HERMIT CRAB AND PELTOGASTER. 607 



the internal ramus attains the same degree of alteration in 

 the first and second appendages, in the third it remains for 

 the most part in a condition of suppressed development (figs. 

 2, o, 4), though in the most marked examples of modification 

 this appendage exhibits the effect in the same degree as the 

 two first (fig. 5). The retardation of modification proceeding 

 backwards in the series is apparently in correspondence with 

 what takes place in the normal development of the female, 

 which reaches its extreme in the fourth abdominal appendage, 

 where the internal ramus remains throughout life in a rudi- 

 mentary state, and where also no alteration occurs in connec- 

 tion with parasite castration. These facts are apparently to be 

 explained by the supposition that the abdominal appendages 

 have been successively pressed into the service of bearing the 

 embryos, and the modification entailed by this is more complete 

 in those longest so employed. 



On account of the completeness of the modified series it is 

 impossible to do more than roughly classify the infected 

 males with respect to the degree of modification. It may be 

 said, however, that at least a quarter of those examined 

 were quite unmodified^ that in considerably more than half 

 the modification range from a condition of bare percepti- 

 bility to one in which the internal ramus, though distinctly 

 small, showed the typical female form, and the remainder 

 alone could be spoken of as completely modified. 



Amongst the infected crabs there is a considerable propor- 

 tion which it is impossible to assign to their sex by merely 

 external examination, for they are entirely without genital 

 apertures, which have to be taken as the external criterion 

 of sex. In these then the modification has been of so long 

 standing duration that the gonad or at least the duct has 

 become completely extirpated, and succeeding moults have 

 closed the genital apertures. As referred to above, a mid- 

 way stage to this condition is to be noticed in certain crabs 

 in Avhich one only of the apertures is closed, this in almost 

 all the cases of both sexes examined being the one opposite 

 the Peltogaster. 



