APLYSIA. 251 



curls round the "tail" of A. The parapodial lobes of A open 

 widely and embrace the body of B, but the anal spout protrudes. 

 The penis of B is then thrust from its sheath and curves 

 downwards and backwards in the form of an inverted U, to be 

 inserted into the common genital aperture (vaginal portion) 

 of A. The whole of the penis is thrust into the vagina and 

 reaches to the base of the vaginal pocket close to the spermato- 

 cyst, as already described. Sperms pass along the spermatic 

 groove of B, along the penis groove, and into the vaginal 

 cavity of A. Individual C, in the same way, inserts the 

 penis into the vagina of B, fixing itself above and behind B. 

 Owing to the curvature of the body of each of the individuals 

 forming the chain, a large chain rolls itself almost into a ball. 

 The coupling lasts for several hours or even days. Judging 

 by the emptiness of the spermatic groove in many individuals 

 that were examined during the long coupling period it is 

 probable that the actual passage of the sperms takes place 

 in a few minutes. 



There is always a certain amount of debris introduced 

 with the sperms, and this, agglutinated by the secretion of 

 the glands lining the wall of the bursa seminalis, is drawn 

 up into the spermatheca, where it is either absorbed or dis- 

 charged through the external aperture. No satisfactory 

 evidence, however, has been obtained on this point. The 

 sperms travel down the vagina to be stored in the spermatocyst 

 at its base. It is not known where the separation of detritus 

 from pure sperm takes place. Mazzarelli states that the 

 spermatheca receives all the spermatic fluid, but the description 

 given above shows that this is not the case. Further, the 

 spermatheca never contains many sperms either during the 

 quiescent period or immediately after copulation. Whenever, 

 however, the spermatheca is full of detritus the spermatocyst 

 is full of pure sperm. It seems probable, therefore, that the 

 separation of the detritus from the pure sperm takes place on 

 their entry into the vagina, and not after an interval. This 



