134 GASTEROrOD SPINNERS. 



when it may sink to the bottom for a certain time ; for, not- 

 withstanding all that has been lately said, I am inclined to 

 go into Dr. Browne's belief of its living there ; and am not 

 unwilling to believe further, that its floating may be during 

 the breeding-season only, for, as we shall afterwards see, the 

 vesicular appendage is likewise a sort of ovarian receptacle. 

 It is no substitute for an operculum, as Cuvier states,* 

 because it does not adhere in the same manner, nor in the 

 analogous place ; for instead of being situated above the 

 posterior part of the foot, it is below :f the organ is, in fact, 

 a special contrivance made for a specific use. J 



The habits of the Litiopa are not less worthy of your 

 notice. This is a small snail, born amid the gulf-weed, 

 where it is destined to pass the whole of its life. The foot, 

 though rather narrow and short, is of the usual character, 

 and, having no extra hold, the snail is apt to be swej^t off its 

 weed ; but the accident is provided against, for the crea- 

 ture, like a spider, spins a thread of the viscous fluid that 

 exudes from the foot, to check its downward fall, and enable 

 it to regain the pristine site. But suppose the shock has 

 severed their connexion, or that the Litiopa finds it neces- 

 sary to remove, from a deficiency of food, to a richer 

 pasture, the thread is still made available to recovery or 

 removal. In its fall, accidental or purposed, an air-bubble 

 is emitted, probably from the branchial cavity, which rises 

 slowly through the water, and as the snail has enveloped it 

 with its slime, this is drawn out into threads as the bubble 

 ascends ; and now, having a buoy and ladder whereon to 

 climb to the surface, it waits suspended until that bubble 

 comes into contact with the weeds that everywhere float 

 around ! § From the observations of Mr. Gray, it appears 

 that the Rissoa parva of our coasts had somewhat similar 

 habits; II and the Cerithium truncatum, which is generally 

 found in brackish water in mangrove swamps and the 

 mouths of rivers, does sometimes suspend itself from the 

 boughs and roots of the mangrove by a glutinous thread.^ 

 We have also a native lacustrine species, the Physa fonti- 

 nalis, which can let itself down gradually, like the Litiopa, 



* Mem. XV. 4. See also Pioc. Bost. Soc. N. Hist. i. 21. 



t Rang, Man. p. 25. 



+ " Mr. Parkinson rightly conjectures that the shells resembling the 

 Helix, or snail, in the older strata, were constituted for swimming, like the 

 lanthina : they could scarcely have used a foot for crawling at the bottom of 

 a deep and agitated ocean." — BakeweWs Geology, p. xxxv. 



§ Rang's Man. pp. 26, 198. Kicner in Ann. des So. Nat. xxx. 223. 



II Proc. Zool. Soc. iii. part i. 116. 



IT Mag. Nat. Hist. n. s. iii. 127. 



