ZOOLOGICAL EXERCISES, 
the siphons. Examine a portion with the quarter 
objective ; notice the rather distant groups of cilia: 
Lift up the border of lobe of the mantle belonging 
to the other side, and observe its attachment to the 
shell, 
4. Below the mantle lie a pair of large leaf-like gills or 
branchice. Cut them off one by one, and examine a 
portion with the microscope. They are formed by 
columns, united together by short transverse pro- . 
cesses, and are richly ciliated. 
5. Below the gills hes the compressed muscular foot, the 
upper part of which is hollowed out into a body 
cavity, which contains the abdominal viscera. 
6. The muscular system.— Observe the posterior adductor 
muscle, just inside the siphons; and the: anterior 
adductor muscle at the opposite end. These two 
large muscles, which had to be cut to remove the 
broken shell, are used for closing the valves. The 
other muscles are :— 
The anterior retractor of the foot, which is 
inserted into the shell near the hinge. 
The posterior retractor of the foot, which is 
inserted close to the posterior adductor. 
The protractor of the foot, which is inserted 
behind the anterior adductor. 
All these muscles arise from the general musculature 
of the foot. Examine a portion with the quarter 
objective; they are non-striated. 
7. The organ of Bojanus,—This ties on the dorsal side ot 
the animal, just above the gills, and passes under the 
posterior adductor. It is renal in function. It 1s 
composed of two parts, an upper and a lower. The 
upper, or dorsal portion, is a transparent sac, which 
