MOLLUSCA PROPER, 193 
CHAPTER XXII. 
MOLLUSCA PROPER. 
THE higher Mollusca or Mollusca Proper comprise those mem- 
bers of the sub-kingdom in which ¢he nervous system consists of 
three principal pairs of ganglia; and there ts always a well- 
developed heart, consisting of at least two chambers. 
In this division are included the following classes :— 
. Lamellibranchiata, without a distinct head. 
Las | 
2. Gasteropoda . oe 
BME with a distinct head and a masticatory 
3. Pteropoda, io a 
f apparatus or “ odontophore. 
. Cephalopoda, 
CLASs [. 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 more or less completely protected 
by a bivalve shell composed of two pieces. They are called 
Lamellibranchiata from the fact that the organs of respiration 
are in the form of leaf-like gills or branchize, 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 Lamellibvanchiata 
is more or less completely enclosed in an expansion of the in- 
tegument which constitutes the “ mantle,’ and which is divided 
into two halves or “lobes,” which are placed on the szdes 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 “ventral,” as in the Brachiopoda. 
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 “in- 
equilateral” (fig. 101, 2). The lobes of the mantle are sometimes 
N 
