THE IANTHINA. 133 



bladders, the Ianthina floats at ease, — not left, however, to 

 be driven at random by every current or breeze which may 

 sweep across its path, for its course is guided by means of a 

 small fin, which runs along each side of the foot a little 

 above its edge. It is only when the " tempest's breath" 

 blows hard that the snail yields to its violence, and suffers 

 wreck on the unfriendly shore. Of this interesting mol- 

 lusk Dr. Browne says, that " it probably passes the greatest 

 part of life at the bottom of the sea, but rises sometimes to 

 the surface." * On the contrary, all recent observers assure 

 us that it has no power of even sinking in the water. The 

 opinion of Browne long prevailed, and we were told that the 

 air-bladders could be thrown off and renewed at pleasure, as 

 the animal wished to sink or rise. Nor am I convinced that 

 this is an erroneous supposition, for certainly the vesicles 

 are often found swimming detached. Cuvier supposed that 

 the snail might be able to compress the apparatus to such a 

 degree as to allow its withdrawal within the shell, when the 

 body would sink by its own weight, and that it would rise 

 again when the snail relaxed its muscular efforts by the 

 natural elasticity of the gaseous contents expanding the 

 vesicles to their full volume. f This conjecture Mr. Bennet 

 has disproved. He found that when the Ianthina was pur- 

 posely irritated, it had no power of retracting its float : — 

 ' On the animal being touched, in ever so slight a degree, it 

 produced the effect of causing it immediately to withdraw 

 itself into the shell, and even at first, on any person moving 

 near the glass of water in which it floated; but then the 

 frothy appendage always remained stationary." J Nor is 

 the float, although undoubtedly the buoyancy depends in a 

 great measure on it, essential to the support of the snail at 

 the surface, for Dr. Browne and Mr. Bennet frequently 

 captured specimens there which had it not; whence the 

 latter infers " that they can float on the surface of the water 

 equally with or without it, although it must be naturally 

 supposed that the animal and its shell cannot be retained so 

 long on the surface without as with it." 



The connexion of this organ with the foot does not 

 appear to be organical ; it is merely fixed in its place by 

 some albuminous secretion, and can be detached without any 

 laceration or wound. The nature of this connexion be- 

 tween the parts greatly supports the conjecture that the 

 animal may cast it off after a season by a natural exfoliation, 



* Hist, of Jamaica, p. 400. t Mem. xv. 5. 



t Med. Gazette for 1834, n. 233. Also Grant in Proceed. Zool. Soc. 

 iii. 14. 



