IV 



RHIZOCEPHALA COMPLEMENTAL MALES 



99 



it and protects it. The remarkable features of this development 

 are, firstly, the difficulty of understanding how the developing 

 embryo is directed in its complicated wanderings so as always to 

 reach the same spot where it is destined to come to the exterior ; 

 and, secondly, the loss after the Cypris stage of all the organs 

 and the resumption of an embryonic undifferentiated state from 

 which the adult is newly evolved. A certain parallel to this history 

 is found in that of the Monstrillidae, described on pp. 64-66. 



The Bhizocephala are hermaphrodite with the possible 

 exception of Si/Ion, which appears to be female and perhaps 

 parthenogenetic, no male having been seen ; but unlike most other 

 hermaphrodite Cirripedes, they reproduce by a continual round 

 of self-fertilisation. This is the more remarkable in that the 

 vestiges of what appears to be a male sex are still found in 

 Sitcc a Una and Peltogastcr] certain 

 of the Cypris larvae in these 

 genera, instead of fixing on and 

 inoculating other crabs, become 

 attached round the man tie -open- 

 ings of young parasites of the 

 same species as themselves, which 

 have recently attained to the ex- 

 terior of their hosts (Fig. 71). 

 These larvae, which remind us of 

 the complemental males in Scal- 

 jii'll/tiit, etc., never produce sper- 

 mato/.oa, but rapidly degenerate 

 where they an; fixed, and appear 

 never to play any role in the repro- 

 duction of their species. The nature 

 of this remarkable phenomenon, together with the sexual condil ion 

 of the Cirripedes in general, will be discussed in the next section. 



Much remains to be elucidated in the life-lii.-lories of these 

 curious animals, and it seems probable that intermediate stages 

 may exist, showing us how the extreme discontinuity <.fde\elop- 

 ment has been reached. Suggestive in this respect is the neuly 

 discovered parasite of the Isopod, Cidath-uro, which the author 

 has named tin/>/<>i-/>is calathurae. 1 This animal does not appear 



1 G. Smith, Fauna u. Flora il. Golfcs v. Neapel, .M..imgr. .\.\i.\., 1906, i>i>. r.n-r.i, 

 119-121. 



. 71. Fourteen Cy]>ri> larvae 

 ti \c-il round the mantle -opening 

 (l of ;i young Sacculina externa, 

 x 20. 



