ASSORTIVE MATING IN CHROMODORIS ZEBRA 277 



former position when the rod is withdrawn. If one individual of 

 a pair be slowly moved away from its mate, the papilla of the 

 untouched nudibranch endeavors to follow its partner in its en- 

 forced retreat. The termination of the preparatory manoeuvers 

 comes when the penis of one Chromodoris is inserted into the 

 central opening of the papilla of its mate. The reproductive 

 apertures are in this case pressed together by muscular contrac- 

 tions, and not held in place, as in some other nudibranchs, 

 either by copulatory clasping organs (Cumanotus; Eliot, '08), 

 or by a thickened mucous secretion (Montagua; Smallwood, '03). 

 The penis is unarmed. 



I have made attempts, but without success as yet, to deter- 

 mine whether or not that individual of a pair which has been 

 most active in conjugation (the moving member of the typical 

 pair previously described) is in every case, or in most cases, the 

 first to engage in insemination. If the pair be undisturbed, 

 mutual insemination frequently occurs, and I believe it to be 

 usually the case here, as in other large nudibranchs. 



Finally, some examples may be cited which serve to illustrate 

 the efficiency of the process of mutual selection with respect to 

 size. Where the difference in the length of two specimens 

 amounted to 5 or 6 cms., the 'trial and rejection' (to use a con- 

 venient but misleading phrase) occupied but a few minutes. 

 The observations here given are typical of many others which 

 were collected. 



Expt. 10.42. April 18, 1917. Chromodorids maintained in the 

 laboratory three and one-half months, without food. Their sizes had 

 decreased to such an extent that most of the specimens were one-half 

 to two-thirds their original length. The following pairs were ob- 

 served to be in copulation at one time, there being in all 13 specimens of 

 different sizes in the one aquarium. 



