THE NEMATODA. PARASITISM 



3°7 



■rag. 



like fore-end. It is generally harmless, but may cause inflammation. 

 It is widely distributed. Oxyuris (male 2-5 mm., female 9-12 mm.) 

 has the same habitat as Trichocephalus. It rarely causes more than 

 irritation. The eggs are ripe when they 

 leave the host's rectum, so that self- 

 infection by him is possible. The animal 

 is cosmopolitan. As a precaution against 

 infection with these parasites and others 

 of similar life-history, the thorough wash- 

 ing of raw vegetables and strawberries is 

 desirable. 



9. A free bisexual generation alternates 

 with a parasitic hermaphrodite. — Rhab- 

 donema nigrovenosum. The herma- 

 phrodite is a blackish worm which lives 

 in the lungs of the frog, where several 

 specimens may often be found in dis- 

 secting. It is protandrous. Embryos 

 escape by the glottis, are passed with 

 faeces, and become sexual adults. Their 

 young wander, and are swallowed with 

 food by a frog. An example of hetero- 

 gamy (p. 207). 



The foregoing paragraphs give 

 only a small sample 

 of the life - histories 

 of Nematoda. These animals 

 have experimented with almost 

 every kind of host, and almost 

 every kind of internal parasitism, 

 and in every line their organisa- 

 tion betrays their adaptation to 

 this way of life. As they are the Fig- 219 

 last important group of parasitic 

 animals with which we have to 

 deal, we may pause here briefly 

 to survey the phenomena of para- 

 sitism. A parasitic organism is 

 one which lives on or in the body 

 of another without conferring any 

 advantage on it. Parasites may be 

 external, as the Greenfly {Aphis) 

 or the Flea, or internal, as those with which we have just 

 dealt. An internal parasite may live (i) in the hollows of 

 the body of its host, as Entamoeba, Tcenia, and Ascaris in the 



Parasitism. 



Oxyuris, some- 

 what diagrammatic, to 

 show arrangement of 

 organs. 



A , Male ; B, female. 



Anus ; g.o., genital opening ; 

 int., intestine ; oes.b., bulb 

 of cesophagus ; ov. , ovary ; 

 ph., pharynx; t., testis; ut., 

 uterus ; v.d., vas deferens ; 

 vag., vagina. 



