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CHAPTER XIX. 

 OF THE CLASS OF PALLIOBRANCHIATA. 



1048. THIS Class, although at present very limited, both as 

 to the number of existing species it includes, and the small num- 

 ber of these which seem to be distributed through the ocean, 

 was formerly of great importance ; since it included a very large 

 proportion of the Bivalve Mollusks of the older rocks. Although 

 an ordinary observer would not detect anything in the structure 

 of the shell, to justify the separation of this Class as distinct from 

 the preceding, yet the necessity for this division becomes evident, 

 when the organisation of the animal is examined. The valves of 

 the shell differ completely in their position from those of the 

 preceding Class, being placed upon the upper and lower surface 

 of the animal, instead of on its sides. The substance of the shell 

 itself is also peculiar, being perforated in nearly all the species 

 by an immense number of minute apertures, passing perpendicu- 

 larly from the inner to the outer surface. These are filled by 

 minute processes of the mantle. The name of the Class is de- 

 rived from the peculiar conformation of the respiratory appara- 

 tus, which here consists of the mantle itself ; this is traversed 

 by blood-vessels which ramify minutely over its surface ; and is 

 furnished, especially along its edge, with vibratile cilia, which 

 produce a continual current in the surrounding water, and thus 

 renew it for the aeration of the circulating fluid. In addition to 

 this singularity of structure, the organs for procuring food offer 

 remarkable peculiarities. They consist of two long spiral arms, 

 one on each side of the mouth ; the existence of which has given 

 rise to the name BRACHIOPODA, or Arm-footed, by which the 

 class is frequently designated. In many species, these are capa- 

 ble of not only being unrolled, but extended beyond the shell to a 

 great distance in quest of food. They are usually furnished with 



