80 BRITISH FRESH-WATER FISHES 
attention to the distribution and characteristics of our native 
sticklebacks. 
The Three-spined Stickleback (Gastrosteus aculeatus) 
Fin ForMULA. TEETH. 
First Dorsal : no membranes, 3 spines. Rather large, villiform, in 
Second Dorsal: 10 to 12 rays. both jaws. 
Pectoral ; 10 rays. 
Anal: 1 spine, 8 or 9g rays. 
Ventral: 1 spine, 1 ray. 
The three-spined stickleback is the smallest of British fresh- 
water fishes, probably never exceeding three inches in length, 
but it makes up for its diminutive stature by exceeding abun- 
dance, and avoids insignificance by reason of its destructive 
habits and indomitable pugnacity. 
In shape this stickleback is very graceful; the upper and 
lower contours sloping symmetrically from the small mouth to 
Appearance. the slender tail, which is set off by the brisk spring 
of the fan-shaped caudal fin. The pectoral fins, which appear 
to be the chief organs of locomotion, are set back about 
one-third of the entire distance between the snout and the 
margin of the caudal fin, and are large and fan-shaped. The 
soft or posterior dorsal fin balances the anal fin, and both 
combine to give the little creature a very smart appearance, 
from the foremost ray in each being nearly three times as long 
as the posterior ray. 
The foremost or spinous dorsal fin has disappeared, and its 
place is occupied by three formidable curved spines, which give 
the fish its distinctive name. Under the skin at the back 
of the head, are three other spines, two with their points 
directed forwards and one directed backwards. The ventral fins 
have become purely defensive, consisting merely of a single 
spine and a small ray. In front of the anal fin is an isolated 
spine, short, stout, and sharp. 
A 
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