MALACOLOGY. 663 



side of the body, as in the vertebrates and insects ; but many 

 possess a fleshy tongue-like appendage, called a foot, which is 

 used, in some cases, either for progression, as in the snail, or 

 adhesion, as in the limpet and chiton, (see Chart, and Plate 

 XVII.) The location of this organ, on the lower part of the body 

 of univalve Mollusks, (see figures of Harpa, Buccinum, and a 

 Haliotes, on the Chart,) suggested the distinctive term for the 

 order, GASTEROPODA. The organs of respiration are always dis- 

 tinct, and present the form of gills, i. e., blood-vessels dividing 

 into parallel branches, which are brought into contact with the 

 air contained either in the atmosphere or in the water. 



The blood of the Mollusks is white, bluish, or limpid. There 

 is always a heart in them, but it is singularly placed; indeed, 

 some of them seem to have several hearts. In no other animals 

 is the circulation more unequal ; but always, however, there are 

 blood-cavities into which the blood-vessels open, and from which 

 other vessels arise and diffuse again the blood into the organs. 

 The stomach is sometimes simple ; sometimes divided into sev- 

 eral parts ; there is always a large liver. 



In some, the' sexes are separated ; and in others united ; all 

 of them produce eggs, which, in some cases, are deposited ex- 

 ternally ; in others, hatched within, so that Mollusks are either 

 oviparous or ovoviviparous. The young of all have, from the 

 first, nearly the form which they present when mature. 



The soft, and usually sensitive skin, frequently forms plaits or 

 folds enveloping the body either wholly or in part. The portion 

 of covering thus formed, is termed the mantle. It is often almost 

 entirely free, presenting two large laminae or lobes, which cover 

 the rest of the animal, as in the Cyprrea; or the two laminae 

 unite so as to form a kind of tube, as in the Solen or Razor- 

 Shell. Sometimes the mantle forms a sort of disk, of which the 

 margins only are free ; or it surrounds the body in the form of 

 a bag. 



In a large number of Mollusks, the soft skin is protected by a 

 sort of calcareous crust, which is secreted from the mantle, in 

 deposits of successive layers, composed of a kind of glutinous 

 substance, mixed with carbonate of lime, differing, as Prof. 

 Dana has shown in his admirable work, (see Narrative of Ex- 

 ploring Expedition,) from the Polyps, (Radiates,) in which the 

 limestone portions form a part of the animal, and are not mere 

 excretory matter, resembling shell. Sometimes the whole shell 

 appears to be horny, but most commonly the calcareous portion 

 predominates, and the inner surface is more compact than the 

 other. 



