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 Rhizocephala are hermaphrodite with the possible 
exception of Sylon, 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 
Saceulina and Peltogaster; certaim 
of the Cypris larvae in these 
genera, instead of fixing on and 
inoculating other crabs, become 
attached round the mantle-open- 
ings of young parasites of the 
same species as themselves, which sa 
have recently attained to the ex- ih 
terior of their hosts (Fig. 71). mn 
These larvae, which remind us of 
the complemental males in Seal- 
pellum, ete., never produce sper- 
matozoa, but rapidly degenerate .. -) _ wourtéen Gusptsaiers 
where they are fixed, and appear fixed round the mantle- opening 
never to play any role in the repro- On eras Ma pea 
duction of their species. The nature 
of this remarkable phenomenon, together with the sexual condition 
of the Cirripedes in general, will be discussed in the next section. 
Much remains to be elucidated in the life-histories of these 
curious animals, and it seems probable that intermediate stages 
may exist, showing us how the extreme discontinuity of develop- 
ment has been reached. Suggestive in this respect is the newly 
discovered parasite of the Isopod, Calathura, which the author 
has named Duplorbis calathurae This animal does not appear 
1G. Smith, Fauna u. Flora d. Golfes v. Neapel, Monogr. xxix., 1906, pp. 60-64, 
119-121. 
