644 
SCANDINAVIAN FISHES. 
, Genus GASTEROSTEUS a . 
Body mackerel-like , compressed ( greatest depth more than half the length of the head), without other carince than 
the lateral edges of the kelly and the elevation of the lateral line on the hind part of the tail; forepart of the 
body {in front of the anal spine) longer than the hind part, including the caudal fin. Number of free spinous 
rays in front of the soft-rayed dorsal fin , less than 13. Ventral fins inserted at the outer , front corner of the 
pelvic hones , which coalesce with each other in the median line of the kelly into a triangular ventral plate , 
tapering behind. Jaw-teeth conical. 
The genus of the Sticklebacks contains the most 
widely-spread of all the Scandinavian fishes. There is 
hardly a single brook in which we fail to see these 
small and lively creatures swimming merrily about, 
often within narrow limits which the heat of summer 
reduces more and more each minute. For economical 
purposes they generally possess far too little importance 
to interest the practical fisherman, who usually knows 
them merely by name. The zoologist cannot content 
himself with so slight an acquaintance. To him a 
knowledge of the Stickleback is quite as important as 
of its far larger and far more useful fellow, the Salmon. 
Both are equally small fractions in the great series 
of natural forms which it is his task to investigate, 
and both are equally interesting, equally advantageous 
subjects of study. When we add to this the wonder- 
ful instinct of the Sticklebacks, which guides them to 
an architectural skill exceptional among fishes, we have 
sufficient grounds for bestowing on them more than 
usual attention. 
To the system atist. they are also of absorbing in- 
terest, for the course of development may be traced 
with especial distinctness in their differences of form 
and changes of growth. Their relation to the preceding 
genus has been expressed in a masterly way by Dr. 
Heincke 6 , in an elucidation of the significance of the 
variations affecting the dorsal spines in the Three- 
spined Stickleback. With this object he had examined 
about 10,000 specimens of the species in question. 
The Three-spined Stickleback is generally furnished 
— and this is one of its most important characters — 
with 3 free spinous rays at the dorsal edge in front 
of the soft-rayed dorsal fin. It had long been known, 
however, that the number of these spinous rays might 
be increased to 4 C ; and a variety of this kind, with 
the four spinous rays comparatively short, had been 
described by Cuvier and Valenciennes'* as a distinct 
Italian species, Gasterosteus tetracanthus. Heincke found 
about 1 percent of the Three-spined Sticklebacks that 
he examined, furnished with four spines; and of this 
number he examined 61 specimens in order to locate 
the supernumerary ray. Each of the spinous rays is 
attached to and articulated, in a singular manner which 
we shall describe more fully below, with one of the 
interspinal plates on the back. These plates are as a 
rule (fig. 157 and fig. 158, B) 6 in number and arranged 
3 
Fig. 158. Gasterosteus aculeatus, gymnurus. Natural size. From 
Italy. After Heincke. 
in a row: 2 small ones just behind the head, 2 larger 
ones, each with a spinous ray, the poster.^, usually 
the larger, 1 smaller plate with no spinous ray, and 
lastly 1 supporting the hindmost spinous ray. The 
largest two of these plates, however, are evidently due 
to the coalescence of at least 2 plates respectively. In 
fig. 159 we see the anterior one broken up into two 
parts, and in fig. 1 60 the posterior of them shows the 
same division. The anterior also rests on the upper 
° The generic name is derived from Artedi, Genera Piscium , p. 52. 
1 Ofvers. Vet. Akad. Fork. 1889, p. 395. 
c Cuv..' Val., Hist. Nat. Poiss., vol. IV, p. 491. 
d 1. c., p. 499. 
