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 usually 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 the ventral valve, outside the 
cardinal muscles. 
In the Terebratulidz 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 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 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 
