UNIVALVE MOLLUSCANS. 269 



entitled to the appellation of the butterflies of the 

 ocean, than the escallop shells which have some- 

 times been so called. The mantle of the bivalves 

 becomes an organ of very different use in the 

 Pteropods; for they, having no means of fixing 

 themselves like most of the bivalves, float con- 

 tinually in the ocean; to compensate for this 

 want, as in innumerable other instances, their 

 Creator has given them the power of expanding 

 this organ as a sail, both for motion and to give 

 some direction to their course; it is attached 

 to the mouth or neck, and is connected in 

 some species with their respiration. Nothing 

 certain is known with respect to their food : 

 probably they absorb the animalcules swarming 

 in the sea-water. 



2. The series of Gastropods begins with ani- 

 mals that have no shell, amongst which the most 

 remarkable seem to be the Scyllcea and the Te- 

 thys, both known to Linne, and by him described. 

 The former is an oblong gelatinous animal, late- 

 rally compressed, elevated above in the middle, 

 where it has two pair of membranous wings or 

 fins. Its inferior surface is hollowed out longi- 

 tudinally, by means of which, and its tentacles, 

 it can embrace the stems of the fuci or sea- 

 wrack, the flowers of which it eats. It is des- 

 cribed as moving very slowly in the water by 

 bending its extremities. It swims on the surface 

 when the weather is calm, but adheres to the 



