90 
SCANDINAVIAN FISHES. 
orbital bone. Both grooves serve to receive the upper 
jaw when the mouth is closed. For this purpose in 
the Tunnies there is an expansion of the furrow in the 
lower jaw at the corner of the mouth, where the broad 
posterior part of the upper jaw may rest. This ex- 
pansion is fairly great even in Anxis. In the true 
Mackerels the upper jaw-bone slides completely under 
the preorbital bone, when the mouth is closed, and 
thus the expansion of the groove in the lower jaw is 
inconsiderable. 
Of the air-bladder there is nothing to remark, 
save that it is sometimes wanting in species very closely 
related to others where it is present. This family, 
however, like the following, is remarkable for the power 
which some of its larger members possess, of raising 
their own temperature considerably above that of the 
surrounding water. Davy states" that on his voyage 
to Ceylon he found the temperature of body in a Bonito 
— “in the deep-seated muscles in the thickest part of 
the fish, a little below the gills” — to be 99° Fahr., 
while the temperature of the water at the surface 
was only 80.5°. He was also informed by fishermen 
that they had observed the same circumstance in the 
Tunny and in other large species of this family 6 . This 
increase of temperature is connected with the great 
muscular activity enjoyed by the Scombroids. This mus- 
cular activity has its physiological conditions in an 
abundance of richly corpuscular blood, a powerful 0 
respiration and circulation, a thick, muscular heart and 
a strong innervation not only of the muscles of the 
body, but also of the organs of respiration. Thus Davy 
found the basal ganglia of the gills ( ganglion branchiate, 
belonging* to nervus vagus ) to be larger in the Scom- 
broids than in other fishes and almost as large as the 
“electric ganglia” in the Torpedo. The quick and ready 
movements of these fishes are also aided by the reduc- 
tion of the scales of the body which generally occurs 
in this family. The scales are sometimes fairly large, 
as in the region of the pectoral fins, and cycloid or 
— as in Scomber kanagurta from the Red Sea and the 
Indian Ocean — with wavy striations and crenulated 
at the margin. Often, however, they are wanting, at 
least externally, either over the whole body or over the 
greater part of its posterior portion, or else they are 
imbedded in the skin, as on the cheeks, where, as well 
as in the corslet, they are generally larger than on the 
rest of the body. All the Scombroids belong to the 
most active of salt-water fishes, and their power of 
wandering so far is closely connected with their wide 
geographical range. 
About 70 species, all belonging to the seas of the 
Tropic and Temperate Zones, are recognised and de- 
scribed in Gunther’s Catalogue; but several of these 
species are only imperfectly known, and on closer ob- 
servation, as Lutken has shown with regard to the 
Tunnies, will probably be explained as alterations due 
to age or varieties of other species. The number of 
forms within the family is large enough, however, to 
admit of sharp disputes not only in the distinction of 
the species, but also in their arrangement in genera. 
Though this may depend to a great extent on our 
ignorance of changes due to age in the different spe- 
cies, an additional reason is the considerable size of 
some of the species, which renders it difficult clearly 
to understand the relations of form. All that we know 
of the changes due to age, is summed up by Lutken d 
in some observations on the spines of the preoperculum, 
the larger jaw-teeth, and the shorter maxillaries of the 
fry (in Scomber colias). He also describes some small 
fishes from 8 to 17 mm. in length, which he refers 
with good reason to some species of Tunny, and assu- 
mes this to be Orcynus germo. These small fishes 
(fig. 25) are short and compressed in form, with sharply 
Fig. 25. Young Tunny, probably Orcynus germo. 
3 times the natural size. After Lutken. 
dentated preoperculum and with the second dorsal 
and the anal fins continuous, but with signs of the 
breaking-up of the posterior part into finlets. The 
mouth is large, its aperture, when shut, rising at an 
angle of 45°. The pectoral fins are broad and short, 
and the ventral short and rounded. With regard to 
the continuation of the changes of form during growth, 
“ Edinb. N. Phil. Journ., vol. XIX (1835), p. 325. 
b Cf. also Carpenter: Man. Physiol ., p. 481; Couch: Fish. Brit. Isl ., vol. II, p. 95; McCoy: Prodr. Zool. Viet., Dec. V, p. 24. 
c The more complicated structure of the gills in the Swordfishes and in one genus of the Mackerel family (Acanthocy biuin), where the 
several branchial laminae- coalesce into one lamina, meshed like a net, is also probably connected with this circumstance. 
d Spolia Atlantica, 1. c., p. 482. 
