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. 



e. 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. 



ry. 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 ft&pattiallint* 

 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 -J 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, 



