S56 MANUAL OF THE MOLLUSCA. 
arrangement of the muscles. The articulated group possess an 
anal aperture ; the unarticulated none. (Hancock.)* 
The valves are both opened and closed by muscles; 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 yalye, behind the dental sockets, where 
there is usually a prominent cardinal process. The teeth form 
the fulcrum on which the dorsal yalve 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 yalye. The pedicle is fixed by a pair of muscles (each 
doubly-attached) to the dorsal hinge-plate, and by another 
pair to the ventral valve, outside the cardinal muscles.+ 
In the Terebratulidee and the other Brachiopods haying 
articulated valyes the muscular system consists of 3 pairs of 
muscles which act directly on the valves, and of 3 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 
help 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 receiyed 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. It has, therefore, been pro- 
posed to substitute the term_adjustor for protractor, and retractor 
for sliding as applied to these muscles. The following table 
shows the names in general use, and those proposed by Mr. 
Hancock :— 
Names in use. Names proposed. Homologous muscles in 
Unarticulated brachiopods. articulated brachiopods. 
Ant. retractors. Ant. occlusors. Ant. occlusors. 
Ant, adductors, Rost: 3; Post. occlusors. 
EOSts 1) iss Divaricators. Accessory divaricators. 
% Philosophical Transactions. 1858. 
7 The muscular system of Terebratula presents a considerable amount of resem. 
blance to that of Modiola (Fig. 214); the anterior and posterior pedal muscles may be 
corapared to the dersal and ventral pedicle muscles, 
