ABSTRACTS 51
in their germination stage even when the drug was used in a concentration of 10 parts per
1,000,000. Khalil also experimented with sodium chloride, lime and ammonium sulphate, and
the results obtained by him were as follows :—
Compound. Lowest dilution causing death of the snails.
Sodium chloride . .
. 1 in 100.
Lime
.
.
.
. 1 in 1,000.
Ammonium sulphate .
. 1 in 1,000 (killed Bullinus only ; Planorbis survived.)
In all the above experiments, the snails were placed for 3 days in tap water containing
the particular concentration of the salt.
In an earlier communication, Khalil (1922) published the results of experimental observations
upon a species of snail (Planorbis corneus) from the ponds at Hampstead, as the Egyptian snails
were not available to him at the time. The minimum period of dryness found to be effective in
rendering the eggs of this species non-viable was five hours, and in no case did this period fail
to be fatal. Crude ammonium sulphate fertilizer, when used in a strength of 1 per 1,000, proved
invariably fatal for the eggs in a minimum period of 6 hours, whilst a solution of one-fifth this
strength was fatal in 20 hours. The adult snails were killed by 3 days' desiccation and also by
24 hours' immersion in 1 per 1,000 ammonium sulphate.
In the case of the Egyptian snails, it was observed by Khalil (1924) that they were susceptible
to room temperature,when kept in the shade, and that snails, protected by the shade of vegetation
along the sides of canals, died in 5 days if outside the water. In view of these findings, Khalil
recommended the utilization of the " summer rotation of irrigation in Egypt causing dryness of the
small irrigation canals".
Lane (1922; in Byam and Archibald, Vol. III, p. 1762) refers to the suggestion made by
Leiper that when smaller irrigation canals become partially dry, the destruction of their molluscan
fauna should be attempted by means of chemicals such as chloride of lime, although this obviously
would be unsuitable for adoption in the case of larger waterways. Lane also refers to the
possibility of a certain amount of success being derived by attaching some kind of wire-gauze filter-
ing apparatus to sluice gates whereby great masses of floating weed with the accompanying snails
could be held up.
The recommendations outlined by Khalil and others do not appear to have been as yet tested as
to their suitability for adoption under field conditions in Egypt, but work conducted on similar lines
under field conditions in other countries has yielded what would appear to be very encouraging
results.
(b) Work in South Africa.
Cawston (1919), in the course of his work upon the life-cycle of liver flukes affecting the sheep
and ox in South Africa, realized that the prevention of these affections called for an attack on the
fresh water snail (Isidora schakoi), and he accordingly recommended the following two agents for
this purpose : (1) Ross's larvicide, consisting of crude carbolic acid, 300 gallons ; resin, 200 lbs.; and
caustic soda, 30lbs.; one part of the mixture which is well boiled in the course of this preparation,
added to 5,000 parts of water, killed the larvæ or cercariæ in less than 5 minutes; (2) lime, in the
proportion of one bag of lime to every thousand cubic yards of water. In a later paper, Cawston
(1922) stated that copper sulphate, when used in a strength of 1 in 500,000, destroyed the majority
of snails. In a paper published in 1924, he recommended, among others, the following two
measures as a preventive for trematode infection in South Africa : (1) The use of lime or "some
larvicide " in small collections of water and (2) the drying up of small collections of water for at
least a week at a time. In a still later paper, Cawston (1926) recommended the burning of rushes