WHY FISH ARE FISHSHAPED.

Oamaru Mail, Volume XXXIX, Issue 10760, 6 May 1911, Page 3 (Supplement)

WHY FISH ARE FISHSHAPED.

That tiie peculiar and typical form I of fish can be- accounted for by the plastic pressure of the water through which they move is the theory propounded, and .supported by hundreds of experiments, by br Houssay. of the Sorbonne, Paris. Some of his results are described in an article contributed by Mr A. Mapnan to La Nature, from which the following is taken : —Study of a fish, Dr Houssay tells u.s, shows a body swollen at its forward end, pointed in tile rear, and also flattened —horizontally in front and vertically behind. From this body emerge- tins, some in pairs, .sumo single. Mr Magnan goes 011: "The fish is adapted to an aquatic medium ; that is to say, its form is the result of the pressure of the water on its plastie body. The resistance opposed by the water to its advance exerts a I

modelling action; this resistance has given to the fish its form and has evolved its fins. . . . Let us try to analyse the mechanism of this transformation.

"When water runs from a reservoir through an oval opening, for example, the liquid takes after its issue the form of a rubber tube twisted 90 degrees on. itself. The name of 'inverted vein' has been given to this biplanar aspect of the liquid. If, on the other hand, a solid elliptical body be drawn through the water, a void space is left behind it, which tends to be filled by the water compressed before it, taking the form of an 'inverted vein.' Now this void space has the shape of a fish that moves head foremast. . ." . The biplanar aspect of a fish's body is thus the prolonged result of this modelling. "Mr Houssay has repr-oduced, in a simple but remarkable experiment, this phenomenon of modelling. He has used an elongated, elastic rubber bag filled' with a. plastic liquid and closed with a solid shutter. The bag, while moving in the water, shutter forward, takes finely the shape of the. inverted vein. ■lt flattens out in front in a horizontal plane and behind in a. vertical plane. "The origin of the fins must ..also be attributed to the modelling action of the water. At a certain speed, the form of the inverted vein is prolonged at its edges into two thin plates, one in front in the horizontal plane, the other in the I rear in a vertical- plane. For a greater speed, the resistance of the water cuts these two plates up into lobes, representing the fins. . . "The cetaeese also give us an example of adaptation to a liquid environment. These are mammals that have returned to aquatic life after having led a. terrestrial existence. The water has Eressed, deformed, and modelled them, lit the adaptation has been less perfect, because of the previous existence of a rigid bony frame. So the stability of these animals is not so good; their body has a tendency to turn about its axis. The catacese correct this automatically by the aid of their brachial pallets and their caudal and dorsal fin 6. These movements tend to cause a coun-ter-rotation which determines a dissymetric pressure of water on the skull and makes it asymetric."

If this theory is correct, the writer goes on to say, the effects of "modelling" should keep on .until resistance is at a minimum. Dr Houssay tests this by making the resistance offered to their motion through the water. Contrary to his hypothesis, the fish-model met with more resistance than the others. This he believed to be due to the absence of fins; and apparently he was right, since, when fins were added to the models, they retarded the other sliapes, but did not affect the fish-form, so that the latter now moved more easily than the others. Having .thus studied what ho considered the typical fish-form, Dr Houssay next .proceeded to investigate the effect of variation from this type. His conclusions arc as follows: " 1. The most stable form is the short form. "2. All forms of fish are made absolutely stable at all speeds by the- aid of tlieir fins. In the short forms; the relaxation or extension of the paired fins is sufficient; in elongated forms the pectoral fins must be stretched. "3. The conical form, at least when, it is not short, cannot be balanced completely at certain speeds, by any method comparable to what takes place in nature.

"It occurs at once to" the mind that all this will apply to the construction of dirigible balloons. These now move in the air ill the fashion of unbalanced models," for every one knows that they pitch formidably. . . . From the. above experiments it would appear that the ideal air-sliip would be a short balloon fitted with fins calculated to stabilise it in the air. "Since publishing those preliminary results, Dr Houssay has continued liis experiments, and his pupils, with whom lie works, say that he has obtained interesting results, yet unpublished. He now moves his models with the heavy side up, and he lias found reasons . . . for the arrangement of the tail sometimes in two equal lobes as in ordinary fishes, and sometimes in unequal lobes. He has also measured by more than 300 curves on a registering-cylinder the strength of living fishes, and all these results form aliarmojiious whole —a complete and well-demonstrated theory-"