BRACHIOPODA. gan 
Names in use. Names proposed. Homologous muscles in 
Unarticulated brachiopods. articulated brachiopods 
Cent. protractors. Cent. aie t 3 
Extr, ot Tope i Vent. adjustors. 
Post. retractors. Post. ” Dorsal 5 
Capsular. Peduncular. Peduncular. 
Ant. parietals. 
Post parietals. 
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 diffi- 
cult 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 
structure. Thus in Waldheimia cranium there are six muscular 
impressions in the dorsal valve; in W. australis there are only 
four, the other two muscles being attached to the hinge-plate, 
not to the yalye. The valve and hinge-plate are never found 
together, and it is, therefore, probable that in the fossil species, 
the shells of which are found without hinge-plates, the muscles 
may haye been arranged as in W. cranium. 
On separating the valves of a recent Terebratula, the diges- 
tive organs and muscles are seen to occupy only a very small 
space near the beak of the shell, partitioned off from the general 
cavity by a strong membrane, in the centre of which is placed 
the animal’s mouth. The large cavity is occupied by the 
fringed arms, which haye been already alluded to (p. 5) as 
the characteristic organs of the class. Their nature will be 
better understood by comparing them with the lips and labial 
tentacles of the ordinary bivalves (pp. 18, 21, and Fig. 208, p,p) ; 
they are, in fact, lateral prolongations of the lips supported 
on muscular stalks, and are so long as to require being folded 
or coiled up. In Rhynchonella and Lingula the arms are spiral 
and separate; in Terebratula and Discina they are only spiral 
at the tips, and are united together by a membrane, so as to 
form a lobed disk. It has been conjectured that the living 
animals haye the power of protruding their arms in search of 
food; but this supposition is unlikely, since in many genera 
they are supported by a brittle skeleton of shell, while the 
* Professor King has shown that the compound naturé of a muscular impression ig 
often indicated by the mode in which the vascular markings proceed from it (as in 
Figs. 176, 181) 
