44 MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 
V. 
THE LIFE HISTORY OF THE CLADOCERA. 
By Robert Gurney. B.A. 
Read 28 th February, 1905. 
Cladocera, or Water-fleas, are to be found in almost every 
collection of fresh water, however small, even if it is not 
a permanent one. In fact, the foulest of farm ponds 
often contain a greater abundance of individuals than the 
clear weedy water of the Broads, though not such a great 
variety of species. 
During a large part of the year all these individuals are of 
one sex — there are no males to be found, and all are alike 
capable of producing eggs and young. The eggs are not laid 
free in the water, but into a cavity between the body and the 
shell called the brood pouch. See fig. 1. Here they develop 
till they are hatched in a form resembling that of the parents. 
Successive broods are produced at intervals, in favourable 
circumstances, of two or three days. Each brood, in the 
case of Daphnia pulex, may consist of about 30 young, and 
broods of as many as 78 have been recorded. Multiplication 
is consequently very rapid. It has been calculated (Ramdohr 
1805) that the offspring of a single Daphnia pidex in 60 days 
might amount to 1,291,370,075 ! In view of such prodigious 
fecundity it is not surprising that it takes but a few days to 
populate a previously untenanted pond. 
It was long ago proved that such reproduction may continue 
for a long time without the intervention of the male sex, but 
the significance of the fact was not understood. Speculations 
on this point at the beginning of the Nineteenth Century were 
naturally affected by the state of knowledge possessed at the 
time as to the phenomena of reproduction. Nothing was 
then known of what is now called parthenogenesis, and it 
was natural to assume that the Cladocera, which were seen 
