MOTION 3i5 



On 25 September 1957 I removed the air-sac from a moribund, 7-in., right-handed specimen at 

 Hastings— the last specimen of a swarm that began to come ashore in westerly winds on 10 September. 

 About the same time, specimens were coming ashore also at St Jean de Luz, Bay of Biscay, and at 

 Almunecar Granada in the Mediterranean. I took the excised air-sac, which I had placed unfixed in 

 a jar of medicinal paraffin, to the Government Chemist's department on 26 September. On the next 

 day, 27 September, Mr D. Green made the following analysis of the contents, whose cubic capacity 

 he estimated at 200 c.c. The gas was liberated from the sac under brine and was found to contain 

 carbon dioxide 05% by volume, oxygen 19-9% and nitrogen (by difference) 79-6%,* as compared 

 with the average composition of atmospheric air: C0 2 , 0-04%; O, 20-99%, and N plus inert gases 



78-98%. 



Schloesing and Richard (1896) gave the following analysis of float-gases shown in Table 6: 



Table 6 



Percentage of 



W 



Gas{c.c.) O N A CO, 



410 15-1 82-02 118 17 



Wittenberg (1958) has recently reported finding, in specimens taken at Woods Hole, a fifth com- 

 ponent, carbon monoxide (traces to 8%). The majority of specimens contained from 1 to 5% CO, 

 which accounted for all the combustible gas present. 



Motion Relative to Wind and Water 

 Physalia has a characteristic orientation relative to the wind. The aboralf half is free of appendages and 

 the animal floats with its long axis at an angle of about 45 ° from the down-wind direction. The tentacles 

 and appendages are borne on a bulge on the oral half of the animal, the tentacles streaming out on the 

 windward side and acting as a drogue or sea-anchor. The bulge is situated either on the left or right 

 side of the float (Text-fig. 5), and I think that left or right-handedness in a particular individual must 

 be established on the first windy day that the larva keeps to the surface. The larval tentacle would 

 cause a drag on the windward side, so that the float would be blown (so to speak) to leeward. As the 

 new tentacles grow— and we know that their development is very precocious— this drag would be 

 increased, and the part of the float from which they are budded would become bowed-out to windward 

 as a bulge, resulting fortuitously in a left- or right-handed individual. 



We may liken the steadily drifting Physalia to a sailing vessel hove-to on either the port or star- 

 board tack. In life the oral end heads up to the wind, an important point to remember when observing 

 the behaviour of living specimens in a breeze. Right-handed individuals appear to be on the port tack 

 and left-handed ones on the starboard tack. 



The animal, being asymmetrical, does not drift straight downwind. I have to thank Dr Henry 

 Charnock, Reader in Oceanography at Imperial College, London, for a tentative explanation of its 

 direction of drift. Suppose we consider a grossly simplified model of a left-handed animal seen from 

 above, with a crest AB and the point of attachment of the appendages (the bulge) at C (Text-fig. 6a). 

 In a given wind a force is exerted at X. This force has two components, one normal to the crest AB 

 and the other along the crest; their resultant is R (Text-fig. 6b). After resting in this position during a 



* Presumably this figure includes the inert gases. 



t Huxley (1859), Haeckel (1888), Schneider (1898) and Okada (1932) all regarded, mistakenly I feel, the aboral (pore) end 

 as anterior and the oral (protozooid) end as posterior. 



