MOLLUSCOIDA : BRACHIOPODA. 



271 



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 

 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 

 Lamellibranchiatd); 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 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 " inequi valve." As regards the contained 

 animal, the position of the valves is anterior 

 and posterior, so that they are therefore term- 

 ed respectively the "ventral" and "dorsal" 

 valves. In the ordinary bivalve Mollusca 

 (Lamellibranchiatd), on the other hand, the 

 two valves of the shell are usually of the 

 same size (equivalve), and they are situated 

 upon the sides of the animal, so that, instead 

 of being dorsal and ventral, they are now 

 termed " right " and " left " valves. The ven- 

 tral valve in the shell of the Brachiopoda is 

 usually the largest, and usually possesses a pro- 

 minent curved beak. The beak is sometimes 

 perforated by a " foramen," or terminal aper- 

 ture, through which there is transmitted a 

 muscular peduncle, whereby the shell is at- 

 tached to some foreign object. In some cases, 

 however (as in Lingula, fig. 97), the peduncle 

 simply passes between the apices of the valves, 

 and there is no foramen ; whilst in others, as 

 in Crania) the shell is merely attached by the 

 substance of the ventral valve. The dorsal or 

 smaller valve is always free, and is never perforated by a 

 foramen. 



In intimate structure, the shell of most of the Brachiopoda 

 consists " of flattened prisms, of considerable length, arranged 

 parallel to one another with great regularity, and at a very 



Fig. 97. Lingula 

 anatina, showing 

 the muscular ped- 

 uncle by which the 

 shell is attached. 



