Balanus balanoides (L.) is a cross-fertilizing hermaphrodite. The spermatozoa which are inseminated into the mantle cavity are initially inactive. Eggs are laid into the two elastic sacs of the oviducal glands (see Walley, 1965), which expand into the mantle cavity as more eggs are laid. The discharge of eggs into the elastic sacs also forces fluid out of the oviducal glands into the mantle cavity where the spermatozoa have been deposited. The fluid activates the inseminated spermatozoa (Walley, White & Brander, 1971; Walker, 1977).

In order to effect fertilization of the eggs activated spermatozoa must pass across the oviducal gland sac wall. How the spermatozoa accomplish this is the subject of the present investigation.