BEACHIOPODA. 357 



Names in use. Names proposed. Homologous muscles in 



Unarticulated bracliiopods. articulated bracMopods 



Cent, protractors. Cent, adjustors. ^ \7- + i • 



Extr. „ Extr. „ J \ ent. adjustors. 



Post, retractors. Post. „ Dorsal „ 



Capsular. Peduncular. Peduncular. 



Ant. parietaJs. 



Post parietals. 



The muscles are remarkably glistening and tecdinons, 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 yalye; in W. australis there are only 

 four, the other two muscles being attached to the hinge-plate, 

 not to the yalve. The yalye and hinge -plate are neyer 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 yalyes of a recent Terebratula, the diges- 

 tive organs and muscles are seen to occupy only a yery small 

 space near the beak of the shell, partitioned off from the general 

 cayity by a strong membrane, in the centre of which is placed 

 the animal's mouth. The large cayity is occupied by the 

 fi'inged arms, which haye been already alluded to (p. 5) as 

 the characteristic organs of the class. Their natui-e will be 

 better understood by comparing them with the lips and labial 

 tentacles of the ordinary biyalyes (i^p. 18, 21, and Fig. 208, ^,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 uj). In Bhynchonella and Lingula the arms are sj)iral 

 and separate ; in Terebratida 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 Hying 

 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 nature of a muscular impression is 

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

 Figs. 176, ISl) 



