312 



MANUAL OF ZOOLOGY. 



CHAPTER XLIV. 



BRACHIOPODA. 



CLASS III. BRACHIOPODA (Palliobranchiata). The members 

 of this class are defined by the possession of a body protected 

 by a bivalve shell, which is lined by an expansion of the in- 

 tegument, or " mantle." The mouth is furnished with two 

 long cirriferous arms. The nervous system consists of a single 

 ganglion, placed in the re-entering angle between the gullet 

 and the rectum, so that the intestine has a " neural flexure.' 7 



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 commonly known 

 as " Lamp-shells," and are all inhabitants of the sea. All the 

 living forms are fixed to some solid object in their adult condi- 

 tion ; but there is good reason to believe that many of the 





Fig. 126. Rhynchonella sn>cata. A, Profile view. B, View of the dorsal surface. 

 C, View of the base, a Ventral valve ; b Dorsal valve ; f Base ; c Beak ; h. Foramen. 

 Lower Cretaceous. 



fossil forms were unattached and free in their fully-grown con- 

 dition. From the presence of a bivalve shell, the Brachiopods 

 have often been placed near the true bivalve Mollusca (the 

 Lamellibranchiata) but their organisation is very much inferior, 

 and there are also sufficient differences in the shell to justify 

 their separation. 



The two valves of the shell of any Brachiopod (fig. 126) are 

 articulated together by an apparatus of teeth and sockets, or are 

 kept in apposition by muscular action alone. One of the valves 

 is always slightly, sometimes greatly, larger than the other, so 

 that the shell is said to be " inequivalve." As regards the 

 contained animal, the position of the valves is anterior and 

 posterior, so that they are therefore termed respectively the 

 u ventral " and " dorsal " valves. In the ordinary bivalve 



