302 BRACHIOPODA. 
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 valve. The valve and hinge- plate are never found 
together, and it is, therefore, probable that i in the fossil species, 
the shells of which are found without hinge-plates, the muscles 
may have been arranged as in W. cranium. 
On separating the valves of a recent Terebratula, the digestive 
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, 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; they are, in fact, lateral pro- 
longations 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 have the power of pro- 
truding 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 food is obtained by means of currents 
created by cilia. Lingula may have the power of slightly extending 
the arms. The internal skeleton consists of two spiral processes 
in the Spiriferidz, whilst in Terebratula and Thecidium it takes 
the form of a loop, which supports the brachial membrane, but 
does not strictly follow the course of the arms. The mode in 
which the arms are folded is highly characteristic of the genera 
of Brachiopoda; the extent to which they are supported by a 
calcareous skeleton is of less importance, and liable to be 
modified by age. That margin of the oral arms which answers 
to the lower lip of an ordinary bivalve, is fringed with long 
filaments (cirri), as may be seen even in dry specimens of recent 
Terebratulz. In some fossil examples the cirri themselves were 
supported by slender processes of shell; they cannot, therefore, 
be vibratile organs, but are probably themselves covered with 
microscopic cilia, like the oral tentacles of the ascidian polypes. 
The anterior lip and inner margin of the oral arms are plain, and 
form a narrow gutter along which the particles collected by the 
ciliary currents” may be conveyed to the mouth. The object of 
the folding of the arms is obviously to give increased surface 
for the disposition of the cirri. 
The mouth conducts by a narrow csophagus to a simple 
stomach, which is surrounded by the large and granulated 
