112 ZOOLOGY. 



lamp. They have a very wide range, being found in all 

 latitudes, and at various depths of sea bottom. They are 

 much more commonly met with as fossils than as recent 

 shells, nearly two thousand extinct species having been 

 described, while less than one hundred living species are 

 known. They are found in all the geological formations 

 from the Cambrian upwards. They are especially abun- 

 dant in the Silurian rocks. 



FIG. 47. ANATOMY OF TEREBKATULA (the animal withdrawn within its shell). 



a a, mantle ; c c, peduncle ; d d' d", muscles which open and close the valves; 

 e, stomach ; i i, intestine ; lc y pseudo-heart; n n, ciliated arms; o, oviduct. 



157. Skeleton. The Brachiopoda, like the Lamelli- 

 branchiata, possess bivalve shells. The valves of the 

 brachiopod shells are always more or less unequal in size. 

 The shells are therefore said to be inequivalve. They 

 are equilateral that is, the two sides of the valves are 

 equal and symmetrical. On the other hand, the valves of 

 the Lamellibranchiata are generally equal in size, but are 

 always inequilateral and unsymmetrical. The valves are 

 dorsal and ventral, instead of being right and left, as in 

 the Lamellibranchiata. The ventral valve is always the 

 larger. 



The valves are often united together, along a hinge 



