SECT. XV CLASSIFICATION OF CRUSTACEA 267 



biological observations of extreme interest^ We limit 

 our own contribution to the subject to a few points 

 of some interest and importance. 



THE CLADOCERA. 



One specimen of Lepidurus glacialis in our collec- 

 tion was in the act of casting its skin. Shining 

 through the shell was a white mass, which turned 

 out to be a group of eggs, thrust in as far as 

 possible under the neck. It was clear that this 

 was not accidental ; the eggs were there in order 

 to develop under the shelter of the cast-off cuticle. 

 The origin of this arrangement may well have been 

 accidental. The Apodidae swim on their backs, 

 so that eggs from the brood pouch might very 

 easily fall into the large dorsal shell, and this 

 would be the more likely, the larger the shell in 

 proportion to the length of the body ; every diving 

 movement of the animal would tend to lodge the 

 eggs further up between the shell and the back. The 

 young hatched out of such eggs may easily be sup- 

 posed to have derived some advantage from their 

 position. We have two cases to consider, first, that 

 in which the eggs hatch out before the cuticle is cast, 

 and develop under the shell of the parent, and second, 

 that in which the eggs do not develop before the 

 shell is cast, the Nauplius swimming about for a time 

 under cover of the exuvia of the parent. 



1 A suggestion as to one of the changes which explain the origin of 

 Branchipus out of Apus will be found on p. 100. 



