THE YOUNG NATUEALIST. 



271 



I movable spines; neither do they comprise the 

 Cirripeda (bernicles), which, however, bear a 

 very close resemblance in many respects to 

 the mollusca. 



Conchology, while referring to shells, and 

 necessarily to the animals which inhabit and 

 construct them, does not cover so wide a field 

 as the study of the whole of the mollusca 

 would do, as these comprise not only all the 

 testaceous or shell bearing mulluscs, but 

 many others, which are either entirely with- 

 out shells, or possess them only in a rudi- 

 mentary form, and although in an intro- 



I ductory article like the present, treating of a 

 branch of conchology only, it is not necessary 

 to say much of those molluscs which are 

 without shells, a brief notice of them is 



! nevertheless desirable. 



Land and fresh-water shells are either 



; Uuivalves or Bivalves. A Univalve is a shell 

 formed of one piece, complete in itself; a 

 Bivalve is formed of two pieces opposed to each 

 other, and joined together by a sort of hinge. 



[ Amongst marine shells there are several 

 species of Multivalves, that is shells formed of 

 several parts, but these do not occur on land, 

 or in fresh water. Some of the Univalves 

 possess what is called on operculum, which is a 



[ sort of door by which the aperture of the 

 shell is closed when the animal retreats 



' within it, the operculum and shell are not 

 however hinged together like the two parts of 

 a bivalve, and it is quite subordinate to the 

 shell itself. 



It is evident that the terms Univalve and 

 '< Bivalve, although sufficiently appropriate as 

 applied to the two classes of shells and their 

 inhabitants, are scarcely applicable to such 

 of the mollusca as are without shells, and 

 that a classification founded on the charac- 

 teristics of the animals rather than the shells 

 is necessary ; the Mollusca are accordingly 

 divided into two classes, called Cephala (with 

 a head) and A cephala (without a head), and 

 this division corresponds with that adopted 

 in Conchology, the Univalves being the shells 

 of Mollusks, which possess a distinct head, 



and called therefore Cephala, whilst the 

 Bivalves are the shells of those which are 

 without a distinct head, or A cephala; whether 

 therefore the terms used are " Univalve " and 

 "Bivalve," or " Cephala " and " Acephala " 

 the classification is the same. It is well to 

 bear this in mind, for whilst the former terms 

 describe to us the leading characteristics of 

 the two classes of shells, the latter teaches us 

 the equally well defined distinction in the 

 animals, and every student of Conchology 

 should desire to know, not only the name and 

 configuration of each species of shell, but as 

 much as possible about the animal also. 



Shells are formed of calcareous matter 

 secreted by what is called the Mantle, this is 

 an elastic skin or integument, developed in 

 Univalves on the back of the animal, and 

 sometimes sufficiently ample to enclose it 

 entirely within its folds ; in Bivalves, the 

 Mantle is a pair of lobes, each of which 

 secretes a shell, and within which the vital 

 organs are situated. The secretion is de- 

 posited in successive layers around the edge 

 of the shell, the outside portion being first 

 formed, and the inner lining being separately 

 secreted, the two forming an oblique layer. 

 These successive layers are generally visible 

 on the outside of the shell, forming concre- 

 tric lines, called lines of growth, in some 

 species they present very marked charac- 

 teristics. 



Amongst the Univalves or Cephals we 

 have both land and fresh-water shells, and 

 the shell-less Molluscs also belong to this 

 class ; the Bivalves or Acephals are all in- 

 habitants of water, and all are provided with 

 shells, they abound in great variety in the 

 sea, but there are very few to be found in 

 fresh-water, only fifteen different species 

 being British. The Univalves number about 

 one hundred British species, two-thirds of 

 these being land and one-third fresh-water 

 shells ; there are in addition eleven British 

 Mollusks, all living on land, which possess no 

 external shell. 



(To be continued.) 



