Summary

1. The velocity gradient of the fluid close to the solid boundary is the most appropriate description of the conditions of water flow which affect the attachment of the larvae of sessile forms to solid objects. The nominal speed of the water movement past the object is of importance only in so far as it influences the velocity gradient in the boundary layer.

2. Experiments in glass tubes on cypris larvae of Elminius modestus and Balanus balanoides show that moderate velocity gradients exceeding 500 sec.^-1 sweep the cyprids past the surface before they can attach. For large objects exposed to turbulent flow, the critical velocity gradient corresponds very approximately to a flow of 1-2 knots.

3. Attachment under conditions of water flow is accompanied by negative rheotaxy, and can occur equally in the light and in the dark.

4. Maximum attachment occurs at or below velocity gradients just great enough for the cyprid to be able to maintain position by swimming along the surface against the current.

5. Once the cyprid has attached it cannot be pulled off the surface even by gradients greatly in excess of those which prevent attachment.

6. Cyprids can migrate in all directions when exposed to moderate gradients, but they do so only with difficulty when the velocity gradient is high, particularly if the water is flowing in the same direction as that in which the cyprids are walking. The direction in which the cyprid migrates is altered only momentarily by changes in the direction of the current, the animal actively resisting the redistribution of forces acting on it.

7. No direct evidence is given in this paper on the influence of water currents on fixation, but a critical comparison with other published work suggests that fixation can occur in places where the velocity gradients are greater than those which limit attachment, provided the cypris is able to migrate there after attachment.

8. Moderate velocity gradients have little effect on the orientation at metamorphosis. Individuals tend to settle with the anterior end pointing downstream rather than in any other position. This orientation is the opposite from that which would be expected if the cyprid were passively orientated by the water current, but is likely to make subsequent feeding more efficient.

9. The ability to attach under conditions of water flow, and the tendency not to attach under stagnant conditions, may have an important influence on the animals' distribution and survival.

10. Solid particles in suspension may profoundly influence the behaviour, hence the results given in this paper may not be relevant to conditions where scouring takes place.