EOLIS. 141 



" Many of the aquatic members of this class (Gas- 

 teropoda) are able to float at the surface, by means 

 of their broad expanded muscular disc or foot. They 

 crawl to the top of the water, up the stems of a plant 

 or the side of a rock, and stretching out the bottom 

 of the foot along the surface, the back being down- 

 wards, it presently dries by contact with the air; 

 while it remains dry, it will float the animal, which 

 then glides along as if on a solid body, crawling, in 

 fact, on the inferior surface of the air ; but, if by any 

 agitation of the water, or by the will of the animal, 

 the foot become overflowed, the state of suspension is 

 ended, and the creature falls to the bottom." 



My first experiment led me to a very different con- 

 clusion. Before meeting with the above passage, I 

 had written as follows : 



The Eolis, I believe, swims on the surface of the 

 water by a very simple process, viz., by ejecting a 

 thick transparent slime, which floats on and partly 

 blends with the water. To this gelatinous substance 

 the animal attaches itself, and so becomes perfectly 

 buoyant, roaming about wherever its will directs. 



The truth of this statement can easily be tested. 

 Take up an Eolis, lay it gently on the surface of the 

 water, try by every means in your power to make it 

 stick there. The result will be a failure, for the crea- 

 ture will assuredly sink to the bottom whenever you 

 release your hold. But, supposing it has thought 

 proper to climb the sides of the tank, and crawl on 

 the face of the fluid contained therein, take a piece of 



