THE BRACHIOPODA OR LAMP-SHELLS 117 



CHAPTER X 



THE BRACHIOPODA OR LAMP-SHELLS 



THESE were at one time included under the 

 Mollusca, on account of their possession of a bi- 

 valve shell. This shell, however, is placed prac- 

 tically back and front of the animal, not to the 

 right and left of it, as is the case with the shells 

 of the bivalve Mollusca. 



The name, arm-footed, was given them in ref- 

 erence to a pair of special structures called the 

 arms, bearing a large number of tentacles; it is 

 now more frequently spoken of as the lophophore 

 (see p. 122), and regarded as comparable to the 

 lophophore of the Polyzoa, spread out into two 

 portions. With the latter group the Brachiopods 

 were formerly united by Huxley, under the name 

 of Molluscoidea. This name is now obsolete, be- 

 cause it is understood that all these creatures are 

 widely different from Molluscs ; but the theory of 

 relationship of the Brachiopoda to the Polyzoa, 

 implied in it, still holds good. 



The chief importance of this group lies in its 

 fossil forms, which are exceedingly numerous, 

 particularly in the Mountain Limestone of the 

 Carboniferous Period ; it is crowded with their 

 shells, especially a form named, from its elon- 

 gated shape, Productus. The shells of Brachio- 

 pods are equal-sided ; that is to say, the right 

 and left valves match ; but they are inequivalve, 

 the ventral valve being much the biggest. It 

 often contains a foramen, or hole, at the beak, 

 for the passage of the pedicle, or stalk, by which 

 the animal is attached to the ground (e.g. Tere- 



