435 



CHAPTER XXII. 



BRACHIOPODA. 



THE Brachiopoda are defined as Molluscous animals in which 

 tJie body is protected by a bivalve shell, which is lined by ex- 

 pansions of the integument or " mantle." The mouth is fur- 

 nished with long, spirally -coiled, cirriferous processes or " arms!' 

 The animal is never composite. (Fig 274.) 



The Brachiopoda are essentially very similar in structure 

 to the Polyzoa, from which they are distinguished by the fact 

 that they are never composite, and by the possession of a 

 bivalve, calcareous, or sub-calcareous shell: They are com- 

 monly known as " Lamp-shells," and are all inhabitants of 

 the sea. All the living forms, save the aberrant Lingula 

 pyramidata (fig. 274, A), are fixed to some solid object in 

 their adult condition ; but there is good reason to believe 

 that some of the fossil forms were unattached and free in 

 their fully-grown condition. From the presence, of a bivalve 

 shell, the Brachiopods have often been placed near the true 

 bivalve Molluscs (the Lamellibranchiata) ; but their organ- 

 isation is very much inferior, and there are also sufficient 

 differences in the shell to justify their separation. 



The two valves of the. shell in any Brachiopod are artic- 

 ulated together by an apparatus of teeth and sockets, or 

 are kept in apposition by muscular action alone. As re- 

 gards the contained animal, the position of the valves is 

 anterior and posterior, so that they are properly termed the 

 " ventral " and " dorsal " valves. One of the valves is always 

 slightly, sometimes greatly, larger than the other, so that the 



