INFESTATION OF F/'ESH-WATER SNAILS. 397 



were placed by themselves in a test-tube of water. Twenty-six 

 hours later many were seen still actively swimming about in the 

 water. The prongs of this cercaria, which is probably C . gladii, 

 are equal in length to that of the tail. The snails were obtained 

 from a pool frequented by cattle and birds. However, if stored 

 for two days apart from fresh-water snails, water becomes free 

 from bilharzia infection. 



Though drying kills this kind of fresh-water snail, others, 

 such as Tiara tuberculata, which is heavily infested with cercariae 

 at the Natal Coast, can resist drought for several weeks at a time 

 and, when the rainy season returns, continue to emit numerous 

 cercariae without being exposed to fresh infection. 



As I was desirous of obtaining some adult bilharzia worms 

 from parasites which I had obtained from fresh-water snails 

 which I had myself experimentally infected, 1 repeated these 

 experiments in February, 1921, using some Physopsis africana 

 and Limnaea natalensis which I had grown from eggs in water 

 kept free from other sources of infection. These snails were 

 placed in a jar of water containing miracidia which I obtained 

 from the urine of a patient who had contracted bilharzia disease 

 through bathing in the pools at Sarnia, and thirty-three days 

 later, on March 21st, mature bilharzia cercariae were found in 

 one Physoppis. One Limnaea which was dead showed no cercariae. 

 The Physopsis was only 10 mm. in length. 



On March 23rd, thirty-five days after these snails were 

 exposed to the miracidia, a free-swimming cercaria was clearly 

 visible to the naked eye and readily identified by a hand-lens, 

 swimming about in the water. No cercariae or other signs of 

 infection were seen in four Limnaeae, one of which was dead 

 before dissection, but numerous mature bilharzia cercariae were 

 found in the onlv remaining Physopsis, which was only 9 mm. in 

 length and was the smallest infested specimen I had seen. The 

 cercaria varied in size according to its movements, but both head 

 and tail were about 0.175 mm. in length, and the prongs of the 

 divided tail were about a quarter the length of the tail. The 

 cercariae were injected into five guinea pigs in order to secure 

 their adult forms, but the results were negative at the end of 

 three months. 



Seven Limnaea natalensis were placed in a jar of water con- 

 taining numerous miracidia on March 9th and kept until May 

 17th, when dissection failed to reveal the presence of cercariae, 

 and the liver substance appeared to be normal. This jar of water 

 also contained a few Physopses which were dead when the Limnaeae 

 were examined. 



On March 31st four Limnaea natalensis from 11 mm. to 

 14 mm. in length, which had been kept in water with the ova of 

 Pasciola since February 14th and 18th, showed no evidence of 

 sporocyst or redia-formation. 



The pools at Sydenham and Mayville contain several cer- 

 cariae which closely resemble S. haematobium, but are not identi- 

 cal with it. Some of these have developed into S. bovis in my 

 experimental animals, and although I do not think that S. bovis 



