CHAPTER XXL 



MOLLFSCA PEOPEB. 



THE higher Mbllusca or Mollusca proper comprise those 

 members of the sub-kingdom in which the nervous system 

 consists of three principal pairs of ganglia / and there is 

 always a well-developed heart, consisting of at least two 

 chambers. 



In this division are included the following classes : 



1. Lamellibranchiata, without a distinct head. 



2. Gasteropoda, ) with ft distinct head and a mast i ca tory 



CLASS I. LAjviELLiBKAisrcHiATA. These are well known as 

 bivalve shell-fish, such as mussels, oysters, scallops, etc., and 

 they are all either marine or inhabitants of fresh water. They 

 are distinguished from the other Mollusks by having no dis- 

 tinct head, and by having the body more or less completely 

 protected by a bivalve-shell composed of two pieces. They 

 are called Lamellibranchiata (Lat. lamella, a plate ; Gr. brag- 

 chia, gill), from the fact that the organs of respiration are in 

 the form of leaf-like gills or branchiae, two of which are placed 

 at each side of the body, constituting what is known in the 

 oyster as the " beard." The body of the Lamellibranchiata 

 is more or less completely enclosed in an expansion of the 

 integument which constitutes the "mantle," and which is 

 divided into two halves or " lobes," which are placed on the 

 sides of the animal, and secrete the shell. The shell, there- 

 fore, of the true bivalves is composed of two valves, which 

 are " right " and " left," and not "dorsal" and "ventral," as 

 in the Brachiopoda. Moreover, the valves of the shell are 

 usually of the same size, so that the shell is " equivalve," and,, 



