MOLLUSCOIDA BRACHIOPODA. 



113 



lino, by two teeth attached to the ventral valve, which 

 are inserted in sockets in the dorsal valve. There is no 

 ligament, the valves being both opened and closed by the 

 action of special sets of muscles. The shells are generally 

 calcareous, but in the Lingula they are of a horny 

 texture. 



The ventral valve is usually provided with a long beak 

 which is often perforated to allow the passage of a mus- 

 cular stalk, called the peduncle (Lat. pedunculus, a stalk), 

 by which the animals are attached to rocks, corals, &c. 

 The Terebratula (Lat. a little bore hole) derives its 

 name from this aperture. This peduncle sometimes 

 passes out between the valves, and in other cases the 

 animal is attached by the beak of the ventral valve, the 

 peduncle being- absent. It is believed that some fossil 



FIG. 48. FOSSIL BRACHIOPODA. 

 Rhynchonella Productus Spirifer. 



groups were alUe to move about freely. The valves are 

 lined by the mantle lobes. The interior of the shell 

 usually contains a calcareous skeleton, attached to 

 the inner surface of the dorsal valve, upon which the 

 arms are supported. In the extinct spirifers, this skeleton 

 was coiled up like a pair of watch springs. In the Tere- 

 bratula it has the form of a loop. 

 15 E. H 



