JJRACHIOPODA. 363 



The BRACHIOPODA inhabit bivalve shells, and for the most 

 part are suspended by a fleshy tubular pedicle, resembling that 

 of the Cirrhopods, to various submarine bodies. Such, at least, 

 is the case in Lingula and Terebratula ; Fig. 170. 



but in the third genus belonging to this 

 class, namely, Orbicula, the pedicle is // .AV.^fe, \ 



wanting, the lower valve of the shell be- 

 ing fixed immediately to the rock where- 

 unto the animal is attached. 



On separating the testaceous valves, 

 the body of the Brachiopod is found to 

 be enclosed between two delicate mem- 

 branes, which exactly line the shell ; and 

 to these membranes, as in the case of 

 other Mollusks, the name of mantle has 

 by common consent been appropriated. 

 The mantle itself is thin and semi-transparent ; but its margins are 

 thickened, and fringed with delicate cilia, the uses of which will 

 shortly become evident. 



When the two lobes of the mantle are widely divaricated, as 

 in Lingula (fig. 170), we perceive the prominent orifice of 

 the mouth (b) placed deeply between them : on each side of 

 the mouth are the two fleshy fringed arms, which in this case 

 can be protruded to a distance out of the shell, and, as Cuvier* 

 supposes, may act as oars, and thus enable the animal slightly 

 to alter the position of its body, or else, as they are most pro- 

 bably delicate organs of touch, they may perform the office of 

 highly sensible tentacula. 



In Terebratula psittacea the arms are enormously developed, 

 fringed upon their outer margins, and quite free except at their 

 origins : when completely contracted, they are disposed in six or 

 seven spiral folds, and, when unfolded, they extend beyond the shell 

 twice its longitudinal diameter. The mechanism by which they 

 are unfolded is described by Professor Owen~[* as being extremely 

 simple and beautiful. The principal stem of each arm is hollow 

 from one end to the other, and contains a fluid, which, being acted 

 upon by the spirally disposed muscles forming the parietes of the 

 canal, is forcibly injected towards the extremity of the arm, and 

 the organ is thus expanded and protruded outwards. 



* M6moire sur 1* Animal de la Lingule. 



f Transactions of the Zoological Society, vol. i. 



