A 



MANUAL OF THE MOLLUSCA. 



PART I. 



CHAPTEE I. 



ON THE POSITION OF THE MOLLUSCA IN THE ANIMAL KINGDOM. 



All knoTTii animals are constructed upon five different types, 

 and constitute as many natural divisions 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 nervous 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 vertebrae ; 

 while in a few it is cartilaginous, and not divided into distinct 

 parts. Yertebrse 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 devoted 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 MoUusca, 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 

 shell, 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 Lingnla, and of a few of the Pteropoda, 



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