their Organs of Respiration. 239 



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 Inferobranchiae, these organs occupy a similar 

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

 the Tectibranchiae, 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 movable corneous lid that sits in 

 the centre of a hollow formed by the large and muscular 

 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 appa- 

 ratus. 



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 * are thus circumstanced. The various modifica- 

 tions of them in the remaining Gasteropoda I will not now 

 dwell upon ; for it will be necessary to describe them here- 

 after, when I explain to you the Cuvierian system : but there 

 is one family which, because of its greatness, may not be 

 passed over in this place, this is the Pectinibranchiae ; an 

 order that includes almost all the marine turreted and con- 

 volute f shells, and a few which are found in fresh water. 

 In it the branchial cavity has a position similar to that in the 

 Pulmonifera, on the upper and fore part of the back; to wit> 

 where it is protected by the body of the shell : but its walls 

 are not smooth and even, like those of the pulmonary cavity, 

 but folded into neat and regular plaits or ridges, that lie 

 parallel to one another, like the teeth of a comb (whence the 

 name Pectinibranchiae), and often part on each side from a 

 central stalk formed by the trunks of the blood-vessels, in the 

 same manner that the barbs of the web of a quill depart from 

 the shaft. The water obtains ingress to this cavity, in such 

 Pectinibranchiae as inhabit shells with entire apertures, by a 

 large slit on the side above the collar ; and, in shells with in- 

 terrupted or beaked apertures, by an imperfect siphon that 



* Lamarck considers the branchiae of bivalves (ConcMferae) as properly 

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

 plausible, more particularly when the cloak is open in front. 



\ Sir E. Home asserts that the lobes of the mantle which cover the 

 shells of the cones and cowries are the respiratory organs of the animal 

 (Comp. Anat., vol.i. p. 55.) ; but this is a mistake: they are true Pectini- 

 branchiae. 



