CHAP. III. TRIBES OF THE GASTEROPODA. 55 



places such genera as Planorhis and Ampullaria in two 

 different orders^ merely on account of a difference in their 

 organs of respiration^ he falls into as great an error as 

 that he has elsewhere committed in uniting the Cyclo- 

 branchia to his Acephala. The order Pulmoiiaria, there- 

 fore, must be clearly abolished. This, indeed, has been 

 already done by Lamarck, whose authority on all ques- 

 tions of natural affinity must, in general, be regarded as 

 superior to that of Cuvier. 



(46.) With the foregoing restrictions, the primary 

 divisions of the order before us will be found to xionsist 

 of the five following tribes: — 1. The Zoophaga of 

 Lamarck, or the carnivorous shell-fish (corresponding to 

 the PectinibrancMa Cuv.); 2. The Phytophaga of the 

 same author, which live chiefly on vegetables, as the 

 snails and slugs; 3. The Scutibranchia Cuv., or lim- 

 pets ; 4. The Cyclobranchia Cuv., or chitons ; and, 5. 

 The Tectibranchia Cuv., or bullas, whose univalve 

 shells, where they exist, are all hid in the flesh of the 

 animal, while their mantle is dilated into two fin-like 

 lobes, with which they can swim. "We shall now state a 

 few general particulars of these tribes, and then proceed 

 to determine their analogies. 



(47.) The Zoophaga are the most pre-eminently 

 typical of the whole of the testaceous MoUusca ; and 

 this holds good, whether we regard the organisation of 

 the animal, or the symmetry and beauty of their ex- 

 ternal shell, with which, in every instance, they are 

 provided. They have only two tentacula ; and the eyes, 

 which are always conspicuous, are sometimes (as in the 

 Stromhidcp) highly developed. The edge of the mantle 

 is almost always provided with a siphon, or tube for 

 respiration, and by which the animal can breathe without 

 protruding its head and foot from the aperture of its 

 shell : this siphon is protected by a corresponding canal, 

 either long or short, at the base of its habitation ; and its 

 presence, in all these MoUusca, constitutes one of their 

 most essential characters. The mouth, also, is very re- 

 markable, — resembling more or less, as Cuvier well 

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