510 



BEACHIOPODA. 



namely Orbicula, the pedicle is wanting, the lower valve of the shell 

 being fixed immediately to the rock whereunto the animal is attached. 



(1340.) The shells of the Brachiopoda are formed upon two plans 

 one having the valves articulated, the other having them unarticulated*. 

 Those with articulated valves (Waldheimia) have the hinge generally 

 furnished with teeth and corresponding sockets, which so lock the 

 valves together that their movements are very limited. In the unarti- 

 culated forms (Lingula) the valves do not move upon each other, for 

 when open no part of their margins are in contact. The two divisions 

 of the Brachiopods thus indicated will be found to be still more marked 

 in their internal organization. 



(1341.) The muscular system in the Brachiopods is very complicated 

 and peculiar in its arrangement. Five or six pairs of muscles have 

 been described in the Terebratulidas, all of which have relation to the 

 movements of the valves upon each other, or to their attachment to, or 

 movements upon, the peduncle. Thus the muscles naturally divide 

 themselves into two groups, the valvular, and those for adjusting the 

 shell upon its peduncle. Of the former there are three pairs, which 

 have been denominated respectively " adductors," " cardinals," and 

 " accessory cardinals." Of the latter there are likewise three pairs, 

 whichh ave been designated the " dorsal pedicle muscles," the " ventral 

 pedicle muscles, and the " capsular muscles ;" the capsular muscles are, 

 however, generally blended into one. 



(1342.) 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 mantlelias'by common 

 consent been appropriated. The mantle 

 itself is thin and semitransparent ; but its 

 margins are thickened, and fringed with 

 delicate setae. 



(1343.) When the two lobes of the 

 mantle are widely divaricated, as in Lin- 

 gula (fig. 256), we perceive the prominent 

 orifice of the mouth (6) 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 Cuvierf supposes, may 

 act as oars, and thus enable the animal slightly to alter the position of 



* " On the Organization of the Brachiopoda," by Albany Hancock, F.R.S. (Phil. 

 Trans. 1858). 



t "Memoire sur 1' Animal de la Lingule." 



Fig. 256. 



Lingula, with the valves sepa- 

 rated : a, b, the pallium or man- 

 tle. (After Cuvier.) 



