1921} Riley: Guide-posts to Medical Entomology 163 
having almost the same figure, that is, a thick head, Fig. viii, a, 
and a small tail b, like young frogs or tadpoles.’’ Swammerdam 
well adds: ‘‘I must confess the sight of these astonished me, 
as I never expected to have met such, and so many miracles in 
one little creature, or that I should have been so well convinced 
of my own ignorance and blindness in a single subject.’ 
In 1831 Mehlis made the remarkable discovery that the 
eggs of certain flukes ‘‘contained an embryo which in shape 
and ciliation resembled an Infusorian; it was occasionally 
provided with an eyespeck and after being hatched swam about 
like an Infusorian.’’ (Leuckart.) 
The discoveries of Swammerdam, a century and a half 
before might well have formed a link in the chain, but this 
link was to be formed anew by Bojanus and by Von Baer. In 
1818 Bojanus described brightly colored worms which we now 
know as rediz, in pond snails. Von Baer in 1824 showed that 
these rediz give rise in their interior to the above-mentioned 
tailed cercariz which, becoming free, swim about in the water. 
Numerous other workers contributed observations but it 
remained for Steenstrup to correlate and interpret the data. He 
pointed out that the embryo escaping from the egg became the 
free-swimming larva, that this entered the snail and formed a 
generative sac (the sporocyst) which gave rise to redie. From 
these in turn arise cercarize which developed into the adult 
flukes. Remarkable advances in the study of the flukes have 
been made since 1842, when Steenstrup published his conclusions 
but the essential facts were made clear by him. It is worth 
pausing to note that these facts which are fundamental to any 
control measures against some of the most dangerous of the 
parasitic worms were obtained through work on forms of no 
economic importance. 
The early workers on insect anatomy occasionally noted the 
presence of parasitic worms within their specimens. Reference 
to such are found in Roesel, DeGeer, and Reaumur. The earliest 
which I have seen is that of Lister who in 1672 not only demol- 
ished the view that horse hairs gave rise to snakes, but showed 
that the so-called ‘‘hair-snake”’ lived for part of its life within 
‘““Black-beetles.’’ Von Linstow’s epochal studies on the develop- 
ment of Gordius were on a species found in Pterostichus niger, 
possibly this same “black beetle.”’ 
