1050 
SCANDINAVIAN FISHES. 
osts“; whereas the others (the posterior six) articulate 
in the Sterlet, according to von Rautenfeld, each with 
a separate cartilage, parallel to the pelvic bone, but 
evidently of the same nature as the radiale, from which 
it has been separated by constriction. Similarly a small 
piece of cartilage is cut off from the distal (outer) end 
of most of the first-mentioned radialia, so that three 
rows of these bones are formed, the innermost row con- 
taining the pelvic bone and, behind this, the inner ra- 
dialia. In the Sturgeon, on the other hand (fig. 289), 
the posterior (3) radialia are distinctly inclined at 
angles to each other, and so arranged that only one 
of the innermost parts ( mtpt ), common to them all, ar- 
ticulates with the pelvic bone, and with the top of this 
part are jointed two cartilages, one belonging to the 
fifth radiale, the other forming a common base for the 
last two (6th and 7th) radialia. All the radialia thus 
lie on one side of an imaginary axis, drawn through 
the inner margin of the pelvic bone, and continued 
outwards by the hindmost (innermost) radiale. The 
pelvic bone is now * * * * * 6 interpreted as being formed by the 
coalescence of the inner parts of the anterior radialia; 
and at the inner end of each pelvic bone we find in 
adult Sturgeons*' a constriction setting off the part (pp) 
which, according to Wiedersheim, is the true rudiment 
of the pelvis of the Selachians and the higher verte- 
brates. The difference in the number of the radialia in 
the Sterlet and Sturgeon is the expression of a conti- 
nued reduction, which has finally brought about their 
disappearance in by far the greater number of the Te- 
leosts. On the other hand, the similarity in structure 
of the original basal parts of the pectoral and ventral 
fins, their composition of articulated radialia, covered 
at the distal ends by the bases of the true (secondary) 
fin-rays, is the expression of the common origin of 
these fins. They have been produced by the differen- 
tiation of a. fold running along each side of the belly, 
now persistent only in the Lancelet, in the same man- 
ner as the dorsal and anal fins have originated from 
differentiations in the region of the embryonic vertical 
fin. The one-sided arrangement of the radialia — set 
more or less distinctly at angles to a more developed 
basal part, which is originally situated at the hind 
margin of the fin — is best explained by a comparison 
with the transformation of the caudal fin from diphy- 
cercy to heteroeercy. Fishes d in general began by making 
their caudal fin - — their earliest organ of locomotion - 
heterocercal, for the attainment of a more highly deve- 
loped musculature, concentrated on one side, to steer and 
accelerate their movements. The same alteration was ex- 
tended to the other vertical fins, dorsal and anal, the 
anterior margin and its basal parts being strengthened 
and developed into organs for cutting and stemming the 
water or into weapons of offence and defence, while the 
posterior parts grew more mobile with more numerous 
divisions, but with the outer (distal) joints arranged in 
series on one side of the more or less confluent basal 
portions (Gegenbaur’s metapterygium). Having once 
been established among the vertebrates, this manner of 
development spread to the lateral fins, to the fore 
and hind limbs of the highest vertebrates. These too 
became unilateral. 
Among the remaining skeletal peculiarities most 
characteristic of the Sturgeon-fishes is the persistency 
of the notochord with only slight alteration, without 
undergoing such coarctations or constrictions as attend 
the development of perfect vertebrae. It is enveloped, 
however, by a comparatively thick sheath (perichord, 
fig. 290, Cs and Ee ), in and upon which there develop 
cartilages representing both neurapophyses ( N ) and 
haemapophyses (77), as well as upper spinous processes 
( Ps ), which parts, however, remain separate. Between 
the apophyses lie strengthening disks (inter cal aria, Ic), 
and the luemapophyses are prolongated into transverse 
processes, bearing cartilaginous ribs, and also grow 
inwards (Fo) under the notochord, where they sur- 
round in the caudal region both the aorta and the 
caudal veins, in the abdominal region only the former. 
Above the spinal cord, which is enclosed on each side 
by the apophyses, runs another similar canal, formed 
by holes through each vertebra, and containing an 
elastic, longitudinal, tendinous band (El). 
a Cf., however, our remarks (above, p. 635) on the pelvic bones of the Hemibranchs, which bones are evidently dermal growths of 
the nature of interspinal plates. The normal pelvic bones of the Teleosts, on the other hand — as appears from their form — are homolo- 
gous with interhcBinal bones (supporting bones of the anal and caudal fins), and we now see that the pelvic bones of the Sturgeons are in- 
termuscular growths, and have the same origin as the supporting bones, of the vertical fins, ontogenically being confluent constrictions of the 
supporting cartilages of the ventral fins. 
6 Cf. Wiedersheim, Das Gliedmassenskelet der Wirbelthiere, Jena 1892, p. 70; Grundriss Vergl. Anat. Wirbelth., Jena 1893, p. 184. 
c As in Polyodon and Scaplrirhynchops , see Wiedersheim, 1. c. 
d Smitt, Ur de hogre djurens utvecklingshistoria (lectures for 1873), Stockholm 1876, p. 230. 
