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MANUAL OF THE MOLLUSGA. 
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CHAPTER I. 
ON THE POSITION OF THE MOLLUSCA IN THE ANIMAL KINGDOM. 
AL known animals are constructed upon five different types, 
and constitute as many natural drvisions or sub-kingdoms. 
1. The highest of these groups is separated from the next 
below it by a sharp line of distinction. In it the main mass of 
the neryous system is placed on the dorsal side of the body, and is 
in no instance pierced by the alimentary canal. It is separated 
from the alimentary canal by a partition, which in most cases 
is bony, and divided into separate parts, known as vertebree ; 
while in a few it is cartilaginous, and not divided into distinct 
parts. Vertebre are a common feature amongst the Vertebrata, 
as this sub-kingdom is called; but they do not form an essential 
characteristic, as the name might seem to imply. Distinct 
organs are deyoted to the functions of respiration and. circu- 
lation; the sexes are generally distinct; each individual is 
generally developed from a single egg. Blood red. 
2. In the second sub-kingdom, or Mollusca, which is well 
exemplified by the common garden snail, the nautilus, and the 
oyster, the soft parts are in most cases protected by an external 
sheli, which is harder than the bones of the vertebrates, and the 
covering of the crab and lobster. It consists almost entirely of 
carbonate of lime, while the bones of the vertebrates contain a 
large proportion of phosphate of lime. The shells of many of 
the Brachiopoda, such as Lingula, and of a few of the Pteropoda, 
B 
