BRACHIOPODA. 301 



dividing the brachiopods into two great groups, the one having 

 articulated, the other non-articulated valves. In the first, more- 

 over, the valves are opened by muscles acting on the cardinal 

 process of the dorsal valve, while in the latter the valves are 

 opened by the pressure of the fluid in the perivisceral cavity. 

 This difference is accompanied by a striking variation in the 

 arrangement of the muscles. The articulated group possess an 

 anal aperture ; the unarticulated none. 



The valves are both opened and closed by muscles (cxxxiv, 1) ; 

 those which open the shell (cardinales) originate on each side 

 the centre of the ventral valve, and converge towards the hinge- 

 margin of the free valve, behind the dental sockets, where there 

 is usuall}' a prominent cardinal process. The teeth form the 

 fulcrum on which the dorsal valve turns. The adductor muscles 

 are four in number, and quite distinct in Crania and Discina ; in 

 Lingula the posterior pair are combined, and in Terebratula the 

 four muscles are separate at their dorsal terminations, but united 

 at their insertion in the centre of the larger valve. The pedicel 

 is fixed by a pair of muscles (each doubly attached) to the dorsal 

 hinge-plate, and by another pair to tlie ventral valve, outside the 

 cardinal muscles. 



In the Terebratulidse and the other brachiopods having articu- 

 lated valves the muscular system consists of' three pairs of 

 muscles which act directly on the valves, and of three pairs 

 which connect the shell, and adjust it with respect to the 

 peduncle. In the unarticulated brachiopods, such as Lingula, 

 the muscles are more complicated than in the former group ; 

 three pairs of protractor muscles keep the valves together, and 

 thus compensate for the absence of the hinge and condyles, 

 which lielp to form this function in the articulated group ; they 

 are so arranged as to co-operate in preventing any displacement 

 of the valves in any direction. Hence the term sliding-muscles 

 which they have received is inappropriate, since they prevent 

 any sliding action. In the lamellibranchs the sliding of the 

 valves is admirably guarded against by means of hinges with 

 teeth and sockets ; in brachiopods the same end is apparently 

 obtained by means of muscles. 



The muscles are remarkably glistening and tendinous, except 

 at their expanded ends, which are soft and fleshy. They are, 

 with few exceptions, non-striated. In the posterior adductors 

 of Waldheimia transverse striations are well displayed. Their 

 impressions are often deep, and always characteristic; but 

 difficult of interpretation from their complexity, their change of 

 position, and the occasional suppression of some and combina- 

 tion of others. There may be considerable changes in arrange- 

 ment of muscles without any important change in the internal 



