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pair has two basal segments, and an inner and outer branch 

 (endopod and exopod). The second basal segments are large, 

 the left longer than the right. The endopod of the right 

 side is a long unjoin ted styliform appendage, while the left 

 endopod is reduced to a small stump. The right exopod has 

 only two segments, the first and second having probably 

 coalesced. The left exopod is three-jointed, and the structure 

 of the last segment, with which the male holds its sper- 

 matophores in copulation, is very complicated. 



In the Pontellidse only the right anterior antenna and the 

 right foot of the fifth pair are specialised. In the last 

 family, Notodelphyidai, the members of which live within the 

 branchial cavities of Ascidians, the chief sexual difference 

 is the presence in the females of a brood-chamber formed by 

 folding of the skin in the back of the fourth and fifth somites 

 of the thorax. In this chamber the eggs are carried and 

 hatched. 



On Darwinian principles an explanation of the copulatory 

 adaptations in this group can be offered with the usual apparent 

 facility. The males, which are unsuccessful in copulating, 

 necessarily leave no progeny, while those that have the most 

 " adapted " grasping organs are the fathers in each generation. 

 The case does not properly come under sexual selection if we 

 confine that term to cases of courtship or combat, but on the 

 other hand, as it is a case of selection with regard to genera- 

 tion and not with regard to the individual life, it belongs 

 rather to sexual selection than to natural selection in the 

 ordinary sense. It is, however, of little importance whether 

 we apply to it the one term or the other. The principle of 

 selection in any case cannot completely explain the facts, 

 because it does not explain why the special modifications are 

 inherited by only one sex. That principle assumes that the 

 necessary variations occurred without explaining why they 

 are not evident in the female sex. On Lamarckian principles 



