ON SURFACE TENSION AS AN AID TO LOCOMOTION 

 AMONG MARINE ANIMALS. 



BY JAMES HOBNELL. 



It has long been known that certain species of nndibranch 

 molluscs can crawl in an inverted position on the surface of water, 

 taking advantage of that peculiarity residing in the surface film 

 known as surface-tension, but as I doubt if the extended range of 

 this habit is generally known, I venture to enumerate several 

 instances that have come .under my personal observation ; some of 

 these have probably not been previously recorded. 



The opisthobranchj molluscs utilize surface tension the most 

 commonly. No species that I have watched fails to practise the 

 habit. Eolis papillosa, Doris tuberculata, D. pilosa, Elysia viridis, 

 Pleurohranchus membranaceus, and- P. plumula, continually do it, 

 and small individuals of Aplysia punctata at times adopt it. Size 

 appears to have little controlling effect, as large Doris, 5 inches long 

 and 3 inches broad, progress thus as easily and as rapidly as the tiny 

 Elysia. Pleurohranchus plumula is, however, the most persistent in 

 the habit, sometimes passing hours together moving or at rest in the 

 inverted position at the surface. Eolis comes next in point of fre- 

 quency in similar habits. 



The little Cowry, Gyproea europosa, is another and even more 

 interesting instance. In confinement, in a tank, it frequently crawls 

 inverted along the surface of the water, and occasionally may be seen 

 to form a little disc of mucus from which it lowers itself gently by a 

 mucous thread till it hangs in mid- water, dangling in the fashion of 

 a spider at the end of its silken cord. The cowry's disc of mucus 

 thus functions as a float, supported not by any inherent lightness as 

 is cork or wood, but by the aid given by the tension of the surface 

 film being sufficiently great to prevent the mucous mass from breaking 

 through. 



This habit of the cowry is to be correlated to that more familiar 

 and natural one so readily verified by any observer who visits the 

 low-tide caves and gullies where, among sponges and ascidians, this 

 animal loves so to live. Here, when the tide recedes, cowries more 

 or less enveloped in their bright coloured mantle lobes, are often seen 

 passively hanging suspended from the gully's roof, or from points 

 and jutting ledges by a stout mucous thread. 



It is noteworthy that Eolis occasionally suspends itself from the 

 surface of water in a similar manner to the cowry, that is by a mucous 

 thread pendant from a float of the same nature. Several times I have 

 noticed this to occurjwhen Eolis has been crawling inverted along 

 the surface. The length of the mucous cord was often as much as four 

 and five inches. 



Passing to another group, we find that many of the smaller 



