22 THE VOYAGE OF H.M.S. CHALLENGER. 



one of these I will point out myself. Those species which are unisexual and have very 

 small and rudimentary males, which, therefore, according to the sketch given above, are 

 the youngest of the hypothetical course in which the different stages of sexual differenti- 

 ation have developed, are at the same time those which closely resemble the species from 

 the oldest geological strata from which species of Scalpellum are known. But I think this 

 objection is weakened by admitting that the sexual differentiation in the genus Scalpellum 

 was already achieved at the period from which these fossil remains date. The somewhat 

 aberrant shape of Scalpellum balanoides — the species in the supposed original condition 

 of true hermaphroditism — is also difficult to explain at first sight. We might have 

 expected to observe the original condition (hermaphroditism without complemental males) 

 in a species as closely resembling Pollicipes as possible, as for example in Scalpellum 

 villosum, Leach, sp. ; or Scalpellum trisjnnosum, Hoek.^ The condition of the genital 

 apparatus and the external shape of the valves (the whole capitulum), however, are two 

 factors which need not necessarily stand in so very close a relation to each other. So it 

 may be easily imagined that the original condition of the sexual apparatus is left in a 

 form in which the external shape of the capitulum has been altered, and on the other hand 

 there is no reason why the sexual relations of a form should not become altered without 

 the external shape undergoing considerable changes at the same time. 



When, however, all these considerations are weighed I do not believe that there are 

 trustworthy grounds for doubting the exactness of the hypothesis that in the genus 

 Scalpellum the hermaphrodite condition is the original, and the unisexual, the secondary 

 stage in the development. 



' I did not observe the male of Scalpellum trispinosum. I suppose that this species is furnished with a comple- 

 mental male with capitulum and peduncle from its resemblance to the species Scalpellwm villosum. I did not study its 

 genital apparatus. I can only say that it is furnished with a weU-deyeloped penis. 



