192 



FRESHWATER FAUNA 



lakes, produces a winter egg, which floats freely on the 

 water and gives birth to a larval form known as a nauplius 

 (Fig. 66 shows the nauplius of a prawn). 



Cyclops and the Copepods generally carry their eggs 

 about with them, the young hatch out as a nauplius ; the 

 eggs of some of the Phyllopods, such as Apus, endure 

 desiccation without injury, and are all the better for it ; 



indeed, unless previously dried 

 they do not develop. The eggs 

 of the Ostracods, Cypris, and 

 its allies will also bear desic- 

 cation. The dispersal of these 

 forms may, therefore, be readily 

 brought about. 



The Amphipods (sand-hop- 

 pers) carry their eggs about, 

 tucked under the abdomen, and 

 the young are born in the 

 mature form with their full 

 number of segments and limbs. 

 The same is true of the Isopods. 

 Palsernon (Fig. 64) is a prawn 

 w T hich lives in fresh water, 

 though not in this country ; 

 it differs from its marine rela- 

 tives (Fig. 65) which are set 

 free in the zoe'a stage (Fig. 66), 

 by hatching out at a later 

 stage of development, in a 

 form more advanced than the mysis (Fig. 67). 



The case of Palaeomonetes, a sub-genus of Palaemon, is 

 precisely similar, as has been shown by Professor Weldon. 

 Palceomonetes vulgaris lives in the North Atlantic, some- 

 times ascending brackish water estuaries or even entering 

 fresh water. After hatching out from the egg it moults 



FIG. 63. Apus cmicriformis, 

 a Freshwater Crustacean, 

 whose eggs require to be 

 dried in order to hatch, 

 (x 2.) From Lamport, 

 "Das Leben der Binnenge- 

 wasser." 



