CLASSIFICATION OF MOLLUSCA. 337 



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. 

 In the Acephalous Mollusks, we find two distinct groups ; in the 

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

 is 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 

 lamellce, 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 Zoophytes. The 

 Tunicata really differ in many important particulars from most 

 of the other Mollusca, especially in the very slight development 

 of the nervous system, which consists of a single ganglion, situ- 

 ated in the vicinity of the oesophagus, and giving off nerves in vari- 

 ous directions. In this respect, they agree with a curious group 

 until recently placed amongst the Zoophytes, but which are now 

 proved to belong to the Mollusks, of which they form a seventh class. 



VII. POLYZOA, a group of minute compound animals, each 

 residing in a distinct cell, and furnished with ciliated tentacles 

 surrounding the mouth. The Polyzoa and Tunicata form the 

 subdivision Molluscoida of some authors. 



