W. H. Leigh-Sharpe 
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inner side of the middle of each half of the abdominal genital segment 
(Fig. 6). The ovisacs, which are merely strings of eggs embedded in 
the cement substance, arise immediately over the oviducal orifice. 
Fertilisation is effected by the passing of spermatophores through a 
pair of openings called the vulvae, situated near the middle line on 
either side of, and at about the level of, the anus. These openings 
lead into a curved tube on either side which is very narrow at its origin, 
and which gradually broadens out to serve as a spermotheca, the broad 
end being in open communication with the oviduct at or near its external 
aperture in such a way as to allow the eggs to be fertilised as they 
pass into the ovisacs. 
The spermothecae once filled with spermatophores by the male 
probably contain enough spermatozoa to fertilise all the eggs laid by 
the female during her lifetime. Only once have I found a female not 
carrying ovisacs, and this probably as the result of accident, and not 
because it was immature, for the ovary contained a full complement of 
eggs. I consider it most probable that the. eggs undergo practically 
complete development in the egg-strings where they are afforded a 
sufficiently safe protection. The time when the metanauplius stage is 
ended coincides with that of the disintegration and dissolution of the 
“ cement ” covering, as in the case of the albuminous envelope surrounding 
the spawn of a frog; and at that moment the ovisac vanishes and the 
larvae swim away freely from the mother into the sea. Almost im¬ 
mediately, in my opinion, a fresh consignment of ova is liberated from 
the ovary, being fertihsed on passing out (with a fresh store of cement 
which actually precedes and surrounds the eggs) by spermatozoa stored 
already in the spermothecae. It may be that the sacless female I 
observed was caught precisely in the interval between the dissolution 
of the ovisacs and the escape of one brood, and the formation of the 
next. Unlike Cyclops the mother does not part with her ovisacs upon 
sudden immersion into alcohol, formalin, or fixative, which is a point 
against their loss being attributed to accident. Again, in this isolated 
case, the ovary was almost completely full of eggs, while usually that 
organ presents spaces, as shown in Fig. 6, which were presumably 
filled by the ova now descended into the egg-strings. 
Precisely why male dogfish are selected by the free-swimming larvae 
as hosts rather than females is not clear. Perhaps it may be due to 
there being far more room and convenience in an extra-cloacal aperture 
kept open by the claspers. Moreover it would seem to be actually 
disadvantageous for the parasite to be situated on the clasper itself 
