FIFTEEN -SP I N ED STICKLEBACK . 
641 
of' 10° simple rays of fairly uniform length, though 
the middle ones are somewhat longer than the outer, 
the tip of the fins, when they are folded, being thus 
evenly, but sharply rounded. When expanded these 
fins are fan-shaped. Their base is almost perpendicular, 
and by the great expansion of the shoulder-blade and 
the coracoid bone is removed farther than usual from 
the clavicle, leaving an even patch, covered with glossy 
skin, in front of it. The length of the pectoral fins is 
generally about equal to the base of the soft-rayed 
dorsal or the anal fin, and is usually somewhat greater 
in the males than in the females, varying between 
about- 10 and 12 % of the length of the body. 
The ventral fins lie a little behind the middle of 
the pelvic bones and a little behind the end of the first 
third * 6 of the body. They are made up of one spinous 
ray, which is distinctly longer than (sometimes nearly 
twice as long as) any of the other spinous rays, and 
behind this ray they have a- triangular fin-membrane, 
set obliquely inwards and backwards, and containing 
in its posterior (inner) part two small, soft rays, the 
last of which is in most cases difficult- to detect. Like 
all the other spinous rays, that of the ventral fins is 
also granulated throughout the greater part of its length, 
with smooth tip; but in this species all the spinous 
rays are without lateral spines and denticles. 
The only external difference that we have been 
able to discover between the sexes of this species, is 
that pointed out by Lilljeborg, namely that the pec- 
toral fins are generally longer in the males than in the 
females; and even this is faint. This uniformity is 
highly remarkable, for in our other Sticklebacks the 
difference between the sexes is striking enough, even 
in the external form. 
The peritoneum is white with a silvery lustre, but- 
dotted with black, which is often the predominant co- 
lour on the dorsal side of the abdominal cavity. The 
intestinal canal is simple and short. Just behind the 
diaphragm the oesophagus, which is lined with longi- 
tudinal folds, passes into the stomach, the inside of 
which is reticulate, and which extends scarcely half-way 
along the abdominal cavity, to about a line with the 
hind extremity of the interclavicles, where it- is bounded 
from the intestine by a strong and muscular contrac- 
tion. The inside of the intestine is thickly lined with 
villi, arranged in oblique, transverse series. In a 
straight- direction, and with a breadth almost equal to 
that of the stomach, but growing narrower behind, the 
intestine extends almost to the end of the abdominal 
cavity, where it bends in a sharp curve obliquely up- 
wards and forwards, soon t-o return in a curve no less 
sharp backwards and downwards to the vent. The only 
trace of pyloric appendages that we have been able to 
discover, is an extremely short process, directed back 
towards the stomach, on the intestine just behind the 
pylorus. 
The liver lies in the form of an undivided mass 
below and along the sides of the oesophagus and sto- 
mach, with a blunt point in a downward and forward 
direction towards the union of the interclavicles. On 
the left side it has a short lateral lobe, which does not 
extend further back than the main lobe; but the lobe 
on the right side is longer, and follows the intestine 
for about half of its anterior, straight- part, or a little 
further. At the end of the middle lobe, under the ter- 
mination of the stomach, a little to the right, lies the 
gall-bladder, and on the left side, above the end of the 
stomach, the spleen. The air-bladder is situated in the 
posterior part of the abdominal cavity, and its anterior 
extremity lies above the end of the stomach. 
The ovaries and testes are paired and saccate. The 
former at least, with their comparatively large and few 
eggs — Kr0yer estimated their maximum number at 
300 — occupy the greater part of the abdominal cavity 
during the spawning-season. 
In coloration the Fifteen-spined Stickleback is one 
of our most beautiful fishes, though under the influence 
of mental agitation, of terror for example, it- often 
suddenly loses its beauty and brightness of colour, and 
does not regain them for a long time. The upper part 
of the body is olive green, shading into brown, the 
lower yellowish with white, silvery belly. The sides 
are, as it were, transparent, showing glimpses of the 
spinous processes of the vertebrae in the form of dark, 
transverse, parallel streaks. Below the lateral line, on 
a ground of lustrous silver, lies a row of large, crescent- 
shaped and oval, olive green spots, which grow smaller 
behind and finally disappear on the tail. A dark brown 
band runs along the side of the head from the cor- 
ner of the mouth to the base of the pectoral fin. 
a Sometimes 9; exceptionally 11. 
6 The distance between the ventral spine and the tip of the snout varies in different individuals between about 34 and 37 % of the 
length of the body. 
