XVII 



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 probable that by their contraction, when the 

 adductors are relaxed, the body may become somewhat thicker 

 and the valves of the shells will slightly open. 



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



FiG. 316. — A semi-diagrammatic 

 figure of the muscular system 

 of Crania (after Blocbmann): 

 a, anterior occlusor; h, poste- 

 rior occlusor; c, superior ob- 

 lique; d, inferior oblique; e, 

 retractor of the arms; /, ele- 

 vator of the arms ; g, protractor 

 of the arms; h, unpaired me- 

 dian 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 

 brings about a certain sliding movement of the shells on one 

 another when they act independently of each other.^ 



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

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

 palaeontologists, and is adopted in Fig. 322. 



