LIFE-HISTORIES OF TREMATODES. 157 



ally in the freshwater snail, Limnaea natalensis, both of which I 

 have found infected in nature as well as determined experiment- 

 ally. The life-history of S. haematobium in South Africa briefly 

 is as follows : — 



The terminal spined eggs pass from the human body with the 

 urine. If they reach water, a ciliated larva or miracidium hatches 

 from each egg and swims about actively. If the pond snail, 

 Physopsis africana, or more rarely, Limnaea natalensis, is encoun- 

 tered, the miracidia penetrate the pulmonary cavity of the snail, 

 enter the liver and settle down there, losing their motility and 

 developing into hollow structures termed sporocysts. From the walls 

 of the sporocyst buds arise which develop into active forked tailed 

 cercariae, which not only invade other parts of the liver but also 

 reach the generative organs of the snail. The cercaria of 

 «S f . haematobium has a body about 240^ long by 100/* broad, its 

 tail is about 200/x long by 45 /a broad, and the caudal forks are 

 about 80/x to 100m long. There is a small oral sucker and the 

 ventral sucker or acetabulum is also small. Three pairs of mucin 

 glands are present, each capped by a hollow piercing spine and 

 opening on the outer margin of the oral sucker. A group of several 

 large gei v m cells lies behind the posterior sucker. 



The cercariae ultimately leave the snail, swim about in the 

 water, and should they come in contact with the skin of a person 

 bathing or paddling in the water, or even drinking it, they attach 

 themselves to the skin, bore through it, dropping their tails in so 

 doing, reach the bloodvessels, and find their way to the liver and 

 mesentery of the human host. In the branches of the portal and 

 mesenteric veins the worms gradually assume the adult form. 

 Laboratory animals exposed to or drinking water containing 

 cercariae have become infected and have died of bilharziasis, after 

 about two months, adult worms being obtained post mortem. 



The adults are unisexual, the males being broader and thicker 

 but shorter than the females. The males that I have obtained 

 experimentally have been from 3mm. to 17mm. long. The suckers 

 are near one another, the ventral sucker being the larger and being 

 pedunculated. The surface of the body is beset with cuticular 

 spines. The mouth opens on the anterior sucker, and the two- 

 bulbed oesophagus communicates with the intestine which bifur- 

 cates behind the posterior sucker. These caecal forks unite far 

 behind and the gut ends as a single canal of short length. The 

 testes are four in number, rounded and large. The edges of the 

 body are folded to form the characteristic gynaecophoric canal. 



The females obtained by me experimentally varied from 5mm. 

 to 30mm. long. They were thread-like, with weak suckers. The 

 ovary is in the posterior half of the body. The uterus is volum- 

 inous and usually contains a number of mature terminal spined 

 eggs at one time. The vitellaria lie in the posterior quarter of the 

 body. The female is carried in the gynaecophoric canal of the 

 male. She migrates from the region of the liver and deposits 

 her eggs in the walls of the human bladder, whence they make 

 their way to the cavity of the bladder and pass out with the urine. 



