MOLLUSCA. 57 



ferring the animal of the mya to the genus ascidia, we shall 

 only mention, that the former has a foot, and possesses a 

 locomotive power ; the latter has no foot, remains immove- 

 ably attached for life upon the substance to which it at first 

 adhered, and depends on the accidental bounty of the waves 

 for all its nourishment. 



The animals of nearly all the univalves are represented 

 as belonging to the genus Umax. But, with the exception 

 of the restricted genus helix and bulimus, the animals of the 

 univalves are all genetically different from the Umax. Their 

 tentacula are generally two in number, with the eyes at the 

 base ; while the tentacula are four in the Umax, with the 

 eyes at the tips of the two longest. These examples will 

 suffice to establish a truth so palpably obvious. 



The principal objection against this system of employing 

 the shell, to the exclusion of the animal, arises from the 

 fact, that nature has not drawn a line of distinction between 

 the mollusca and the testacea. Thus, many of the vermes 

 mollusca of Linnaeus include shells in their bodies, as the 

 aplysia ; and many of his vermes testacea likewise are soft 

 on the outside, the shell being inclosed by the integuments, 

 as the helix laevigata, now constituting the genus VELUTJNA. 



We have another objection to this artificial system, and 

 one which we consider of great force ; that, wherever it 

 prevails, the form and habits of the animal are overlooked. 

 How fully is this truth illustrated in the works of the testa- 

 ceologists of this country ! We might examine all their 

 writings, from the Pinax of Merret to the Descriptive Ca- 

 talogue of Maton and Raket, including the works of Pen- 

 nant, Berkenhout, Da Costa, and Donovan, and learn little 

 more, besides the habitat, than that to every shell there is 

 an animal attached. Of this charge the names of Lister 



