The Exterior of the Fish 23 
hypersensitive end-organs may well have been established. 
The sensory tracts along the sides of the body are certainly 
well situated to determine the direction of the approach of 
friend, enemy, or prey.” 
The Fins of Fishes.—The organs of locomotion in the fishes 
are known as fins. These are composed of bony or cartilaginous 
rods or rays connected by membranes. The fins are divided 
into two groups, paired fins and vertical fins. The pectoral fins, 
one on either side, correspond to the anterior limbs of the higher 
vertebrates. The ventral fins below or behind them represent 
the hinder limbs. Either or both pairs may be absent, but 
the ventrals are much more frequently abortive than the pec- 
torals. The insertion of the ventral fins may be abdominal, as 
in the sharks and the more generalized of the bony fishes, thoracic 
under the breast (the pelvis attached to the shoulder-girdle) or 
jugular, under the throat. When the ventral fins are ab- 
dominal, the pectoral fins are usually placed very low. The 
paired fins are not in general used for progression in the water, 
but serve rather to enable the fish to keep its equilibrium. 
With the rays, however, the wing-like pectoral fins form the 
chief organ of locomotion. 
The fin on the median line of the back is called the dorsal, 
that on the tail the caudal, and that on the lower median line 
the anal fin. The dorsal is often divided into two fins or even 
three. The anal is sometimes divided, and either dorsal or 
anal fin may have behind it detached single rays called finlets. 
The rays composing the fin may be either simple or branched 
The branched rays are always articulated, that is, crossed by 
numerous fine joints which render them flexible. Simple rays 
are also sometimes articulate. Rays thus jointed are known 
as soft rays, while those rays which are neither jointed nor 
branched are called spines. A spine is usually stiff and sharp- 
pointed, but it may be neither, and some spines are very slen- 
der and flexible, the lack of branches or joints being the 
feature which distinguishes spine from soft ray. 
The anterior rays of the dorsal and anal fins are spinous in 
most fishes with thoracic ventrals. The dorsal fin has usu- 
ally about ten spines, the anal three, but as to this there is 
much variation in different groups. When the dorsal is di- 
