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has seen a nais, whose intestine was filled with Daphniae still 
living, which it had taken very dexterously when swimming. 
Their mode of generation is pretty nearly unknown. It is 
said, however, that they are oviparous, and that towards 
spring, is perceptible, towards the two thirds of the length of 
their body, and underneath, an elongated mass of a different 
colour from the intestine, and which, seen through the micro- 
scope, appeared to contain an innumerable quantity of eggs. 
This is to be seen for a longer or shorter time, which depends 
upon the heat of the season ; but in general it has disappeared 
at the commencement of summer. The naides can be mul- 
tiplied artificially, by cutting their body transversely into 
many pieces. Such, at least, was the result of the experi- 
ments of Trembley and of Rcesel ; M. Bose, however, tells us 
that he has repeated them without success. 
The ancients appear to have been acquainted only with the 
most common species of Hirudo, and these animals were 
not yet employed in medicine, at the time of Hippocrates. 
Pliny designates them very well under the name of Hiru- 
dines , and of Sanguisugce , distinguishing two species. Many 
new species have been added in modern times, and they have 
been divided into generic groups. Many scientific men have 
studied their organization, the knowledge ol which yet ap- 
pears hardly complete, if we may judge from the very great 
and singular differences of opinion existing on many points of 
consequence respecting it. 
A part of their history, which had been much neglected 
until later times, but the study of which has been necessitated 
by their dearness, and the difficulty of procuring them for 
medicinal purposes, is their mode of reproduction and preser- 
vation. M. Lenoble, a physician of Versailles, appears to be 
the first who noticed that the medicinal leeches formed a sort 
of cocoon, which Bergman had before observed in relation 
