CHAP. II. TESTACEOUS MOLLUSKS GENERALLY. 31 



shutting of the shelly valves, which produces a jerk. 

 In some very few instances among the typical Testacea, 

 as in the genus lanthina, or oceanic snail^ the animal 

 has a cellular organ attached to the belly, by which it 

 floats on the surface of the ocean, or sinks to the bottom, 

 at its own pleasure. The power of swimming, how- 

 ever, is chiefly found among the aberrant groups, such 

 as the cuttlefish (^Cephalopoda), the tritons (Nudi- 

 branchia), and the Tectibranchia : the first of these 

 may really be said to possess Jins ; while the naked tri- 

 tons, no doubt, use the appendages of the body for the 

 same purpose. The power of adhesion is also differently 

 bestowed : in the cuttlefish and Planar ia^, it resides in 

 the innumerable suckers which terminate the arms of 

 one, and are placed on the under side of the other. In 

 the limpet {Patella), the ear-shell {Haliotis), and the 

 chiton, it originates in the excessive breadth of the 

 disk upon the belly, which covers a surface equal to that 

 of the whole animal and its shell : so firmly, indeed, 

 do these genera adhere to the rocks or other substances 

 upon which they are found, that they can only be sepa- 

 rated by great force. It is among the limpets that we 

 find the power of locomotion at its lowest ebb ; for they 

 seldom remove far from the spot on which they were 

 born ; and many, from the shape of the shell corre- 

 sponding to the surface of the rock, appear never to 

 have done this : finally, in the genus Hipponix, we 

 arrive at a positive certainty that the animal is fixed, 

 because it adheres by a separate distinct plate, which 

 thus, in point of fact, renders it a bivalve shell. At- 

 tachment, however, is much more prevalent among the 

 bivalves, where we have entire families fixed to marine 

 substances, either by one of these valves, as the oysters, 

 or by a packet of strong fibrous threads. The attached 

 genera are much less numerous than the others, and are 

 affixed in different ways. Some, like the muscles {My- 

 tilus), are merely connected into little bunches or fa- 

 milies, by slender and scattered threads, strong enough 

 to keep them together ; others, as the Pinnce, or wing- 



