The form and function of the reproductive system in the New Zealand sacoglossan Elysia maoria Powell is described and compared with the reproductive systems of other members of the suborder Elysiacea. E. maoria has numerous gonadial follicles lying in the parapodia. Sperm-producing and ova-producing follicles are distinct and in definite zones. Above them are the diffuse and branched prostate glands, and below them are the reticulate albumen glands. Multiple bursal sacs lie in two or more pairs near the centre of the body. There are two genital apertures and no vagina. Male reproductive structures mature, and male reproductive behaviour begins, before female. Full sexual maturity is reached when an animal is about 20 millimetres long.
Impregnation is hypodermic, the penis of one animal being thrust apparently into the haemocoele of its partner. Most insertions were at the posterior end of the body. The method of penial penetration is uncertain, for the penis in this species, unlike that in other Elysiacea in which hypodermic impregnation occurs, bears no hollow spine. The fate of sperm at copulation is not known. In spawning the ova pass to the exterior through the glandular oviduct where they receive an albumen coat and a membrane and are bound into a ribbon by two mucous coats. The spawn bands, which are flat white coils attached to the green alga on which the animals live, hatch about two weeks after spawning. Larvae kept in the laboratory showed no tendency to settle after two weeks, suggesting that the planktonic stage may be fairly long.
Comparison with other Elysiacea shows E. maoria to be one of the primitive members of the suborder, with its very diffuse reproductive system, large number of small eggs, and long planktonic stage.