140 NEMATHELMINTHES 



intestine are already 2 to 3 mm. long. Yon Linstow has recently 

 suggested that the larval stages may be hatched out in the body 

 of the millipede Julus guttulatus, whose habits might easily lead 

 it to eat the eggs of the parasite in manured gardens, etc., and 

 which is itself sometimes unconsciously eaten when hidden in 

 fruit or vegetables. This would account for the frequent presence 

 of the parasite in pigs, and also for the fact that in man it is 

 commonest in children who are apt to eat windfalls, and in 

 maniacs and people with perverted tastes. 



A. meyalocephala, which is found in the horse, ass, zebra, ox, 

 etc., attains even greater dimensions than the foregoing. The 

 male rarely exceeds 7 inches in length, but the female some- 

 times reaches 1 7 inches. They are found in the small intestine of 

 their hosts. Cobbold l succeeded in rearing larvae, which attained 

 a high degree of organisation when the eggs were placed 

 amongst moist horse-dung, and it seems probable that the larvae 

 pass into the body of their hosts in drinking water ; at any rate 

 no intermediate host has yet been found, and Davaine, who fed 

 cows, and Leuckart, who fed horses with the unhatched eggs, both 

 failed to infect the animals they experimented on. A. mystax, 

 'which lives in cats, dogs, and other Carnivora, has also been 

 found in man. It is provided with fin-like extensions on the 

 side of its head (cf. Fig. 62), and varies much in size in different 

 hosts. When first found in man it received the name of A. 

 alata. It becomes sexually mature in about three weeks. 



One of the most remarkable cycles of development amongst 

 the many curious life-histories met with amongst Xematodes, is 

 that presented by Rhabdonema (Ascaris) niyrovenosum. The 

 free form of this, formerly known as a distinct species, Rhabditis 

 nigrovenosa, lives in the excrement of frogs, and attains sexual 

 maturity in a very short time. The sexes pair, and the fertilised 

 ova give rise to embryos which hatch out within the body of the 

 mother, and then begin to devour her internal organs. After the 

 destruction of the mother, the embryos escape and live in water or 

 slime, and sometimes burrow into water snails, but they undergo 

 no change until swallowed by a frog. Then they make their way 

 into its lungs and grow enormously, attaining a length of almost 

 an inch. This form, parasitic in the frog, is a protandrous 

 hermaphrodite, which first produces spermatozoa and afterwards 

 1 Cob)x>ld's Parasites, London, 1879, p. 24*5. 



