1 24 ELEMENTARY BIOLOGY. [CHAP. 



of the anterior adductor and below that of 

 the anterior retractor. Its fibres spread out in 

 a fan-like manner over the upper part of the 

 foot, some of them extending over the sur- 

 face of the liver. 



c. The lesser retractors. Several very small 

 muscles arising from the shell just in front 

 of the umbo and spreading over the surface 

 of the liver. 



. The intrinsic foot-muscles: forming the 

 greater part of the ventral portion of that 

 organ. 



77. Small muscles attached to each mantle-lobe, 

 at some little distance from its swollen free 

 edge and fixed to the shell along a linear 

 impression, which runs from one adductor to 

 the other and is termed the pallial line, 

 b. Tease out in glycerine a bit of one of the mus- 

 cles which has been treated with 0-5^ chromic 

 acid solution. Examine with \ inch obj. It 

 is composed of spindle-shaped flattened cells, 

 in each of which lies an elongated nucleus : 

 the substance surrounding the nucleus is clear, 

 but the rest of the cell is granular and con- 

 tains a great number of small particles arranged 

 pretty definitely in transverse rows. While these 

 muscular fibres agree in form with those of 

 smooth muscles^ in minute structure they ap- 

 proach striped muscles. 



12. The shell or exoskeleton. 



a. Its two hardened lateral pieces or valves ; each 

 with a straight dorsal and a curved ventral edge, 



