BRACIIIOPODA. 



47 



BRACHIOPODA. 



These animals, which for a long time were classed with the [Case 

 Mollusca, are now considered by some naturalists, chiefly upon -''"-J 

 embryological grounds, to be more nearly related to Annelids or 

 marine worms. Others maintain that their affinities lie rather 

 with the Polyzoa and Tunicata, with which they form a distinct 

 class, termed Molluscoida. The Brachiopoda offer considerable 



Fiff. 



British Brachiopods (Terebratula and Crania). 



external resemblance to bivalve Mollusca, from which, however, 

 they differ in being always equilateral, never quite equivalve, and 

 in having neither a foot nor gills, respiration being performed by 

 the lobes of the mantle and two ciliated arms near the mouth. 

 These are generally supported by a shelly appendage, by the 

 form and disposition of which the various genera are distin- 

 guished. Brachiopods are marine, and always attached either by 

 the surface of one of the valves or by a peduncle which passes 

 through a hole at the beaked end of the shell, as in the Lamp-shells 

 (Terebratula). The valves are unconnected by a ligament and not 



