ZOOLOGICAL EXERCISES, 
these numbers are combined into the following fin 
formula :— 
P. 14; Was D. 4) 45; A. “. 
in which the numbers above the horizontal line are the 
spinous rays, those below mean the soft rays, and the 
perpendicular line shows the division between the 
two dorsal fins. 
2. Internal structure—Remove the skin in longitudinal 
strips from one side, beginning near the caudal fin. 
Notice the lateral or trunk muscles, divided by zig- 
zag lines into vertical segments, called myocomma. 
The muscular fibres are nearly horizontal. The 
myocomma are divided into two halves by the 
scleral spines which spring from the transverse pro- 
cesses of the vertebrae. Notice, just under the skin, 
a fine white cord lying along them. This is the 
lateral branch of the vagus nerve. 
3. Remove the muscles from the abdominal cavity, cutting 
down to the ribs. Notice the branching nerve on 
the posterior side of the pectoral fin. It arises on 
each side of the occipital vertebra, and is the first and 
second spinal nerve, or the spinal recurrent. Remove 
the scapular arch cutting through the scapula. Re- 
move the operculum, and notice the gills lying below. 
There are four branchial arches carrying gills on their 
lower and posterior sides, and gill rakers on their 
upper and anterior sides. The rakers on the anterior 
arch are in two rows, the anterior being long; the 
posterior row, and the single row on the other arches 
are short. These rakers act like a sieve, and prevent 
the food going out of the pharynx with the water. 
Remove them, cutting through the branchial arches 
