54' SHELLS AND SHELL-FISH. FART I. 



almost every thing, has been this_, — that the reader is 

 at a loss to know by what other principle they are 

 united, even according to the system of the learned au- 

 thor. Some crawl on their belly, others do not ; some 

 have shells, others none ; many have a distinct head, 

 but many have not ; the eyes are present, or they are 

 wanting : all the organs, in short, of respiration, deglu- 

 tition, and reproduction, are admitted to vary ; and this, 

 in such an extraordinary manner, and in species so ob- 

 viously allied to each other, that they have no claim to 

 a primary consideration. True it is, that all these ani- 

 mals possess an aortic heart ; but this, properly speak- 

 ing, is more the universal character of the testaceous 

 Mollusca as a class, than one by which such an immense 

 number of animals, totally different in all other respects, 

 can be distinguished. The order, as it noAv stands, may 

 be said to embrace the whole of the Mollusca, except the 

 Dithyra, or bivalves, and the Cephalopoda, or cuttlefish. 

 " There is. no doubt, some great error," observes jNJr. 

 MacLeay, " yet undetected in the principles upon which 

 we are accustomed to arrange the 3Iollusca, and that 

 we shall never arrive at the truth, by looking, like M. de 

 Blainville, solely to the position and structure of the 

 organs of respiration ; or, like M. Cuvier, to the method 

 of reproduction, as when he unites the Cyclohranches to 

 the Acpphala or Dithyra. 



(45.) Rejecting, therefore, all those animals from the 

 true gastropods, which have neither a distinct head, as 

 the Tubulibranchia, or whose branchia are naked, as in 

 Doris, we shall retain an assemblage of mollusks, nearly 

 all of which are furnished with univalve shells*; and 

 whose head, distinguished from their body, is provided 

 with tentacula and eyes ; the flattened part of the belly 

 serving them as a foot. Thus restricted, we shall find 

 that nearly all the remaining divisions of M. Cuvier, 

 under certain modifications, indicate so many natural 

 groups. It is clear, however, that when M. Cuvier 



* Excppt in Chiton, where the valves are separate and dorsal only, and 

 some of the Tectibrancliia. 



