36 SHELLS AND SHELL-FISH. PART I. 



mentary eyes ; it presents us, in fact, with all the cha- 

 racters of a testaceous gastropod, except one, that of 

 being covered by a shell. In the Tectibranchia, again, 

 we have a group still more remote from the typical 

 structure ; for many of these have the mere rudiments of 

 a shell covering a small part of their body, and they swim 

 as much as they crawl. Yet their evident connection to 

 the Volutidae (by means of the Cyprceidce) on one side, 

 and to the chitons on the other, points them out as truly 

 belonging to this order. 



(31.) The DITHYBA of Aristotle, or bivalve shells, 

 form the next, or sub-typical division. This comprises 

 animals destitute of an apparent head, but still inclosed 

 in a shell, and still moving on the belly. There is not, 

 however, in this latter part, any flattened disk ; and the 

 shell, instead of being spiral, is generally composed of 

 two pieces, or valves, joined together by a horny liga- 

 ment. The Dithyra, moreover, are further distin- 

 guished by having neither tentacula, nor the least 

 vestige of eyes. We meet with some, indeed, as the 

 Chelisomce, which have no external shelly covering ; 

 and others, where the bivalve shell is inclosed in a 

 tube, as the family of Ttibulibranchia, thus assuming 

 the disguise of a univalve. But these remarkable de- 

 viations are few, and point out, as we have just men- 

 tioned, the extreme limits of the tribe. 



(32.) The NUDIBRANCHIA, or tritons (part of the 

 naked Gasteropoda of M. Cuvier), form the third of 

 our principal divisions, and are all no less strongly cha- 

 racterised. Destitute of any covering, and presenting 

 a marked peculiarity in their external branchia, their 

 appearance is so very different, that every unprejudiced 

 observer cannot but wonder they should, up to this day, 

 have been mixed up with the true gastropod shell-fish. 

 These animals, in fact, are entirely flattened, naked, 

 soft, and generally ornamented with beautiful colours ; 

 they move about by means of their foot, or concave 

 underside of their belly, in the depths of the ocean ; 

 on the sides, and often on the upper part of their 



