532 Natural History of Molluscous Animals : — 



" To taste the freshness of heaven's breath, and feel 

 That light is pleasant, and the sunbeam warm.'* 



When thus suspended they will sometimes relax their hold 

 iind drop at once to the bottom, from which, in general, they 

 emerge by crawling up some solid body: but occasionally I 

 have seen them rise up direct through the water ; a fact 1 can 

 explain only by supposing that they have the power of com- 

 pressing, in the first instance, the air in their pulmonary 

 cavity, and of again allowing it to expand and dilate so as to 

 render the body lighter than the medium in which they live. 



One pretty lacustrine species, the Physa fontinalis, can let 

 itself down gradually by means of a thread affixed to the sur- 

 face of the water * ; a manner of proceeding which finds an 

 analogy only in some land slugs, which have been observed 

 to spin a line of the glutinous secretion from their skin, and 

 thus let themselves down from trees and over precipices. 



I have said that many freshwater Molliisca occasionally 

 float at ease, but there is a marine genus to which this is 

 habitual, nor does it seem certain that it can change its place 

 in any other way. This genus is the lanthina, which, by the 

 aid of a spongy organ, attached to the posterior part of the 

 foot, and composed of little vesicles, apparently filled with 

 air f , floats without any exertion, and probably directs its 

 course by means of a small membrane, which runs along 

 each side of the foot, a little above its edge. The common 

 ■species is an inhabitant of the seas of the West Indies, and it 

 has sometimes been driven on the shores of Scotland and of 

 Wales, no willing visitant, you may believe, of these northern 

 climes, yet treated by British naturalists as a native of them. 

 As the animal is really one of much interest, I will transcribe 

 for you what Brown says of it in his Account of Jamaica : — 

 " The creature probably passes the greatest part of life at the 

 bottom of the sea, but rises sometimes to the surface, and to 

 do so, it is obliged, piscium more [after the manner of fishes], 

 to distend an air-bladder ; which, however, is formed only for 

 the present occasion, and made of tough viscid slime, swelled 



tible undulation ; but their progression, by means of some concealed rotation 

 "or unknown mechanism, is not more slow than that of land snails." 



* Montagu, Test. Brit. p. 227. 



-|- Cuvier thinks that this organ bears some analogy to the opercula of 

 other univalves, and that it may be a vestige of an operculum which has 

 undergone such modifications in its form and structure as we frequently 

 observe in the productions of nature. {Soiverby^s Genera, No. v.) I con- 

 -sider this as an example of those false analogies or affinities which so much 

 ■abound in modern works on natural history, and which seem got up for no 

 other purpose than to prop a favourite theory. 



