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peratures, the miracidia become first pyriform, 
then immovable and finally die after a short 
lapse of time, unless they are soon removed. 
In the latter case they may reassume their 
activity. They are always more sensitive to 
cold than the cercariae and this explains the 
fact that an infection, already started in the 
antennae of a snail, may be interrupted in 
consequence of cold weather. 
Penetration of the Miracidia in Mol- 
lusks. 
In order to watch the penetration I put a 
few snails in a deep PETRI’s dish, adding 
water containing myracidia. A binocular mi- 
croscope is very useful for this purpose but 
any lens with a weak power may be used. 
At first the miracidia continue their evolu- 
tions, sometimes passing close to the snails 
without taking any notice of them. Then they 
come nearer and nearer to the species they 
prefer. Suddenly, one or the other sticks on 
an exposed part. Others follow, but many 
remain refractory for a long time. 
The species experimented upon belong 
to Planorbis, Physa, Limnaeus, Ancylus and 
Ampullaria, With rare exceptions (perhaps 
due to a previous contact with other species) 
the three last were completely left alone. The 
Physa and the redblooded Planorbis showed 
equal attractlon, the adults being prefered ; 
contrary to my expections, the bigger they 
were the more attraction they seemed to 
offer. The point of predilection is the anten- 
na which is very well suited for further ob- 
servations. The rest of the head and the post- 
antennial region are also frequently chosen. 
The well exposed pallium is little attacked 
in Planorbis, somewhat more in Physa; in 
both the foot is neglected. 
As shown in the drawing the miracidium 
on penetrating has an elongate, almost cy- 
lindrical form. Its movements are similar to 
those of a rock-drill or the piston of a syrin- 
ge. The time required for complete penetra- 
tion is 10-15 minutes ; afterwards the miraci- 
dium is seen extended in the subcutaneous 
layer, showing its cilia but with the glands 
empty. Sometimes after several attempts the 
miracidium drops again or leaves the point 
of attack without penetrating. 
Up to date, only four species of Planor- 
sis are known as intermediate hosts. They 
are Pl. Boissyi in Egypt, guadaloupensis in 
the Antilles, Venezuela and the North of 
Brazil, olivaceus, found in Bahia and Sergipe 
and used in most of my experiences, and 
finally centimetralis mihi, found in the nor- 
thern states and also in Minas. The other 
species described in my monograph of the 
brazilian Planorbis are not suitable. In some, 
as also in Physa, the penetration is normally 
effected, but evolution is arrested after the 
first stages. 
The miracidia, which try to penetrate in 
other regions than the antennae or near to 
the eye, fail to pierce the skin or die shortly 
afterwards; at least I never found the next 
stages in other points, though possibly a mira- 
cidium might pass through the respiratory 
opening and reach the visceral part. 
My studies were all made on the anten- 
nae which are easily observed and may be 
removed without danger to the snail. Even 
the detached antennae of Physa and Planor- 
bis (which due to their epithelial cilia move 
like living organisms) are easily infected. 
Successful and unsuccesful attempts to 
penetrate produce, in the antenna, a local reac- 
tion caracterised by swelling and hyperaemia, 
which is easily appreciated, on account of 
the red colour of the blood in the larger Pia- 
norbis. This reaction is certainly due to the 
gland secretion, the inoculation of which must 
soften the tissue and help the miracidium in 
penetrating. The snails are generally indo- 
lent; when attacked by a larger number of 
miracidia, they try to retract the exposed parts 
or to leave the water. 
This first reaction vanishes in a few hours, 
leaving at most One or more small nodules. 
Miracidia which find no occasion to pe- 
netrateyin a snail, sink after some time to 
the ground and die, having first expelled the 
contents of the glands. 
