MOLLUSCA. 167 



lusca which possess a shell, this is secreted by the integument, 

 or by what is technically called, the mantle, and, in all cases, 

 it is composed of carbonate of lime. The methods in which 

 the lime is arranged differ in different cases, but all living 

 shells have an outer covering of animal matter, which is 

 known as the " epidermis." In a great many of the higher 

 Mollusca, such as the whelks, periwinkles, snails, and others, 

 the shell consists of only a single piece, when it is said to be " uni- 

 valve." In many others, such as oysters, mussels, scallops, etc., 

 the shell is composed of two pieces, and is then said to be 

 " bivalve." In a few forms, the shell consists of several pieces, 

 and it is then said to be " multivalve." The more important 

 variations in the shells of the Mollusca will be noticed in 

 speaking of the different classes of the sub-kingdom. 



In accordance with the nature of the nervous system, the 

 Mollusca are divided into two great divisions, known respec- 

 tively as the Molluscoida and Mollusca proper. In the Mol- 

 luscoida, the nervous system consists of a single ganglion, or 

 principal pair of ganglia, and there is either no circulatory 

 organ or an imperfect heart. In this division are included the 

 three classes of the Sea-mosses {Polyzoa), the Sea-squu-ts [Tw- 

 nicata), and the Lamp-shells and their allies (Brachiopoda). In 

 the Mollusca proper, the nervous system consists of three 

 principal pairs of ganglia, and there is a well-developed heart, 

 consisting of at least two chambers. Under this head come 

 all the ordinary forms of shell-fish. 



