MOLLUSC A. 



55 



The natural order of arrangement, if followed, would 

 have placed the marine gasteropods between the 

 Lamellibranchs and the air-breathing or pulmoniferous 

 gasteropods. The prominent characteristics of these 

 being, that while they are univalves, like the Pulmoni- 

 fera (snails) , they have organs for breathing the air in 

 water instead of lungs, or pulmonary sacs, as they 

 ought to be called. These organs, though not similar 

 to the gills of the Lamellibranchs in appearance, are in 

 reality quite similar when viewed from an analytical 

 point of view. The gills of the Lamellibranchs are 

 tubes united in sheets in the oysters,* but in the 

 marine snails they are commonly in the shape of free, 

 floating fringes of tubes, or rows of projecting plates, 

 thus exposing a much greater surface to the water than 

 if only one side was uncovered (Fig. 27). 



When a shell is present, it is always contained, 

 either in a hollow of the mantle lying under the shell, 

 or above in the fold between the creeping disc and the 

 mantle border, as in the Limpet, Patella f (Fig. 50). 

 In all of these animals the heart receives the blood 

 from the gills in front, as described above, and sends 

 out the great artery (aorta) behind ; and they are called 



* In several types this is not so complete, and they are more 

 or less free and disunited, but still maintain the complicated 

 folds and aspect of the Lamellibranchs, as has been described 

 above. 



t The species on our coast belong to Acmea and other 

 genera, which have the gills in plumes, situated in the mantle 

 cavity above the head, though their shells (Fig. 35) are similar 

 to those of Patella, and were formerly described under that 

 name. 



