Origin of the Fins of Fishes.

Otago Witness, Issue 2572, 8 July 1903, Page 80

Origin of the Fins of Fishes.

One of the most interesting problems in vertebrate morphology, and one of the mo«t important from its wide-i caching relations, is that of the derivation of the fins of firfies This, resolves itself at once into two problems, the oiitrin of the median fine which appear in the lancelets, at the very bottom of the fish-like series, and the ongin of the paired fins- or limbs, which are much more complex, and which fiivt appear with the primitive shaiks In the process of development the median or vertical fins are doubtless older than the paiied fins or limbs, whatever be the origin of the latter. They ari^e in -a dermal keel which is developed in a web fitting and accentuating the undulatory motion of the body. In the einbiyo of the fish the continuous veitical fin fiom the head along the back and aiound the tail precedes any tiace of the paired fins. In this elementary fin-fold elementary suppoits, the rudiments of fin-rays, tend to appear at intervals. These are called ray-hair-. They aie the prototype of fin-rays in the einbyro fi.-h, and doubtless, similaily pieceded the latter in geological time. "In the development of fishes the caudal fin becomes moie and moie the teat of propulsion. The fin-rays are strengthened, and their basal supports are more and more specialised. That the vertical fins, dorsal, anal, and caudal, h<ive their origin in the median fold of the skin admits of no very serious question. 11l the lowest forms which bear fins these structures are dermal folds, being supported by very feeble rays. Doubtless at

first the vertical fins formed a continuous folc 1 extending around the tail, this fold ultinnteh- broken by atrophy into distinct dorsal, anal, and caudal fins. In the lower fishes, as in the earlier sharks, there ' is an approach to this condition of primitive continuity, and in the embryos of almost all fishes the same condition occurs. The question of the origin of the paired fins is much more difficult, and is still far from settled, although the majority of recent writers have favoured the theory that these are parts of a once continuous lateral fold of skin corresponding to the vertical fold wliich forms the dorsal, anal, and. caudil. In this view the lateral fold is soon atrophied in the middle, while at either end it is highly specialised, at first into an organ of direction, then. into fan- | shaped and, later, paddle-shaped organs of locomotion. Finally, from the jointed i paddle, which Gegenbaur has called the i archipterygium, there has developed, on the one hand, the rayed pectoral and vental fins of ordinary fishes, and on the other, in land-cieeping animals, jointed legs and arms. As to this the evidence of paleontology <s conflicting. An early shark of the Devonian, Cladose- ' laclie, has fan-shaped paired fine so formed and placed. Another shark almost as old, Pleuracanthus, of the Carboniferous, has fins which fit best a totally different theory of origin. Its jointed or archipterygial fin has no resemblance to a fold of skin, but accords better with Gegenbaur's theory i that the pectoral limb was at first a modified septum or gill arch. Sharks still older than either (Heterodontidse) in the Silurian, so far as we can judge by their teeth, are closely related to forms bearing the more specialised type of fin found in the typical sharks of to-day. Evidently none of these three, as seen in the rocks, represent the real beginning of paired fins in the life of the past. — David Starr Jordan", in the Popular Science Monthly.