626 Natural History of Molluscous Animals : — 



to be separated by any dissection {Ibid., vol. v. p. 16.) ; and 

 this excretion, which is common to many other naked mol- 

 lusca, will probably be found to be the urinary discharge. In 

 some univalved mollusca, the source of this discharge has 

 been ascertained. Swammerdam detected in the snail a little 

 oblong triangular part, placed near the heart, which he calls 

 the " sacculus calcareus." This organ has a pretty large 

 duct, which runs into the intestine; and Swammerdam be- 

 lieves it to be a gland whereby the calcareous matter of the 

 blood is drained from the body, and deposited in the intestine ; 

 " and accordingly we Bnd that such a matter is sometimes 

 mixed with the excrements." (Book of Nature, p. 49.) The 

 organ is found, in a modified shape, in many other mollusca; 

 and some naturalists * have imagined that the shell was 

 formed by it, misled apparently by the name given by Swam- 

 merdam ; 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 {Mem., vol. xi. p. 26.) : but Mr. Jacobson has 

 proved that it performs the functions of a kidney. " Chemical 

 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 calcareous 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." {Edin. Journ, Nat. and Geogr, Science, 

 vol. iii. p. 325.) 



7. All molluscous animals excrete a mucous fluid to lubri- 

 cate the skin, furnished by the skin itself, or by some crypts 

 situated in it.f This mucus is, in general, possessed of no 



* " The formation of the calcareous matter of their shells, which takes 

 place in a peculiar viscus lying near the heart (sacculus calcareus Swamm. 

 glandula testacea Poll).''* Blumenbach's Man. Comp. Anat., p. 251. transl. 



f " In the gasteropodous mollusca of the genera Z/imax, Ariow, Helix, 

 and Bulimus, we find under the mouth, between the two inferior lips, and 

 the protuberance of the disk of the foot, the orifice of a canal, hitherto 

 unobserved, which runs along the whole of the foot. This anatomical 

 arrangement is not very distinct in the genus Succinea, which approaches 

 nearer to the Lymnae^se in internal structure. In the Arion empiricorum, 

 which is entirely black, we perceive a trace of this canal, which appears in 

 the form of a whitish band. The canal is not simple; it receives many 

 little ducts, which come from the muscular sac in which the viscera are 



