CHAP. II. TESTACEOUS MOLLUSK8 GENERALLY. 3l 



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 fins ; 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 Planaria, 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 Pinnae, or wing- 



