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



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

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

 the youngesl of the hypothetical course in whieli the different stages of sexual differenti- 

 ation have developedj are at the Bame time those which closely resemble the species from 

 the oldest geological strata from which species of Scalpettum are known. But 1 think this 

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

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

 aberrant shape of Scalpettum 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 Potticipes as possible, as, for example, in Scalpettum 

 vittosum, I. each, sp. ; or Scalpettum trispinosum, Hoek. 1 The condition of the genital 

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

 factors whieli 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. 



W Inn, however, all these considerations arc weighed I do not believe that there are 

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

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

 stage in the development. 



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

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

 genital apparatus. I can only say that it is furnished with a well-developed penis. 



