' THE DAPHNIID M. 
465 
who keep fresh-water aquaria. The Daphnia pulex is very- 
abundant in ditches and ponds, and it is recognised by the 
carapace consisting of two shell-like valves, which enclose the 
body, and allow the large antennae to protrude externally. It is 
the type of an order, and is only interesting to us because it does 
not undergo metamorphosis. The young Daphniidce are born 
from the egg with ten legs and with the external appearance of 
the adults. Shell moulting occurs several times, and there is 
much that is interesting in the creature’s having two kinds of 
eggs, but the absence of transformation is the most remarkable 
part of its economy. 
The Daphnia is classified with certain other Crustacea on 
account of the absence of true branchiae and the presence of 
foliaceous feet, which act as respiratory organs ; but the Phyllopoda, 
which are thus associated with the water fleas by a common 
physiology, have a different method of evolution. They undergo 
transformations, and one genus ( Nebalia ), a species of which lives 
in the sea, are born as Nauplii and become Zoca before arriving at 
maturity. The Phyllopoda have very numerous segments and 
limbs, and these are added to the bodies of the Nauplii , one after 
the other, from before backwards, and there is no sharp line of 
division- between the different regions of the body, such as the 
thorax and abdomen. Fritz Muller considers them as zoea which 
Daphnia pulex. 
Cypris fusca. 
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