CLASSIFICATION OF MOLLUSCA. 337 



to rank as a distinct Class. They are especially distinguished 

 by the form of the foot ; which, instead of being a horizontal 

 disc, is compressed vertically, so as to form a fin, which is turned 

 upwards, like the dorsal fin of Fish, instead of being situated on 

 the under side of the body. 



876. In each of the foregoing Classes, we observe a consider- 

 able variation in regard to the relative size, and even the very 

 existence of the Shell ; for, whilst there are some species in all of 

 them, which are entirely destitute of this protection (such being 

 called naked Mollusks), there are others which possess it in a 

 slight degree, having it generally concealed in a fold of the 

 mantle ; whilst in others, again, it completely envelopes the body 

 when they desire to withdraw themselves under its protection. 

 In nearly every case, the Shell, where it exists, is Univalve ; 

 that is, composed of but one piece. In the -Acephalous Mollusks 

 on the other hand, we find two very distinct groups ; in the first 

 of which the shell is always present ; whilst in the second it is 

 as invariably absent. The first is therefore named Conchiferous, 

 or shell-bearing; and the latter Tunicated, the shell being 

 replaced by a leathery or membranous tunic. The Conchiferous 

 Acephala, with scarcely an exception, have Bivalve shells ; and 

 they are again divided into two classes, according to the arrange- 

 ment of their respiratory organs. 



IV. LAMELLIBRANCHIATA, having the gills arranged in four 

 lamella, or riband-shaped folds, which run parallel to the edges 

 of the shell. To this group belong all the ordinary Bivalves. 



V. PALLIOBRANCHIATA, having the respiratory surface formed 

 by the mantle itself. The animals of these shells differ in several 

 other important particulars from the preceding. This class is 

 very small at present, in comparison with the other Bivalves ; 

 but in the more ancient periods of the earth's history, a very 

 large proportion of the Bivalve Mollusks seems to have belonged 

 to it. 



VI. TUNICATA, a group including all those Acephalous Mol- 

 lusks which are destitute of a shell. In this class, we find many 

 points of structure, which lead us towards the higher Zoophytes. 



