120 ELEMENTARY BIOLOGY. . [XL 



S. The intrinsic foot-muscles: forming the greater 

 part of the ventral portion of that organ. 



e. Small muscles attached to each mantle-lobe, 

 at some little distance from its swollen free 

 edge and jSxed to the shell along a linear 

 impression, which runs from one adductor to 

 the other and is termed the pallial line. 



h. Tease oiit 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 af which lies an elongated nucleus: the 

 substance surrounding the nucleus is clear, but 

 the rest of the cell is granular and contains a 

 great number of small particles arranged pretty 

 definitely in transverse rows. While these mus- 

 cular fibres agree in form with those of smooth 

 muscles, in minute structure they approach 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, 

 and an anterior larger and postei-ior smaller end : 

 note the soft uncalcified ventral edge of each 

 valve. 



h. The umbo ; a small blunt eminence on the dorsal 

 border of each valve near its anterior end. 



c. The ligament : an elastic uncalcified part of the 

 exoskeleton behind the umbones, uniting the two 

 valves and tending to keep their ventral edges 

 slightly separated. 



