P. Manson-Bahr and N. H. Fairley 51 
debilitated, and were then chloroformed. The mature worms were then 
extracted in large numbers from the liver and mesenteric veins. 
Both sexes of the worms were then found to be adult, the uterus of the 
females containing ova. Specimens were measured while alive, preserved by 
fixing in hot 70 per cent, alcohol, and then cleared in creosote. The morpho¬ 
logical differences verified in this manner entirely corroborate those detailed 
by Leiper (1916) and on these observations Table III has been compiled. 
Transmission experiments under artificial conditions with the object of 
determining whether Planorbis acts as a strictly selective intermediate 
host for S. mansoni and Bullinus ( Species contortus and dvbowski) for 
S. haematobium. 
For this purpose experiments were undertaken in order to breed Planorbis 
in the laboratory, and thereby to ensure its freedom from previous infestation. 
This was found to be easily feasible, and a number of laboratory-bred snails 
were obtained from adults, which had been placed in a vessel three months 
previously. It was found that on an average, Planorbis took, in the atmosphere 
of Cairo in the spring and summer months, about three months to attain to 
maturity. 
Thirty-six of these snails were placed in a vessel together with the central- 
fugalized urinary sediment containing ova of S. haematobium. On examination, 
two months later, a batch of nine were dissected and no cercariae were found. 
The remainder w ere then placed together with lateral spined. ova of S. mansoni, 
occurring in the urine of an infested soldier. Subsequently after a further 
period of six weeks 21 per cent, of the survivors were found to be heavily 
infested with cercariae which gave measurements identical with those already 
recorded for S. mansoni. 
Similar attempts to breed Bullinus in captivity were not attended with 
success, but a large supply of these snails was obtained in August, 1917, from 
a sedimentation tank, fed by filtered water from the sweet-water canal where 
there was no chance of faecal or urinary infection; in order to ensure that these 
snails were not infested by cercariae of any description, one hundred were 
dissected and found to be normal. Attempts were then made to infest a large 
number by means of lateral spined ova obtained from the urine with entirely 
negative results. 
The Seasonal Incidence of Infectivity of Planorbis and 
Bullinus with the two Species of Schistosomidae. 
The accompanying tables show that considerable numbers of snails all 
obtained from the same locality (el Marg) were dissected for twelve conse¬ 
cutive months, and one remarkable fact emerges, namely, that the largest 
I number harbouring cercariae of both forms occurred in the late autumn 
months, and especially in December. It was further noted that in these months 
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