1050 



SCANDINAVIAN I'lSHES. 



osts"; wliei'eiis tlic others (the posterior six) articiilHte j more or less distiinth' at aiiuies to a inoi-e developed 



in tlie Sterlet, according- to von Haitknfei.I), each with I basal ])ai-t, which is ori,ifinalh' situated at the hind 



a separate cartilage, parallel to the jielvic hone, but i margin of tlie tin — is best exjiiained l)y a comparison 



evidently of the same nature as the radiale. from which I with the transformation of the caudal fin from diphy- 



it has been separated by constiiction. Similarly a small [ cercy to heterocercy. Fishes'' in general began by niakin<f 



])ieee of cartilage is cut off from tiie distal (outer) end their caudal tin — their (earliest organ of locomotion — 



of most of the tirst-meiitioned radialia, so tiia,t three heterocercal, for tiie attainment of a more liighlv deve- 



rows of these bones are formed, the innermost row con- lojjed musculature, concentrated on one side, to steer and 



taining the pelvic bone and, behind this, the inner ra- 

 dialia. In the Sturgeon, on the otlier hand (tig. 289), 

 tlie post(n'ior (:'>) radialia are distinctly inclined at 

 angles to each other, and so ari'anged tliat only one 

 of the innermost parts ()iifpt), common to them all, ar- 



accelerate their ino\eraents. The same alteration was ex- 

 tended to the other vertical tins, dorsal and anal, the 

 anterior margin and its basal parts being strengthened 

 and dc\eloped intt) organs for cutting and stemming the 

 water or into weapons of offence and defence, while the 



ticulates with the jjelvic bone, and with the top of this | posterior parts grew more mobile with more numerous 



])art are jointed two cartilages, one belonging to the 

 fifth radiale, the other forming a common base for the 

 last two ((ith and 7th) radialia. .Ml the radialia thus 

 lie on one side of an imaginary a.xis, drawn through 

 the inner margin of the pelvic bone, and continued 

 outwards hx the liindmost (innermost) radiale. The 

 pelvic bone is now'' interpi-eted as being formed by the 

 coalescence of the inner parts of the anterior radialia; 

 and at the inner end of each ]>elvic lione we find in 

 adult Sturgeons' a constriction setting off' the part (jJJ)) 

 ^vhich, according to Wiedershei.m, is the true rudiment 

 of the pelvis of the Selachians and the higher verte- 

 brates. The dift'erence in the immber of the radialia in 

 the Sterlet and Sturgeon is the expression of a conti- 

 nued reduction, which has finally brought about their 

 disap])earance in bv far the greater number of the Te- 



divisions, but \\itli the outer (distal) joints arranged in 

 series on one side of the more or less confiuent basal 

 portions (Gegenbaih's mt'tupterygium). Having once 

 been e.stablished 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 onlv slight alteration, without 

 undergoing such coarctations or constrictions as attend 

 the development of perfect vertebra*. It is enveloped, 

 however, by a comparatively thick sheath (perichord, 

 fig. 290, Cs and Ee). in and upon \vliicli there develop 

 cai'tilages representing both neurapophyses (N) and 

 ha>ma])ophyses (7/), as \\ ell as upper spinous processes 



leosts. On the other hand, the similarity in structure (/'s), which parts, however, remain separate. Between 



of the original basal parts of the ))ectoral and ventral the apophvses lie strengthening disks (iiifrrcaloria, Ic), 



fins, their composition of ai'ticulated radialia, covered : and the luemapophyses are prolongated into transverse 



at the distal ends by the bases of the true (secondary) processes, bearing cartilaginous ribs, and also grow 



fin-rays, is the expression of the common origin of , inwards (Fo) under the notochord, where they sur- 



these fins. They have been i)roduced bv the differen- j round in tlie caudal reirion both the aorta and tlic 



.... . ■ . I . " . . 



tiation ot a fold running along eac-h side of the belly, ] caudal veins, in the abdominal region tmly the former. 



now ])ersistent only in the Laneelet, in the same man- Above the si)inal cord, which is enclosed on each side 



ner as the dorsal and anal fins have originated from by the apophyses, runs another similar canal, formed 



dift'erentiations in the region of the embryonic vertical by holes through each vertebra, and containing an 



fin. The one-sided arrangement of the radialia. — set elastic, longitudinal, tendinous band (EI). 



" C'f., however, DUr ri'innrks (above, p. G'^5) on the pelvic bones of the Heniibrani-bw, wliich bones .are evidently ilennal growlli-^ of 

 the nature of interspinal plates. The normal pelvic bones of tlie Teleosfs, on the otlier hand — as appears from their form — are hoinolo- 

 gons with interhiemal bones (supporting bones of the anal and caudal fins), and we now see that the pelvic bones of the Sturgeons are in- 

 terinuscnlar growths, and have the same origin as the supporting bones of the vertical tins, ontogenically being confluent constrictious of the 

 supporting cartilages of the ventral fins. 



'• Cf. WiEPERSHElM, Das Oliedmnssenskdet der WirbeUliieir, Jena 1892, ]i. 70: (Iriindriss VenjI. Aiiat. WirbeWi.. Jena 1893, p. 184. 



'■ As in Polyodon and Scaphirhynchops, see Wiedehsheim. 1. c. 



" Smitt, I'r de hogre djnrens utvecklingshistoria (lectures for 187.'!), Stockholm 1876, p. 2.30. 



