196 IXVEllTEBEATE ANIMALS. 



CHAPTEE XX. 

 MOLLUSCA PROPEB. 



The higher MoUusca or iloUusca Proper comprise those members of 

 the sub-kingdom in which the nervous system consists of three prin- 

 cipal pairs of (janglia ; and there is always a well-developed heart 

 consisting of at least two chambers. 



In this division are inchided the following classes: — 



1. Lamellihriincliiata , without a distinct head. 



2. (Jasteropvda, j . . . 



n pf J ( With a distinct head and a masticatory 



,' ,1 1 , \ I apparatus or " odontophore." 



4. t ephalopoda, ; 



Class I. Lamellibranchiata. 



These are well known as Bivalve shell-fish, such as Mussels, 

 Oysters, Scallops, &c., and they are all either marine or inhabitants 

 of fresh water. They are distinguished from the other Molluscs by 

 having no distinct head, and by having the body protected by a, 

 bivalve shell composed of two pieces. Thej' are called Lamellihran- 

 chiata from the fact that the organs of respiration are in the form of 

 leaf-like gills or branchise, two of which (or stJinetimes one only) are 

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

 oyster as the "beard." The body of the LnmeUihrancliiata 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, therefore, of the true Bivalves is composed of two 

 valves, which are "right" and "left," and not "dorsal" and "ven- 

 tral" as in the Bnn-hiopoda. Moreover, the valves of the shell are 

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

 lastly, the shell is more developed on one side than the other, so as 

 to become "inequilateral" (fig. 138). The lobes of the mantle are 

 sometimes quite free along their ventral margin ; but at other 

 times they are more or less united to each other, and leave only two 



