Duration of the incubation period.

Because artificial fertilisation was unsuccessful with O. lutaria, the duration of the incubation period was studied by regular examination and measurement of larvae liberated by oysters held in the laboratory.

A larva takes approximately five days from fertilisation to develop into an early veliger. The time of development between an early veliger and a middle veliger without pigment spots is approximately three days. The shell measurements of early veligers are 0.22mm × 0.18mm (length × height) and total measurements are between 0.32mm × 0.29mm and 0.33mm × 0.32mm. The shell measurements of the middle veliger are between 0.30mm × 0.22mm and 0.31mm × 0.19mm and total measurements are between 0.40mm × 0.29 and 0.35mm × 0.32mm. The page 20

Text-fig. 5.—Fig. A—Veliger with foot fully extended. Fig. B—Veliger with velum and foot in swimming position. Fig. C—Veliger with velum and foot withdrawn. Fig. D—Spat settled for approximately 72 hours.

Abbreviations: A., anus; a.a., anterior adductor muscle; an.s., anterior suture; a.s., apical sensory organ; b.g., byssus gland; diss.s., dissoconch shell; e.s., eyespot; fo., foot; g.p., gill plate; h., hinge; he., heart; int., intestine; lab.p., labial palp; m.e.m., mantle edge; mo., mouth; p.a., posterior adductor muscle; p.g., pedal ganglion; prod., prodissoconch; p.s., posterior suture; rec, rectum; r.v., radial vessel; vel., velum; vis.m., visceral mass.

page 21 total time taken for an early veliger to develop into a veliger with pigment spots is unknown, but observations made on oysters held in the laboratory in sea water between 18.0°C. and 20.0°C., suggest that it is five to eight days. A fully developed veliger measuring 0.47mm × 0.35mm is liberated after about a further six days. Thus the total time of development within the mantle chamber appears to be about 21 days.

The stage of development reached by the larva under natural conditions before it is liberated is discussed in the section on the free swimming larva, but briefly if O. lutaria follows other incubatory species of oysters, the larvae will be liberated when they have attained a size of about 0.32mm × 0.26mm. Larvae of O. lutaria with these measurements were liberated in the laboratory, but more fully developed larvae complete with foot and pigment spots were also observed being liberated from the parent indicating that if conditions are favourable the parent oyster will incubate the larvae through all the stages of development until the latter are ready to settle.

Liberation: In O. lutaria the developing larvae are liberated by violent contraction of the posterior adductor muscle of the parent. This fast closure of the shell forces the larvae out through a gap in the inner pallial fold as in other incubatory species.

Liberation of the larvae in some cases was completed within an hour; in other instances the liberation took as many as four days to complete. The only regular feature of liberation that was observed was that the majority of larvae were liberated in the first two or three rapid closures of the shell and that subsequent liberations yielded very much lower number of larvae. Towards the end of liberation, larvae appeared to "spill" out of the shell and over the left valve to the dish beneath. The last few larvae were always held in strands of mucus that was probably secreted by the gills during the process of feeding.