SCOLECIDA. 



123 



muscular pharynx («). The intestine is freely suspended in the 



body-cavity, and a distinct anus is present. There are two lateral 



water-vessels which open ventrally in front by a common pore. The 



nervous system is a gangliated cord 



surrounding the gullet. The sexes 



are generally in separate individuals ; 



and the males are usually fewer in 



number and smaller in size than the 



females. 



Considerably more than a thou- 

 sand species of Nematoid worms are 

 known, of which a large number are 

 permanently free. Good examples 

 of the free Nematoids are the little 

 " Vinegar-eel " {Anguillula aceti), the 

 " Paste - eel " [Anguillwla glutinis), 

 and the common Anguillula fluvi- 

 atilis of impure water. 



A large number of the Nematoid 

 worms are internal parasites, inhab- 

 iting the alimentary canal, the air- 

 passages, the solid organs and tissues, 

 or the blood-tubes of various ani- 

 mals ; but it will be sufficient here 

 to mention some of the commonest 

 forms of these. The most abundant 

 and familiar of these parasitic types 

 is the common Bound- worm {Aacaris 

 lumbricoides), which may be six or 

 eight inches in length, and is found 

 inhabiting the small intestine of all 

 races of men, but principally of 

 children, often in great numbers. 

 Another almost equally common type 

 is the small Thread-worm (Oxyuris 

 vermicularis), which is less than half 

 an inch in length, and is found in- 

 habiting the large intestine, gene- 

 rally near its termination. It is 

 generally found in children ; but an 

 allied type {Trichocephahis dispar) 

 is often found in the large intestine of adults. Less common 

 than any of the preceding is the singular Guinea-worm (Filaria 

 medine7isis), which is coniined to the intertropical regions of both 



Fig. 78. — Nematoiilea. A, Rhabditis 

 bioculata, female, enlarged. B, 

 Portion of the alimentary tract of 

 Oxyuris vermicularis, enlarged, g 

 Gullet ; V Muscular gizzard ; s Chy- 

 litic stomach, or anterior end of 

 the intestine (i) ; o o Ovaries ; p 

 Genital pore. 



