MANUAL OF NATURAL HISTORY. 113 



which they create currents to bring them food In 

 arranging the classes of Mollusca, the characters 

 are principally determined by the organs of loco- 

 motion ; in the orders the nature of the branchial 

 apparatus is taken advantage of ; in the diagnosis 

 of families the lingual dentition and peculiarities of 

 configuration form the leading points ; in genera, 

 the form of the shell is chiefly considered ; and in spe- 

 cies, the colour and sculpture of the shelly envelope. 

 The molluscous tribes, with a few exceptions, re- 

 produce by eggs which are deposited by the mother 

 in situations best adapted to insure incubation, and 

 in numerous instances, are protected by a leathery 

 envelope or gelatinous nidus ; the young fry, when 

 first excluded from the egg, are enclosed in little 

 shells fitted with a lid, and swim about by the aid 

 of fins ; as the animal grows older the fins disap- 

 pear, and frequently the shell falls off* and gills 

 and tentacles become developed. The senses of the 

 Mollusca are not acute; they are mostly dumb, 

 their vision is imperfect, their smell limited, their 

 taste implied, their touch obtuse, and their energies 

 are mainly directed to the procuring of food, and 

 the multiplication of their species. The intensity 

 of their instinct, and the power of their volition 

 and motive power will, however, be found to vary 

 in proportion to the complexity of their organiza- 

 tion, being feebly expressed in the lowest or Annu- 

 loid forms, and more perfectly manifested as we 

 approach the Vertebrate type, as shewn in the class 

 of Cephalopods. 



