582 Natural History of Molluscous Animals : ey, 
* 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 
and 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 I 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 Mollisca occasionally 
float at ease, but there is a marine genus to which this is 
habitual, nor does it seem certain that i it can change its place 
in any other way. This genus is the Ianthina, 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 +, floats without any exertion, and probably directs its 
course by means of a small membrane, which runs along 
each side of the foct, 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, pzsczwm 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. (Sowerby’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. 
