BRACHIOPODA. 211 



process, which renders them less oblique ; the upper valve is restored to its 

 place by two pairs of retractor slidiug-muscles, which are perhaps the equi- 

 valents of the dorsal pedicle muscles of Terebratula* The muscles are 

 remarkably glistening and tendinous, except at their expanded ends, which 

 are soft and fleshy ; their impressions are often deep, and always character- 

 istic ; but difficult of interpretation from their complexity, their change of 

 position, and the occasional suppression of some and combination of others. f 

 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 occu- 

 pied by the fringed arms, which have been already alluded to (page 8) 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. 24, 27, fig. 171, p-1')-) ; 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 TlhjnclioneUa 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 conjec- 

 tured that the living animals have the power of protruding their arms in 

 search of food ; but this supposition is rendered less probable by the fact 

 that in many genera they are supported by a brittle skeleton of shell. The 

 internal skeleton consists of two spiral processes in the Spiriferidce (fig. 132), 

 whilst in Terebratula and Thecidlum it takes the form of a loop, which sup- 

 ports 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 calca- 

 rious 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 {ci)ri'\.), as may be seen even in dry 

 specimens of recent TerebratulcB. In some fossil examples the cirri them- 

 selves 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 {cilio-brachiata of Farre). 

 The anterior lip and inner margin of the oral arms is plain, and forms a 



* In Discina one pair of the retractor muscles seems to be actually inserted in the 

 pedicle. Mr. Hancock compares the pedicle muscles with the retractors of the 

 Bryozoa ; he objects to the hypothesis of the sliding movement of the valves. 



t Prof. King has shoAvn that the compound nature of a muscular impression is 

 often indicated by the mode in which the vascular markings proceed from it (as in 

 figs. 140, 145.) 



I Called cilia at p. 8, but this term should be restricted to the microscopic organs 

 which clothe the cirri. 



§ Spiriferarostraia and Terebratula pectunculoides, in the British Museum. 



