412 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



Strong bundles of muscular fibres, usually unstriated, pass 

 transversely from one valve of the shell to the other, and 

 bring them together ; while they are divaricated by the 



Fig. 118.— Anor?onta.~ Vertical and transverse section of the body through the heart; 

 /, ventricle ; g, auricles ; c, rectum ; p. pericardium ; h, inner, ?*, outer gill; (/, ves- 

 tibule of q, the organ of Bojanus ; £, foot, AA, mantle lobes. 



elastic reaction of the ligament. Of such adductor muscles 

 there may be either one or two. When there are two (Di- 

 myaria), the anterior adductor lies in front, and on the hae- 

 mal side, of the oesophagus; while the posterior adductor lies 

 in front, but on the neural side, of the rectum. Hence the 

 alimentary canal, as a whole, lies between those two muscles. 

 When only one adductor muscle exists (Monomyaria), it is 

 the posterior. 



The foot is retracted between the valves of the shell by 

 two or three pairs of retractor muscles, of which the anterior 

 and posterior pairs are usually attached to the shell, close to 

 the anterior and posterior adductor impressions. The pro- 

 traction of the foot appears to be effected by the compression 

 of the blood by the intrinsic muscles of the walls of the meso- 

 soma and of the foot itself. 



Each lobe of the mantle is attached to the corresponding 

 valve of the shell by a series of muscular fibres, the attach- 

 ments of which give rise to a linear impression, which runs 

 from one adductor to the other, and constitutes the pallial 



