274 W. J. CROZIER 



Sometimes conjugation is effected with less trouble, or more 

 speedily, because the specimens are of nearly identical size and 

 are from the first so situated that their right sides are in con- 

 tact (figs. 17, 18); in other cases, difficulty is experienced in 

 bringing the reproductive openings together. Two nudibranchs 

 differing greatly in size are, under ordinary conditions, unable 

 to accomplish this (fig. 19), and, following futile attempts at 

 copulation, will (usually after a short time) wander apart. 



The whole character of the animal's behavior in these experi- 

 ments supports the contention that the nudibranchs are 'at- 

 tracted' to one another as a result of reactions to specific secre- 

 tions; the 'epidemics' of conjugation in aquaria, referred to in a 

 previous section, may also be cited in this connection. The con- 

 summation of pairing depends upon these specific reactions (in- 

 cluding those of the terminal papilla of the genital ducts), and 

 upon the mechanical condition that the papillae may be brought 

 together. 



A certain amount of equalization of 'size' does occur, but it 

 is not efficient in promoting copulation between individuals dif- 

 fering several centimeters in length. The openings of the repro- 

 ductive ducts are situated in the same relative position in speci- 

 mens of all sizes, the distance from the mouth and height above 

 the surface of the foot (when the animal is quietly creeping) 

 being each directly proportional to the total length of the ani- 

 mal (fig. 20). Contractions of the general body musculature, 

 and movements of the protruded genital papilla, are both in- 

 volved in the efforts to overcome differences in the sizes of indi- 

 viduals endeavoring to mate; the latter movements are the less 

 important. These nudibranchs can contract to such an extent 

 that the length 'over all' is about 80-85 per cent of its normal 

 extent when not specially contracted; the distance between 

 mouth and genital atrium can be similarly shortened. In the 

 copulation of two specimens of equal size the animals are some- 

 times contracted to a certain extent, but the usual condition in 

 such a case does not involve noticeable shortening. The prin- 

 cipal effect of the shortening is to bring about an elevation of 

 the genital orifice. It may in this way be employed by a smaller 



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