24 The Exterior of the Fish 
vided all the rays of the first dorsal and usually the first ray of 
the second are spines. The caudal fin has never true spines, 
though at the base of its lobes are often rudimentary rays 
which resemble spines. Most spineless fishes have such rudi- 
ments in front of their vertical fins. The pectoral, as a rule, 
is without spines, although in the catfishes and some others a 
single large spine may be developed. The ventrals when ab- 
nominal are usually without spines. When thoracic: each 
usually, but not always, consists of one spine and five soft 
rays. When jugular the number of soft rays may be reduced, 
this being a phase of degeneration of the fin. In writing de- 
scriptions of fishes the number of spines may be indicated by 
Roman numerals, those of the soft rays by Arabic. Thus 
D. XII-I, 17 means that the dorsal is divided, that the an- 
terior portion consists of twelve spines, the posterior of one 
spine and seventeen soft rays. In some fishes, as the catfish or 
the salmon, there is a small fin on the back behind the dorsal 
fin. This is known as the adipose fin, being formed of fatty 
substance covered by skin. In a few catfishes, this adipose fin 
develops a spine or soft rays. 
Muscles.—The movements of the fins are accomplished by 
the muscles. These organs lie along the sides of the body, 
forming the flesh of the fish. They are little specialized, and 
not clearly differentiated as in the higher vertebrates. 
With the higher fishes there are several distinct systems of 
muscles controlling the jaws, the gills, the eye, the different 
fins, and the body itself. The largest of all is the great lateral 
muscle, composed of flake-like segments (myocommas) which 
correspond in general with the number of the vertebrae. In 
general the muscles of the fish are white in color. In some 
groups, especially of the mackerel family, they are deep red, 
charged with animal oils. In the salmon they are orange-red, 
a color also due to the presence of certain oils. 
In a few fishes muscular structures are modified into electric 
organs. These will be discussed in a later chapter. 
