Bilharzictsis 
36 
in a remarkable manner the historic investigations of Leiper, Thompson and 
Cockin, both as regards the duality of the species of Schistosomidae, and as 
regards their selective affinity for the two genera of fresh-water molluscs. 
The data which have been obtained from the accidental infestation of the 
troops in different parts of Egypt in association with the local snail fauna 
amply confirm those obtained from experimental animals. 
In addition, certain observations were made on the distribution of the 
different species of snails and their habits which have a bearing upon the 
prophylaxis of the disease, and it is with this end in view, and also with the 
idea of promulgating further investigations on the general prophylaxis of 
bilharziasis, that we submit this paper for publication. 
III. THE MORPHOLOGY AND LIFE CYCLE OF THE TWO 
SPECIES OF EGYPTIAN SCHISTOSOMIDAE. 
(Schistosomum haematobium —Bilharz v. SiebolcI, 1852.) 
(Schistosomum mansoni —Sambon, 1907.) 
In Schistosomum haematobium infestations terminal spined ova are de¬ 
posited in the excreta, both urine and faeces, of an infested subject. On reaching 
water a ciliated miracidium is hatched out, which in a period of thirty-six 
hours at a maximum must find the correct species of snail of the genus Bulli- 
nus, failing which it dies. If a suitable snail is encountered, the miracidium 
enters it by piercing its soft parts, probably through the pulmonary chamber, 
and eventually reaches the digestive and the hermaphrodite glands. Here it 
is converted into a morula and later becomes hollowed out into elongated 
finger-like sporocysts; these, by a process of budding of cell masses known as 
“germ balls” or blastospheres, produce cercariae; they may also by a process 
of exogenous budding produce secondary cysts. When mature, the cercariae 
are ejected into water, and in the subsequent twenty-four hours must meet 
their definitive host, usually man, and after invading his tissues, either via the 
skin or the upper alimentary tract, make their way to the portal veins in which 
they develop into adult worms, both male and female. 
In Schistosomum mansoni infestations lateral spined ova are excreted in 
the faeces of an infested subject, less commonly in the urine as well. The inter¬ 
mediate host in this case also is a fresh-water snail, but of the senus Planor- 
bis. In other respects the life cycle resembles that described above for S. hae¬ 
matobium. 
The Ova. 
The ova of the various species of schistosomes affecting man are passed 
to the exterior in the excreta, and in this fashion reach water. These ova 
are of a dark yellow brown colour, and of different shape; they are non- 
opereulated, but possess either a spine or the rudiments of one. The ovum is 
covered by a chitinous shell, lined with a thin shell membrane, its interior 
