APLYSIA. 245 



sheath (pn. s.) is a muscular cul-de-sac whose blind end, to 

 which the penis is attached, is posterior. The walls of the 

 sheath are attached by numerous muscles to the body wall 

 in this region, and there is a large retractor muscle situated 

 posteriorly. On slitting up the sheath the penis itself is seen 

 attached to the blind end by its base, with its free end directed 

 forwards. In shape it is spatulate or like a leaf with the 

 edges curled upwards. The surface is chitinised, but does not 

 bear nodosities as in Aptly sia depilans and A. limacina. A deep 

 groove runs along its dorsal side. On the ventral side is a linear 

 longitudinal groove of uniform diameter throughout. This is 

 the seminal groove, which terminates close to the tip of the 

 penis and can be traced backwards to its base, then forwards 

 along the right ventral side of the penis sheath and over the 

 edge of the penial aperture, where it is continuous with the 

 lateral seminal groove on the exterior of the body. When the 

 penis is everted by afflux of blood and relaxation of the muscles 

 attached to its base, it is thrust from its sheath, the latter 

 becoming evaginated. The seminal groove is thus straightened 

 out. When the penis is withdrawn the groove becomes S 

 shaped. There is no prostatic gland in Aplysia punctata. 



It is now necessary to describe the internal structure of 

 the parts of the reproductive system. The hermaphrodite 

 gland is a racemose gland consisting of innumerable acini, 

 each of which produces eggs and sperms, which, when ripe, 

 are discharged into the cavity of the acinus. The various 

 acini are connected with the little hermaphrodite duct. Young 

 individuals are protandric and produce only sperms, but 

 later both eggs and sperms are found in the same acinus, 

 though the latter are always the more numerous. The ova 

 are discharged in an unripe condition, so that their fertilisation 

 by the sperms of the same individual is impossible at this 

 stage. The ovum is a large cell with distinct nucleus and 

 nucleolus, and is crowded with yolk granules. The sperms 



