646 



MOLLUSCOIDEA. 



" ligament " of the Bivalve Molluscs being developed. One pair of 

 " divaricators " is of much smaller size than the other, and the name 

 of " accessory divaricators " is given to these. The valves, on the 

 other hand, are held together by a pair of " adductor " or " occlusor " 

 muscles (fig. 491, a), which pass from one valve to the other, in the 

 neighbourhood of the beaks. The adductors bifurcate about the 

 middle, so as to produce a large quadrangular impression on the 

 internal surface of the dorsal valve (fig. 492, b, a a) and a single 

 divided impression towards the centre of the ventral valve (fig. 

 492, a, a). There are also, in some cases, muscles connected with 

 the movements of the peduncle ; and others (the " dorsal " and 

 " ventral adjustors "), which have the function of erecting the shell 

 and also that of attaching the peduncle to the shell. In the Inar- 



Fig. 492.— A, Interior of the ventral valve of Waldheimia Jlavescens: f, Foramen; d, Delti- 

 dium ; t, Teeth; a, Adductor impressions; c, Impressions of the divaricator muscles; c', Acces- 

 sory divaricators ; b, " Ventral adjustors" ; b' Peduncular muscle. B, Interior of the dorsal valve 

 of IValdheimia Jlavescens: c c' ', Cardinal process; b b' , Hinge-plate; .y, Dental sockets; /, 

 Loop ; q, Crura of the loop ; a «, Adductor impressions ; c, Point of attachment of the accessory 

 divaricator ; b, Point of attachment of the peduncular muscles ; ^, Septum. (After Davidson.) 



ticulated section of the Brachiopoda, as in Lingula, the arrangement 

 of the muscles by which the valves are opened and shut is still more 

 complicated. 



The " arms " or " brachial processes " are in some cases simply 

 coiled up within the shell, but in other cases, as previously noted, 

 they are supported by a more or less complicated calcareous skeleton, 

 which is known as the "loop" or "apophysis" (fig. 492, b). When 

 present, the loop is always attached to the dorsal valve of the shell, 

 and though it serves to support the brachial membrane, it does not 

 strictly follow the course of the arms. In its simplest form, the loop 

 is a thin ribbon-shaped calcareous lamella, the two ends of which 



