MOLLUSCA. 61 



destitute of a head ; some of them have no locomotive 

 power ; and, in others, the organ of motion is a fleshy foot, 

 which the animal can protrude at pleasure. These circum- 

 stances point out the connection which subsists between 

 the organs of the animal, and the external forms of the shell; 

 a connection which, in every system, ought to be carefully 

 attended to. 



It is somewhat difficult to point out, among the univalves, 

 the true subordination, or relative importance of the cha- 

 racters employed by conchologists in describing them. We 

 have much to learn of their anatomy, and hence we cannot 

 with certainty point out the relation of the parts of the shells 

 to the organs which those parts protect. The form and struc- 

 ture of the mouth of the shell, however, may be expected to fur- 

 nish characters of the first-rate importance, and have always 

 attracted the notice of the student of testaceous bodies. The 

 very shape of the animal, together with its ordinary habits, 

 must necessarily depend, in a great measure, on the form of 

 the mouth. 



In many genera, the mouth of the shell towards the base 

 is produced, and terminates in a groove or beak. These 

 univalves are termed canaliculated, and are readily distin- 

 guished from those whose mouth is entire. The differences 

 in the form of the shell in these two divisions is an index of 

 equally remarkable differences in the form of the animals. 

 The canaliculated shells contain animals possessed of an 

 elongated tube for the purposes of respiration, and this canal 

 is destined for its reception and protection when expanded. 

 The animals whose shells are destitute of this canal, are 

 likewise destitute of this lengthened respiratory tube. Cir- 

 cumstances of this kind induce us to believe, that shells, 

 agreeing in external form, in general, contain animals of a 



