166 Miscellaneous. 



median part of the posterior half of the ovaries nearly at the centre 

 of figure of the parasite ; they give origin each to a long deferent 

 duct, which reaches the posterior margin and turns round it to open 

 in the supra-peduncular region. Succuliiui Fraissei therefore belongs 

 to the group of mesorchideons SaccuUnce, the type of which is Sacctt- 

 lina corcnlam, Kossm., parasitic upon Atergntis floridus. 



At first it appeared to me that only the females of Stenorlupichus 

 were infested by the parasite, which appeared the more surprising 

 because in StenorJv/nchus plialanyhun the number of males is much 

 greater than that of the females. A more careful examination 

 showed me that the male sex has no indemnity, although it seems 

 to be less frequently attacked (about one in six). 



In the females the influence of the parasite makes itself felt ex- 

 ternally by a profound modification of the four pairs of ovigerous feet. 

 These appendages are much reduced, although we cannot attribute 

 their atrophy to wearing caused by the friction of the Saccidina. I 

 have indeed ascertained that in adult females in which the recently 

 evaginated Saccidina was stUl small, the ovigerous feet already pre- 

 sented the poor appearance of aborted organs. 



I soon observed infested Stenorhi/nchi, apparently quite similar 

 to the preceding, in which these feet did not exist at all, but in 

 which I easily found copulatory styles, greatly reduced it is true, 

 and a different position of the genital aperture. These individuals 

 were males, the tail of which, however, had all the external charac- 

 ters of the female appendage, and seemed arranged so as to shelter 

 the parasite as perfectly as it shelters the ova in the other sex. 



Moreover, the secondary sexual characters of these infested males 

 were also modified in the same direction as the primary characters. 

 The chehe, instead of being strongly developed, were reduced, and 

 did not much exceed the head as in the normal males ; in one word 

 they presented the same arrangement as in the females. All these 

 peculiarities are the more striking because in the normal condition 

 Stenorhi/nchus is one of the Brachyurous Decapods in which sexual 

 dimorphism is best shown. 



To find facts comparable with those just set forth we have to 

 invoke the efi'ects produced by castration in the higher Yertebrata, 

 and the appearance in eunuchs of certain secondary sexual cha- 

 racters usually belonging to the female sex. 



From another point of view, the false finality, unfavourable to the 

 crab, which causes the appearance in one sex of a character of the 

 other sex with the apparent object of protecting a parasite, is not 

 the only example presented to us by nature of this sort of struggle 

 between natural selection and sexual selection. Do not we see the 

 stamens of Melandfi/um dioicum, normally aborted in the female sex, 

 become developed nevertheless when the plant is infested by (Jsti- 

 laijo antherarum, so that the plant becomes, apparently, hermaphro- 

 dite in order to allow of the fructification of the parasite ? 



It is probable that the observations which I have made upon the 

 Sacculina of Stoiorhi/nchus may be extended to other species, and 

 especially to the SaccuUna neglecta of Inachiis scorpio, which, accor- 

 ding to Fraisse, only infests the females. Therefore I make a point 

 of declaring that I abandon the argument which 1 had drawn from 



