246 MUCUS. 



formed by it, misled apparently by the name given by 

 Swammerdam ; for no opinion was ever more groundless 

 or hastily offered. Cuvier considers it as the source of 

 the mucus which snails excrete so profusely when forced 

 to withdraw suddenly into their shells, and with which they 

 fix their shells to smooth bodies : * but Mr. Jacobson has 

 proved that it performs the functions of a kidney. " Che- 

 mical analysis of the matter secreted by this organ, has led 

 him to discover in it uric acid, ammonia or calcareous salt, 

 and water. His experiments were made on the great snail 

 (Helix pomatia). He was unable to discover any trace of 

 uric acid in any other part of the animal. And as, in the 

 superior animals, the kidneys are the only organs which, 

 in a state of health, secrete uric acid; and as the calca- 

 reous sac of the snails has many other anatomical relations 

 with the kidneys, Mr. Jacobson concludes that this sac 

 represents the kidneys, and must be so considered in all the 

 mollusca which are provided with it."f 



7. Mucus. All molluscous animals excrete a mucous 

 fluid to lubricate the skin, furnished by the skin itself, or 

 by some crypts situated in it. This mucus is, in general, 

 possessed of no remarkable properties : it is usually colour- 

 less, but in some species milky or yellow, as you are aware 

 is exampled among our native slugs ; and the Clios, a genus 

 of marine pteropods, envelope themselves, when in danger, 

 with a whitish cloudiness that appears to exude from the 

 whole surface of the body. The smell also which certain 

 mollusks exhale, is probably a principle of this mucus. The 

 Octopus moschatus is distinguished for the " amber scent of 

 odorous perfume," which that cuttle exhales so strongly as 

 to fill quickly a whole apartment, whether the animal be 

 dead or alive, and whence it derives its specific designation.^ 

 I have already told you that the Aplysia of southern Europe 

 stinks disagreeably ; but, according to Rapp, the Tethys en- 

 tices us not more by its singular beauty, than by its odour, 

 which he compares to that of roses. Rondeletius might 

 doubtless be quoted for a very opposite virtue in Tethys, 

 but not by an advocate of the tribe. Helix pomatia smells 

 strong of hemlock, in the beginning of June, a smell which 



* Mem. xi. 26. 



t Edinb. Journ. Nat. and Geogr. Science, iii. 325. 



J Bosc maintains that ambergris derives its scent from this cuttle, on 

 which the whale feeds. Hint. Nat. des Vers, i. 48. 



"Odore est valdc ingrato et pisculento, nauseam movet, splendore 

 diutius inspectantibus dolorem oculorum capitisque adfert, id quod in me- 

 ipso sum expertus." RONDEL, de Pise. 527. 



