Copyright J. Anderson www.nudibranch.org/Scottish%20Nudibranchs/aplysia-punctat…
Two A. punctata mating, upper (U) and lower (L).
Seminal groove emerges from common genital aperture of U (1) and traverses neck and head (2) to base (3) of everted penial sheath and its right ventral side (4) to the extended penis inserted into common genital aperture of L; concealed by anterior of L’s right parapodium (5).
Anterior of foot (6) of U grips mantle and shell via foramen of L. Parapodium (7) of L opened wide to allow U to grip its mantle and shell.
Anal/exhalent funnel (8) protrudes to function during copulation.
Raised walls along sides of seminal groove meet firmly at edges, but do not fuse, to form tube lined with cilia that propel the contents.
A. punctata often extrude spawn during coupling, and it often passes from the common genital opening along the seminal groove, escaping through a wrinkle part-way along or continuing to emerge (confusingly) at the penial aperture.

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