THE BRACHIOPODS 



IN long past ages these creatures were far more abundant than 

 mollusks, and their shells are among the commonest fossils of the 

 oldest rocks. Now, however, they are nearly all extinct, and are 

 usually found only at considerable depths, or along tropical shores. 

 At first sight one would mistake these animals for clams or mus- 

 sels, but they are more closely related to worms than to mollusks. 

 In Brachiopods the two halves of each shell on either side of a 

 middle line are similar, wdiereas in clams and mussels the two 

 halves of each shell are not alike in shape. 



A still greater difference, which has l^een discovered through 

 careful study, is that the shells of the Brachiopods grow on the 



back and lower side of 

 the animal and the 

 head faces the gape of 

 the shell, whereas in 

 the mollusks the shells 

 grow on the right and 

 left sides, and the 

 ventral side of the bod^^ 

 faces the gape of the 

 shell. The mouth in 

 the Brachiopods is 

 flanked by two curi- 

 ously coiled and featJi- 

 ered arms which lie 

 witliin the cavity be- 

 tween the shells, and 

 are supported by skel- 

 etal rods attached to 

 the upper shell. These 

 serve as gills and also to capture the minute creatures upon which 

 the Brachiopod feeds. In Brachiopods the posterior end of the 



bodv is sometimes stalk-like and projects backward either through 



1^ 



Fig 43; Parchment Shells growing upon a stone. From 

 a depth of I611 feet off (Gloucester Harbor, Mass. 



