542 ORGANS OP RESPIRATION. 



of the back in carinaria. In patella and chiton, they form a 

 circular range of branchial laminae around the body, between 

 the margins of the mantle and foot ; but in pleurophyllidia, 

 they are confined to the two sides of the body, the circle 

 being interrupted before and behind. In the haloitis, they 

 constitute two unequal penniform organs, placed under an 

 extended fold of the mantle, on the left side of the body ; 

 and the same character is found in the pectinibranchiate 

 gasteropods, inhabiting nearly all univalve, unilocular, turbi- 

 nated shells, and forming the largest order of this class. 

 The branchiae when confined to the right side in gasteropods, 

 are commonly single, and when confined to the left side, 

 they are thus generally double penniform organs. The two 

 penniform pectinated gills of the pectinibranchiate species, 

 are placed in a recess on the left side of the back, under the 

 open parieties of the mantle, and, as usual, near to the 

 auricle and ventricle of the heart, which receive the aerated 

 blood. 



The muscular and ciliated syphon, prolonged from the 

 left side of the mantle, and extended through the canal 

 or groove of the shell, directs the respiratory currents in- 

 wards over their surface ; and the water, interrupted by the 

 closed cavity of the mantle behind, after bathing the entire 

 ciliated branchial laminae, and the ciliated respiratory cavity 

 in which they are lodged, passes outwards over the right side 

 of the palleal cavity, and over the muciparous follicles and 

 the excretory orifices there placed. The development of 

 the vibratile cilia over all parts of the branchial apparatus 

 and external soft parts of gasteropods accords, as usual, with 

 the limited means otherwise provided for renewing the 

 stratum of the surrounding element in immediate contact 

 with their surface. Here also they are seen on the mucous 

 lining of the alimentary canal ; they are early developed on 

 the mantle and branchiae of the embryos in ovo, and serve, 

 for a time, as natatory organs in the young when hatched ; 

 and by the continuance of their action on portions detached 

 from the body, they exhibit the same independence of voli- 

 tion as in other classes. 



A transition from the branchiated to the pulmonated 

 gasteropods is effected by the naked, amphibious, marine 

 onc/iidium } in which, besides about twenty pairs of small 



