MOLLUSCA— CLASS II. BRACHIOPODA. 175 



Testa semper bivalvis, ligamento cardinali nullo, musculis internis valide 

 adhaerens ; vel statim vel pediculo aut tendine tibroso affixa. 



The animals which we are now to consider differ materially in their 

 system of organization from those of the preceding class. They have 

 received the above title on account of their being provided with two long 

 spirally-twisted brachia or arms, and are distinguished by other not less 

 important particulars. The soft parts are differently arranged within the 

 shell, the valves are not united by any ligament, and there is a marked 

 change in the arrangement of their breathing apparatus. Although Pallas 

 has given a short anatomical description of the Terebratula, it was not 

 until the appearance of Cuvier's memoir on the anatomy of the Lingula 

 that the true characters of these remarkable animals became known ; it 

 was then determined that the Brachiopoda should be set apart in a sepa- 

 rate and distinct class. The anatomy of the Terebrutula and Orbicul/e 

 has since been most elaborately set forth by Professor Owen in the 

 ' Transactions of the Zoological Society,' and agrees in all its essential 

 particulars with that of the LingulcE previously described by Cuvier ; sub- 

 ject, however, to certain modifications arising from the different situations 

 they inhabit. The Lingultf, which are provided with a long pedicle, 

 commonly live near the surface, and are found at low water partially, if 

 not entirely, buried in the sand for the protection of their fragile shells 

 against the violence of the tides ; the Terebratulce, on the contrary, are 

 found in deep water, attached in clusters to fragments of rocks and coral- 

 lines by a bunch of short fibrous tendons issuing through an orifice in 

 the shell. 



The essential points in which these animals differ from other bivalve 

 mollusks are as follows : First , in the position of the soft parts within the 

 shell : in the Tropiopoda the back is placed against the hinge, and the 

 sides against each valve ; whilst in the Brachiopoda, the dorsal part of 

 the visceral mass is against one valve, and the ventral part against the 

 other. Secondly, in being provided with a pair of retractile brachia or 

 arms : for in the place usually occupied by the branchiae are two long spi- 

 rally-twisted arms, generally more or less fringed, and so strongly resem- 



