CHAPTER XIX. 



OF THE CLASS OF PALLIOBRANCHIATA. 



965. THIS Class, although at present very limited, both as to 

 the number of existing species it includes, and the small number 

 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 name of 

 the Class is derived from the peculiar conformation of the respira- 

 tory apparatus, 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 addi- 

 tion 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 capable of not only being unrolled, but extended beyond 

 the shell to a great distance in quest of food. They are usually 

 furnished with numerous vibratory filaments, for the more certain 

 capture of the prey, and probably also for assisting in the main- 

 tenance of the respiratory current an extraordinary provision, 

 which is rendered necessary by the great depth at which these 

 animals live, and the consequent enormous pressure of the 

 water around. All the existing genera of this class are attached, 

 in some way or other, to solid bodies. In Terebratula and 



