504 



Forty-fiiTb Report on the State Museum. 



these is a single small pair which is attached to both valves {pro- 

 tractors of the free valve), which effects a slight forward and 

 backward movement of the upper valve. Two other small mus- 

 cular pairs are attached to the brachia, and have their origin on 

 one of the valves {protractors and retractors of the brachia). 

 There is also a small unpaired muscle originating on the surface 

 of the upper valve, close to the margin and in the median line ; 

 this is attached to the viscera, and according to Joubin, by 



Fig. 119. —Muscular system of Discinisca, the muscles remaining attached to the pedicle-val ve- 

 er, posterior adductor; 6, anterior adductor; c, median unpaired muscle; d, posterior pro. 

 tractor; e, anterior protractor; /, retractor; g, intestine; h, oesophagus; i, arms. (Joubin.) 



its contraction the passages from the genital organs are 

 opened. 



In Discinisca the arrangement of the muscles is very similar, 

 there being two pairs, instead of a single pair of protractorSy 

 which slide the upper or brachial valve out ward over the edges 

 of the attached valve, while a single pair of retractors draws the 

 valve into place again. There is also here an unpaired median 

 muscle which is probably an organ of retraction of the pedicle, 



56 



