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 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 onlj' 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 Spiriferidse, 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 dr}-^ specimens of recent 

 Terebratulse. 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 oesophagus to a simple 

 stomach, which is surrounded by the large and granulated 



