28 SHELLS AND SHELL-FISH. PART I. 



find a few bivalves which assume the appearance of 

 univalves ; and others which, from the valves being 

 more than two, were called by Linnaeus Multivalve 

 shells. This term the great Swede applied not only to 

 those animals which really belonged to the Testacea, but 

 to the Cirripedes, or barnacles, which have nothing to 

 do with shells, properly so called. The term, there- 

 fore, is altogether objectionable, and will not here be 

 employed. 



(24.) Such are the leading external distinctions of the 

 class of animals now before us. Their typical characters 

 are three ; two positive, the other negative. The first is 

 that of crawling upon their belly, yet without feet; 

 the second is their being covered with a beautiful calca- 

 reous shell, which is their constant habitation, and 

 which they have the power of enlarging, to suit the 

 progressive growth of their soft bodies ; the third is, 

 that, if deprived of this covering, they have not the 

 least vestige of rings or joints on their bodies : the 

 mouth, indeed, may be circular, and even the branchia, 

 but this is the only analogy they have to the Radiata. 

 In proportion as nature recedes from this typical 

 eminence, the structure of the shell becomes imperfect, 

 until, in the aberrant groups, the animal is either naked, 

 or is but partially protected by such a covering. 



(25.) The Testacea, considered anatomically, may 

 be described as soft inarticulate animals, almost always 

 breathing by branchia, or lungs, which vary, however, 

 in the most singular diversity of ways both as to form 

 and situation ; this variation, moreover, takes place in 

 groups so closely related to each other in all other re- 

 spects, that it is perfectly clear no natural arrangement 

 can be founded upon the organs of respiration. The 

 least organised, in fact, such as the PlanaricB, &c., have 

 no branchia whatever ; so that they only possess two of 

 the characters of the class, — namely, a flattened disk or 

 disks, which act as a foot, and a total absence of joints 

 in their body. Their blood is white, and its circulation, 

 observes Cuvier, is always double ; that is, their pul- 



