476 RECENT BRACHIOPODA chap. 



valve, and it probably belongs to the ventral side of the body. 

 It is in CisteUa, and doubtless in other genera, in close organic 

 connexion with both valves, and it seems to consist of an un- 

 usually large development of the supporting tissue which occurs 

 so frequently in the body of Brachiopods. The surface of the 

 peduncle is produced into several irregularities and projections 

 which fit into any depressions of the rock upon which the 

 animal is fixed. 



In CisteUa there are four pairs of muscles, two connected 

 with opening and closing the shell, and two with the movement 

 of the body upon the stalk (Fig. 314). The most considerable 

 of these muscles are the two occlusors, which have their origin, 

 one on each side of the middle line of the dorsal valve, and their 

 insertion by means of a tendon into the ventral valve. In the 

 species in question each of these muscles arises by a double 

 head, the two muscles thus formed probably representing the 

 anterior and posterior occlusors of other forms. The contraction 

 of these muscles undoubtedly serves to close the shell, which 

 is opened by a small pair of divaricators arising from the 

 ventral valve, and inserted into a portion of the dorsal shell 

 which is posterior to the axis of the hinge. Contraction of 

 these muscles would thus serve to approximate the posterior 

 edges of the valves and divaricate the anterior edges and thus to 

 open the shell. 



The adjusters are four in number, a ventral pair running 

 from the ventral valve to be inserted into the stalk, and a corre- 

 sponding dorsal pair from the dorsal valve. The simultaneous 

 contraction of either pair would tend to raise the valve, whilst 

 the alternate contraction of the muscles of each side would tend 

 to rotate the shell upon the peduncle. The muscles of Wald- 

 heimia fiavcscens are shown in Fig. 329, and described briefly 

 on p. 502. 



The muscles of the Ecardines differ from those of the Testi- 

 cardines inasmuch as they do not terminate in a tendon, 

 but the muscle fibres run straight from shell to shell. They 

 are also more numerous. In Crania there is an anterior and 

 a posterior pair of occlusor muscles, and two pairs of oblique 

 muscles, which seem when they contract together to move the 

 dorsal shell forwards, or when they contract alternately to 

 slightly rotate it. In this genus there are also a pair of pro- 



