270 BRANCIIIFEROUS MOLLUSCA. 



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

 walls, and more especially its roof, 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, 

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

 aquatic Pulmonifera (Limneus, 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, the organs are likewise of greater extent and com- 

 plexity. 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 

 meanderings 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 because complexity seems unnecessary where fresh 

 doses of unbreathed fluid are continually brought into mo- 

 mentary contact with them, and without any effort on the 

 creature's part. 



The mollusca which have their branchiae entirely exposed 

 belong to two orders, the Pteropoda and Gasteropoda. A 

 very few, as the Actaeon (Aplysia viridis, Mont.) and the 

 nearly allied genus Limapontia, appear to have no other 

 respiratory organ than the common integuments, which have 

 suffered no modification ; but in the genus Cribella of Au- 

 douin and Milne-Edwards the skin of the mantle is, for this 

 purpose, wrinkled on each side, and perforated with an infi- 

 nite number of pores.* In a greater number the branchiae 

 are actually blended with the locomotive organs, as in Clio, 

 a member of the former order, whose fin-like expansions are 

 supposed to perform the office, not of progression only, but 

 also of ventilating the blood as it circulates through the fine 

 regular network with which their surfaces are covered. The 

 Glaucus (Fig. 44) affords another example of the same union 



* Edinb. Journ. Nat. and Gcogr. Sc. iii. 244. 



