The prosobranch snail, Hydrobia ulvae, lays eggs on the shells of neighbouring individuals where they are held together and protected by a covering of sand grains. The eggs develop within this egg mass, hatching as veliger larvae for which there are several descriptions (Henking, 1894; Thorson, 1946; Pilkington, 1971; Fish & Fish, 1974). It is clear that there is considerable variation in the size, pigmentation and swimming behaviour of veligers from different localities. The material used in the present study was from a population of snails in the Dovey estuary, Wales, which have free-swimming veliger larvae similar to those described by Thorson (1946). These have a single whorl to the shell; a shell length at hatching of 140–170 µm; a semi-lunate band of purplish black pigment on each velar lobe; a narrow band of pigment between the eyes and black pigment associated with the hindermost part of the intestine. In contrast, those from Ply-mouth hatch with one and a half shell whorls and a colourless velum (Lebour, 1937; Pilkington, 1971); a shell length of 340–390 µm and show complete suppression of the pelagic phase, the velar cilia being short and incapable of lifting the larva off the bottom (Pilkington, 1971). It has been suggested by Pilkington (1971) that the differences in size and swimming behaviour might be the result of the Plymouth larvae hatching at a later stage. The present account is the result of investigations into the effects of temperature and salinity on the survival of embryos to hatching and the duration of embryonic development.