THE DAPHNIID/E. 



465 



who keep fresh-water aquaria. The Daphnia pidex 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 DapJiniidce are born 

 from the &^^ 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 havine two kinds of 



Daphnia pidex. 



Cypris fusca. 



eggs, but the absence of transformation is the most remarkable 

 part of its economy. 



The Daphiia is classified with certain other Crustacea on 

 account of the absence of true branchia; and the presence of 

 foliaceous feet, which act as respiratory organs ; but the PJiyllopoda^ 

 which are thus associated with the water fleas by a common 

 physiology, have a different method of evolution. They undergo 

 transformations, and one genus (Ncbalia), a species of which lives 

 in the sea, are born as Naiiplii and become Zo'ca before arriving at 

 maturity. The PJiyllopoda have very numerous segments and 

 limbs, and these are added to the bodies of the Naiiplii, one after 

 the other, from before backwards, and there is no sharp Hne of 

 division between the different regions of the body, such as the 

 thorax and abdomen. Fritz Miiller considers them as zo'ca whicn 



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