4 BRACHIOPODA. 



It deservedly gained him the Royal medal, and is the 

 more praiseworthy because he never, I believe, had the 

 good fortune to see a living specimen. This oppor- 

 tunity, however, has frequently occurred to me ; and I 

 will endeavour, with the aid of Mr. Hancock's and other 

 treatises, to present a few remarks on the structure and 

 habits of this extraordinarv class. 



It has been usual to consider the valves of the shell 

 in Terebratula as covering the front and back of the 

 animal, the perforate valve being ventral, and the imper- 

 forate valve dorsal. When the Terebratula is attached 

 by its peduncle the perforate valve is uppermost. But 

 the analogy between the Brachiopoda and the Verte- 

 brata is very slight. The back of a Terebratula is really 

 that part which lies behind the arms and mouth, and is 

 close to the apex or point of attachment. Instead, 

 therefore, of calling the valves " ventral" and " dorsal," 

 it would seem more correct to describe them as "upper" 

 and " lower " — the larger and deeper valve being perfo- 

 rate and uppermost, and the smaller and shallower valve 

 being imperforate and lowermost. In the Brachiopoda 

 the valves are articulated across the back ; in the Con- 

 chifera the valves are united by a ligament or cartilage 

 along the back. The arms occupy two-thirds of the 

 shell. They resemble the mainspring of a watch, and 

 are not capable of being protruded or unrolled. I have 

 never observed the cirri, with which they are clothed, 

 to project much beyond the edges of the shell in the 

 living animal. The great extent of these brachial organs 

 is very remarkable. In Rhynchonella psittacea the arms, 

 when forcibly stretched out, are said to be more than 

 four times the length of the shell, and to support about 

 3000 cirri. In Terebratula caput-serpentis the cirri 

 open and fold together somewhat like a butterfly-net. 



