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 (Filarid), 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 



Trichina, 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. One of 

 the most familiar is the so-called 

 " vinegar - eel " (Anguillula acetL 

 Fig. 42, A). 



FIG. 42. Nomatoda. A. Vinegar-eel 



OBDEE Vin. ROOTERA (Lat. 



rota, wheel; jero, I carry). The living in stagnant water. 

 Rotifera, 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 Rotifera 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 



