276 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



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muscle, ex. Outer lamella of inner gill, i. Cerebral ganglia, in. Inner 

 lamella of inner gill. j. Cerebro-pedal commissure, k. Cerebro-visceral 

 commissure. I. Parieto-splilancnic ganglia. Ip. Labial palpi, m. Rec- 

 tum, mo. Mouth, pg. Pedal ganglia, t. Reproductive orifice. 



1. The four long flat gills (Fig. 142, o, g, i, g ; Figs. 

 143 and 144, c), two on each side, which are attached to 

 other structures above, but hang down into the branchial 

 chamber, like longitudinal curtains, with their ventral 

 margins free. 



2. Hanging down into the space between the gills is the 

 soft abdomen (Figs. 142 and 143, a, h). Its walls are 

 muscular, and the anterior and posterior foot retractor 

 muscles form its anterior and posterior faces, and suspend 

 it between the valves of the shell. 



3. On its ventral surface these muscles unite to form 

 the foot (Figs. 142, 143, and 144, 6), which is usually 

 quite small in a specimen which has been opened, but 

 is capable of great extension, and usually protrudes in the 

 livino^ animal from between the ventral edofes of the shell. 



4. Notice that the inner gill of each side is a little 

 larger than the outer, and its anterior edge rests between 

 a pair of flat, triangular, lip-like processes, the labial palps 

 (Figs. 142 and 143, Z,^). 



5. Above the foot, and just below the anterior adductor 

 muscle, these palps are continued across the front of the 

 abdomen, and between them is the large oval opening of 

 the mouth (Figs. 142 and 143, m, o). 



i. Pass a bristle into the dorsal siphon and notice that 

 it lies above the gills, and does not pass into the branchial 

 chamber. Remove the animal from both valves of the 

 shell, and cut, with a pair of scissors, along the line where 

 the inner edges of the inner gills join each other. Spread 

 the specimen out, under water, as shown in Fig. 143 and 



