296 SEXUAL DIMORPHISM 



condition, at any rate when actually discharging their 

 reproductive products, swim about actively in the water 

 instead of crawling on the ground, or lurking under stones 

 and in crevices. The transformation can be studied in 

 the common British form Nereis pelagica, the mature or 

 " epitokous " forms of which were originally described, as 

 Heteronereis grandifolia. The mature form has, in addition 

 to the peculiarities already mentioned, larger eyes and 

 larger cephalic palps than the immature form. So far no 

 differences between the sexes have been mentioned, we have 

 been considering a metamorphosis somewhat analogous with 

 that of the tadpole into the frog. Before proceeding further 

 it may be pointed out that the facts harmonise very 

 completely witb. the views maintained throughout this 

 work. The ordinary Nereis form does, to a certain extent, 

 employ its parapodia as Unlets for swimming, and it is not 

 unreasonable to suppose that greater activity in swimming 

 during the actual discharge of the sexual products preceded 

 the evolution of the structural peculiarities of the mature 

 form. The changes in the parapodia are such as might well 

 be produced by merely greater use of them. Greater exercise 

 of them as Unlets may well have been the true cause of the 

 hypertrophy of the various lobes. It is perhaps more diffi- 

 cult to understand how mere exertion should have led to the 

 replacement of the original chsetse by a new set of different 

 shape. But it is not impossible that the mechanical action 

 should have had the effect of -loosening the old chaetae and 

 stimulating the cells at their base to secrete new, which in 

 their development were subjected to pressures and strains 

 that made them flattened and expanded. Here, as in so 

 many other cases, the most cogent argument is, that the view 

 I maintain explains the relation between the metamorphosis 

 and the change of habits. There is no special reason why 

 mere reproductive maturity should be accompanied by this 



