250 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



filling of these spaces with blood, aided by muscular action, 

 the penis becomes tense and is thrust out of its sheath, appear- 

 ing large and swollen. Interlacing muscle fibres occur, but the 

 main muscle is a stout hollow band which occupies the centre 

 of the penis and is continuous with the retractor muscle at 

 the base of the sheath. 



Copulation and Oviposition. 



It is often the case in hermaphrodite animals (e.g., Earth- 

 worm, many Gastropods) that the passage of sperms from one 

 individual A to another individual B is effected simultaneously 

 with the passage of sperms from B to A. The possibility of this 

 depends to a large extent upon the relative positions of the male 

 and female apertures, which must be so arranged that lateral 

 coupling can take place. In Aplysia the mutual interchange 

 of reproductive elements between two individuals does not 

 occur, and if only two individuals come together one acts as 

 male and the other as female. After an interval the roles may 

 be reversed. Usually, however, a far more remarkable thing 

 takes place. A number of individuals, varying from three 

 to as many as ten, form a chain. (Fig. 5.) A member of the 

 chain fixes itself above and behind the preceding member of 

 the chain. The lowermost individual of the series acts as 

 female only, the topmost as male only, and all the intermediate 

 ones function as male for the preceding (i.e., lower) and female 

 for the succeeding (i.e., higher) individual of the chain.* 



Copulation may be observed in Aplysia punctata in spring 

 or in late summer. The first individual of the chain (A) fixes 

 itself by its foot to weed or rock. Individual B crawls over 

 the visceral hump of A and takes up a position with the 

 aperture of the penis on a level with the common genital 

 aperture of A. B then attaches the anterior portion of its foot 

 to the mantle of A and grasps it with considerable tenacity. 

 The posterior part of the foot of B, however, remains free and 



* Coupling in chains occurs also in other Gastropods, e.g., Crepidula. 



