ASSORTIVE MATING IN CHROMODORIS ZEBRA 271 



tactile and chemical senses seem the main, if not the only, 

 channels through which the maneuvers of conjugation may be 

 controlled and directed. 



The mating behavior of certain nudibranchs other than Chro- 

 modoris throws an interesting light upon this question, although 

 the material available for comparison is very limited. In the 

 littoral nudibranch Cenia, as described by Pelseneer ('99, p. 

 518, footnote 2), the three widely separated external genital 

 openings of one member of a mating couple are simultaneously 

 brought into close relation with the appropriate openings of the 

 other individual, the posterior female orifice of each receiving 

 the 'opposite' penis. If it be true that this type of conjugation 

 is necessary in Cenia (further observations on the point would 

 be valuable), and if mutual equalization of the sizes of conju- 

 gants be inadequate to overcome natural variation in the sizes 

 of reproducing individuals, then in this species there is an ob- 

 vious mechanical basis upon which assertive mating might be 

 required to operate. There is the difficulty, in the case of Cenia, 

 and of other ''Elysiens" which I have been able to study, that the 

 conjugating population is of very nearly uniform size, which is 

 decidedly not the case with Chromodoris. The important fact, 

 however, which indeed forms the basis of selection in pairing, is 

 the necessity for the reproductive openings to be brought into 

 close contact. In at least one nudibranch (Cumanotus) there 

 are present somewhat unusual 'clasping organs,' which help to 

 insure the close approximation of the respective male and fe- 

 male apertures (Crawshay, quoted by Eliot, '08). This is neces- 

 sary both in cases where the male and female openings are dis- 

 tinctly separated, and in species where they are concentrated 

 upon a relatively small papilla (Montagua, Smallwood, '03; 

 Chromodoris), and is of course the condition obtaining in some 

 other gastropods (cf. Meisenheimer, '07). C. zebra conjugates 

 after the fashion employed by other members of its family. 

 Two individuals come together with their right sides in con- 

 tact, and when the central aperture of the genital papilla of one 

 specimen is closely applied to that of its mate, the short conical 

 penis of one is inserted into the aperture of the other and insemi- 

 nation then proceeds. 



