236 Natural History of Molluscous Animals : — 



was the first to perceive their utility in this respect; and 

 when it is considered that their position mainly determines the 

 arrangement of the other viscera, and must consequently exert 

 a powerful influence over the habits of the animals, you will 

 feel disposed to admit that a happier choice could not have 

 been made, the more particularly as the organs in question 

 are in general easy to detect, and exhibit sufficient variety in 

 location and form for every systematic purpose. 



Molluscous animals are either Pulmbniferous, and breathe 

 atmospheric air only, or they are Branchiferous, and respire 

 it through the medium of water. In the former, the respi- 

 ratory organ is a simple cavity, commonly situated on the 

 anterior part of the back; but sometimes, as in Testacellaj 

 near the tail. The air is admitted by a small circular aper- 

 ture that opens outwards on the neck under the margin of 

 the cloak, and which the animal opens and shuts at pleasure. 

 Externally the cavity is protected either by a thick fold of 

 the cloak, often strengthened with a horny or calcareous 

 plate, or by the body-whorl of the shell ; while its interior 

 walls, and more especially its floor, are covered with a fine 

 vascular network, formed by the minute ramifications of the 

 pulmonary vessels, which thus expose the blood freely to the 

 influence of the air, alternately introduced and expelled by 

 the alternate dilatation and contraction of the cavity itself. 

 All the terrestrial Mollusca, such as slugs and snails, and the 

 great bulk of the Gasteropoda that inhabit fresh water, pos- 

 sess a respiratory apparatus of this kind; and, since these 

 aquatic Pulmonifera (Lymneus, Planorbis, and Ancyllus may 

 be quoted as examples) are necessitated, from time to time, to 

 inhale the fresh and uncombined air, so they will be found 

 uniformly to be the denizens of shallow waters, and to spend 

 a large portion of their lives at the surface. 



The Branchiferous Mollusca have the aerating organs 

 greatly more diversified in every respect ; and, to countervail 

 the disadvantages of breathing a medium little impregnated 

 with air, they are likewise of greater extent and complexity. 

 When placed within the body, the branchiae, if distinct, are 

 divided into multiplied lobes and leaflets; or, if a mere 

 cavity, the surfaces are folded into innumerable plaits, all 

 calculated to afford ampler space for the display and mean- 

 derings of their blood-vessels, and to expose a wider surface 

 to the contact of the water : but, if the branchiae are external 

 and exposed, they are, it may be, less complicated, only be- 

 cause complexity seems unnecessary where fresh doses of 

 unbreathed fluid are continually brought into momentary 



