- 

 worm ( Oxyuris) of the human subject, both of which inhabit 

 the alimentary canal, and the guinea-worm {Filaria), which 

 spends a portion of its existence in 

 the cellular tissue of man, especially 

 of the legs, and which attains a 

 length of several feet. More dan- 

 gerous than any of these is the 

 Triehina, which spends its immature 

 stages encysted in the muscles of 

 some such animal as the pig, and 

 only attains maturity and becomes 

 capable of producing eggs, when in- 

 troduced into the alimentary canal 

 of some other warm-blooded verte- 

 brate animal. When this takes 

 place, a train of symptoms are 

 originated which sometimes re- 

 semble rheumatic fever, and appear 

 to be very generally fatal. 



Of the free Nematode worms, 

 which are never parasitic at any 

 time of their lives, about two hun- 

 dred species have been described, 

 most of which inhabit fresh water 

 or the shores of the sea. Qne of 

 the most familiar is the so-called 

 " vinegar - eel " {Anguillula aceti, 

 Fig. A A). 



Fig. 42. — Ncmatoda. A. Vinegar-eel 

 {Angidllida aceti). B. Dorylai- 

 mus stagnaUs, a free Nematode, 

 living in stagnant water. 



Oedee VIII. RoTiPEEA (Lat. 

 rota, wheel; fero, I carry). — The 

 Motifera, or "wheel-animalcules," 

 derive their popular name from the fact that the anterior end 

 of the body is furnished with one or two circlets of cilia 

 (Fig. 43) which, when in motion, vibrate so rapidly as to 

 produce the illusory impression of a quickly-rotating toothed 

 wheel. The Motifera are almost all aquatic, and are mostly 

 inhabitants of fresh water. They are all microscopic in size, 

 none attaining a greater length than one-thirty-sixth of an inch. 

 In the females there is a distinct mouth, intestinal canal, and 



