SOURCES OF INTESTINAIi WORMS. 
169 
the so-called 
Echinococcus — 
heads or scolices 
from the in- 
terior of an 
hydatid cyst 
X 150 diam. 
stage of development^ within the human body, is a point 
which, so far as my experience goes, remains yet to be ascer- 
tained. I certainly see no reason for supposing that we escape 
its invasion ; on the contrary, I am pretty confident that it has 
been hitherto overlooked, merely because observers generally 
are, as yet, entirely unfamihar with its characteristic features. 
Whatever difficulties, however, exist in regard to detection 
of the larv8e of the two kinds of Tanice just referred 
to, none need occur in the case of a third species. 
I allude to the familiarly known hydatid (fig. 9). 
These bodies are the young of the Tcenia echino- 
coccuSj and we are exposed to their terrible 
ravages by drinking water containing the eggs of 
this tapeworm. The eggs at the time of their 
being swallowed must be mature, or, in other 
words, must already contain six-hooked em- 
bryos, if we are to become their parasite bearers. 
The tapeworm itself is extremely minute, and fi&. 9 .— One of 
resides in the alimentary canal of the dog. If 
neither dog nor wolf existed, we should probably 
be free from the invasions of this parasite. The 
eggs escape their canine host, and, by various 
possible and easily explained ways, are trans- 
ported into ditches, ponds, cesspools, lakes, and streams. 
They are so minute, that hundreds might exist in a glass of 
water without their being recognized by the naked eye ; yet 
any one of them gaining access to our stomach would be suffi- 
cient to endanger life. The larvae, though generally preferring 
to make their way to the liver, are not at all particular, and, 
consequently, will often select the other internal viscera, the 
brain, or even the interior of the bones. I know of eight 
instances where they took up their residence in the tibia. It 
is of little moment, however, to say more on this head, save 
for the purpose of showing the frequency of the hydatid 
disease amongst us. Here, again, we have no accurate data 
to rely on; yet, so far as my observations have extended, I 
can positively assert that many lives are annually sacrificed 
from this cause. I say sacrificed,^^ because the disease is 
preventible. No one need drink water impregnated with 
the ova of entozoa, even though the water may look pure. 
Artificial reservoirs to which dogs have no access cannot con- 
tain the eggs of Tcenia echinococcus j neither, in like manner, 
are we likely to obtain them in well or spring water. But 
open waters, into which the carcasses of dogs are occasionally 
thrown, or into which tapeworm-eggs may have been carried 
by rain and other agencies, are always liable to contain them. 
The eggs may also be introduced with food which has been 
immersed in or washed with water from this source. It comes 
