MOLLUSCA. 197 



Oyster, Mussel, and Clam, and are characterized by the 

 possession of lamellar gills. The shells differ from those 

 of Brachiopods in being placed on the right and left sides 

 of the body, so that the hinge is on the back of the ani- 

 mal, and in being generally unequilateral and equivalved. 

 They are sometimes termed CONCHIFERA, or shell-fish, 

 (Lat., concha, a shell ; fero, I carry.) 



The shells of Mollusks are epidermal structures. The 

 mantle, or loose skin, secretes calcareous matter in. lay- 

 ers, converting the epidermis into shell. The micro- 

 scopic structure is so characteristic that a thin section of 

 a fragment often suffices to determine the group to which 

 it belongs. A large class of shells is formed like the 

 Oyster, of three parts; the external epidermis, brown 

 and of a horny texture ; the prismatic portion, consist- 

 ing of minute columns set perpendicularly to the surface ; 

 and the internal nacreous, or pearly layer, made up of 

 very thin plates whose edges overlap and form wavy 

 lines. In many cases the prismatic and pearly layers 

 are traversed by minute tubes. The pearls of commerce, 

 found in the mantle of some Mollusks, are similar in 

 structure to the shell ; but what is the innermost layer 

 in the shell is outside and much finer in the pearl, which 

 is formed around some nucleus, as an organic particle 

 or grain of sand. 



Shells of one piece are called " univalves," as the snail. 

 Others, as the Clam, are of two parts, and are called 

 " bivalves." The ribs, ridges, and spines on the outside 

 mark successive periods of growth, and correspond with 

 the age of the animal. Figs. 96 and 97 show the princi- 

 pal parts of ordinary bivalves and univalves. The valves 

 17* 



