THE MUSCULAR SYSTEM 



477 



tractors and a pair of retractors, and two levators of the arms, whose 

 function is to draw forward or retract the arms, and an unpaired 

 median or levator ani muscle. In addition to these bundles of 

 muscles there are certain muscles in the body wall, and it seems 

 proljable that liy their contraction, when the adductors are 

 relaxed, the body may become somewhat thicker and the valves 

 of the shells will slightly open. 



In Lingida (Fig. 322) the muscular system is more com- 



FiG. 316. — A semi - diagrammatic 



figure of the muscular system of 

 Crania (after Bloclimami) : a, 

 anterior occlusor ; b, jDosterior 

 occlusor ; c, superior oblique ; 

 (/, inferior oblique ; e, retractor 

 of the arms ; /, elevator of the 

 arms ; g, protractor of the arms ; 

 /(, unpaired median muscle. The 

 dorsal valve is uppermost. 



plicated ; in addition to the anterior ( = anterior laterals) and 

 posterior ( = centrals) pairs of occlusors, there is a single divari- 

 cator ( = umbonal), whose contractions in conjunction with those 

 of certain muscles in the body wall press forward the fluid in the 

 body cavity, and thus force the valves of the shell apart ; and 

 there are three pairs of adjustor muscles. These latter are 

 called respectively the central ( = middle laterals), external 

 ( = external laterals), and posterior ( = transmedians) adjustors, 

 whose action adjusts the shells when all contract together, and 

 l)rings about a certain sliding movement of the shells on one 

 another when they act independently of each other.^ 



^ Hancock's nomenclature is here used. The corresponding names used by King 

 and Brooks are placed in brackets. Their nomenclature is used by many palaeon- 

 tologists, and is adopted in Fig. 322. 



