76 THE NERVOUS SYSTEM OF MOLT.USKS, 



Some of these tactile appendages, as well as some of 

 the ocelli, send their nerves to the branchial ganglia, 

 while others, situated on the anterior borders of the 

 mantle, send filaments to the labial ganglia. The latter 

 also receive filaments from the so-called labial appen- 

 dages, whose function is uncertain, though it has been 

 suggested that they may be organs of taste or smell. 

 Lastly, in close relation with the pedal ganglia or 

 ganglion, there are two minute saccules (fig. 23, s), to 

 which an auditory function is usually ascribed. 



Thus we find amongst these headless Mollusks a distri- 

 bution of specially impressible parts or sensory organs, 

 such as cannot be paralleled among any other animals. 

 The functions which we shall find pertaining to the 

 ' brain ' in other creatures are in them distributed in a 

 very remarkable manner — so that such organisms may 

 be said to be brainless as well as headless. 



The Pteropods constitute another interesting class of 

 Mollusks, which lead us on from the comparatively 

 sluggish Lamellibranchs to the Gasteropods and the 

 Cephalopods — organisms which possess definite and wide- 

 reaching powers of locomotion, as well as a distinct head 

 carrying sense-organs and a more or less developed 

 brain. 



The possession, by many members of this class, of two 

 fin-like muscular expansions attached to the side of the 

 head induced Cuvier to give them the above class name. 

 According to Owen, " All the species of Pteropoda are of 

 small size ; they float in the open sea, often at great 

 distances from any shore, and serve, with the Acalepha3, 

 to people the remote tracts of the ocean. In the latitudes 

 suitable to their well-being, the little Pteropoda swarm in 

 incredible numbers, so as to discolour the surface of the 

 sea for leagues ; and the Clio and the Limacina con- 



