3°° 



MANUAL OF ELEMENTARY ZOOLOGY 



unite in a short, muscular vagina. The eggs are produced 

 in immense numbers — some 1500 a day — fertilised in the 

 upper part of the uterus, enclosed in 

 a chitinoid shell, passed into the gut 

 of the host, and by it voided with 

 the feces. They need a period of 

 ripening before they can bring about 

 a new infection. They will stand 

 drought, and, by being swallowed 

 with food or water, may reach a 

 new host. Usually they hatch in his 

 intestine, but in warm, damp places 

 may do so in soil. 



Ascaris lu?nbricoides is found 

 throughout the world, and can only 

 be avoided by care taken in regard 

 to the cleanness of raw foods and 

 drinking water. It may cause little 

 trouble to the host, or be the source 

 of diarrhcea and nervous complaints, 

 the latter apparently through some 

 poisonous substance which it forms 

 or causes to be formed. Santonin, 

 thymol, and other vermifuges are 

 used against it. 



Though many nematode worms 



are known, none of them has been 



found to differ anatomically from 



Fig. 207. — A diagram of Ascaris in any important respect : all 



the nervous system are eitherj like it> p aras j tiC) or live 



in damp spots where decaying matter 

 is plentiful; and the same simple 

 a, Anus ; Ag, anal ganglion; organisation is adapted by slight 

 Bn, vemrai nerve; c, differences to the needs of each of 



lateral ganglion on cir- _ . ^^.v.^ ui wiv ui 



them. Their life-histories, however, 

 are as diverse as they are remark- 



of a nematode. — 

 From Sedgwick, 

 after Biitschli. 



cumoesophageal ring ; 

 Rn, dorsal nerve ; 5", 

 lateral nerve ; Si, sub- 

 lateral nerve ; Sm, sub- able, probably because it is onlv bv 



median nerve. -, . ... , ■' . J 



strange and various shifts that they 

 can obtain entry to their several hosts. The following are 

 brief outlines of examples of the principal types of nematode 

 life- history : — 



