BRANCHIFEROUS GASTEROPODS. 273 



There are other Gasteropods in which, although not so 

 fully exposed as in the preceding order, the gills are still 

 only slightly concealed by some lap or fold of the cloak. 

 The genera Patella,* as now restricted, and Chiton afford 

 examples where the branchiae, in form of a cord composed 

 of pyramidal processes, or of close-set and parallel transverse 

 leaflets, encircle the body more or less completely, lying in 

 a furrow between the foot and cloak, and merely covered 

 by the prominent margin of the latter. In an order which 

 Cuvier calls Inferobranches, these organs occupy a similar 

 position, but limited often to one side of the body ; while, 

 in the Tectibranches (an order of which Aplysia may be 

 selected as the type) the gills, almost free, and like some 

 miniature arbuscle, occupy a position on the back, where 

 they lie, scarcely hidden, under a moveable corneous lid that 

 sits in the centre of a hollow formed by the large and mus- 

 cular dorsal fin, intended apparently to collect the water as 

 in a crater, that it may not pass away too rapidly, and until 

 it has thoroughly penetrated the intricacies of the branchial 

 apparatus. 



The mollusca with internal branchiae . are more numerous 

 than the preceding ; for some Pteropoda, the greater number 

 of the Gasteropoda, all the Cephalopoda, and all the acepha- 

 lous tribes f are thus circumstanced. The various modifica- 



the digestive function ; but their branchial character is established by Sou- 

 leyet (Reports on Zoology, p. 426), and by Hancock and Embleton. 

 Vibratile cilia, distributed over the entire cuticular surface, aid their func- 

 tion. — Ann. and Mag. N. Hist. xii. 236. 



Messrs. Hancock and Embleton say : — u The function of respiration we 

 believe to be performed by the whole surface of the skin, including the 

 papillte ; the skin of the back and of the sides between the papillce, and the 

 entire surfaces of these latter organs, present the phenomenon of ciliary 

 vibration. The papillfe we regard as one modification among many, of in- 

 creasing the surface for a respiratory purpose, and thus are to be regarded as 

 a specialized breathing apparatus, to which the rest of the skin is subsi- 

 diary." — Ann. and Mag. Nat. Hist. ser. 2, i. 103. 



" Now the whole, or nearly the whole of the blood that passes to the 

 auricle of the heart comes, as we have shewn, in the section on the circula- 

 tory organs, directly from the skin, and as we know that the blood thus cir- 

 culating in the skin and papillae is separated from the oxygenated water of 

 the surrounding sea, by a very thin layer, in the papillae by an exceedingly 

 delicate membrane, we have little hesitation in saying, that it is in the 

 papillae essentially, and in the rest of the skin secondarily, that the function 

 of arterialization cf the blood is carried on previously to the return of that 

 fluid to the heart."— Ibid. 104. 



* Blainville, however, maintains that Patella is pulmonifcrous. — Manuel, 

 p. 125. 



t Lamarck considers the branchiae of bivalves (Conchifcra) as properly 

 external (Hist. Nat. v. 417) ; and this view of their position is plausible, 

 more particularly when the cloak is open in front. 



T 



