321 



(It may be of interest to record here some observations that I made in May 1956 at Villefranche on 

 the sailing-behaviour of Velella, about which little seems to be known. The observations were made 

 from the causeway that links Le Port with La Darse. Small numbers of Velella were drifting slowly 

 in the light breeze close into the rocks, so that I could look down on them at range of about 15 yards. 

 After watching several individuals closely for 2 hr. or so, I came to the following tentative conclusions: 

 Velella orientates its long axis to the wind in much the same way as Physalia. Having a rather short 

 axis, it appears to be not very stable so that now and then it turns slowly, but eventually takes up its 

 original orientation again at an angle of about 40 to the wind. Right- and left-handed forms occur as 

 in Physalia, but no further opportunities arose, as I hoped they would, to continue a study of Velella.) 

 Since this report was begun, Woodcock (1956) published an interesting short article on 'Dimor- 

 phism in the Portuguese man-of-war', partly to give further details in support of his contention that, 

 in the North Atlantic, the animal is consistently driven by the wind to the left of the down-wind 

 direction. Whether Woodcock's figures would satisfy a statistician that left-handed individuals 

 predominate there, and if so to what extent, it is not for me to say, but in view of the large population 

 which must exist there I feel doubtful. I would agree that, supposing roughly equal numbers of the 

 two kinds started off on their drifting-voyages round the average cyclonic wind-system of the North 

 Atlantic, left-sailing Physalia might tend to sail away from the centre of the system, often in the 

 direction of coastal waters. This was pointed out to me previously by Dr Mackie, but it would be a very 

 big task to collect evidence of this. I have already started to fix the positions and tack of a few speci- 

 mens observed at sea, in the hope that one day a meteorologist might be induced to work out the 

 estimated positions of the same animals on a few days both before and after the time of observation. 

 It is only in this way that I think we shall obtain data on their circulatory movements. It seems to me 

 very improbable that ecological factors can act selectively for tack on each generation, or that dimor- 

 phism can be due to genetic differences. That anything like a ' tack '-change can be caused by 'migra- 

 tion of the tentacles on the pneumatophore ' seems to me quite out of the question, although as the 

 animal grows older, small branches of the cormidia do grow out to leeward. I very much like 

 Woodcock's chart of the Kaiwi channel, which shows clearly the effect of a change of wind on the 

 landfall of Physalia. His statement that off Deewhy Beach, Sidney, the right-sailers outnumber the 

 others is interesting ; but I doubt whether it will be confirmed, as Woodcock seems to hope, that most 

 Physalia in the southermost hemisphere sail to the right. 



But I can give him one more record of southern hemisphere specimens— sixty-seven 4-cm.-long 

 Physalia* collected by Prof. T. A. Stephenson at Muizenberg Beach, False Bay, S. Africa (prevailing 

 winds in summer, south-east). Only one of these was right-handed. 



For statistical purposes it is useless just to count animals of either tack from the southern hemi- 

 sphere. Woodcock (1944) said 'of twenty-two south latitude animals preserved at the Museum of 

 Comparative Zoology, Harvard University, and at the U.S. National Museum in Washington, 



* B.M. Reg. No. 1934, 3-I4- 1 - 



