436 



MOTION. 



magnitude, swims in an inverted position; it 

 is propelled exclusively by its posterior legs, 

 which are lengthened, and move in a plane 

 parallel to the axis of the body. The thighs, 

 legs, and tarsi are nearly of equal length ; 

 the two phalanges, which are slightly flattened, 

 are furnished with hairs to strike the water 

 with greater force, and to vary the surface pre- 

 sented in the effective back strokes. When 

 the insect is poised freely in the water, its 

 centre of gravity lies in the vertical line, pass- 

 ing downwards from the centre of the figure. 

 From the singular circumstance of its swim- 

 ming on its back, it has derived the appellation 

 of Notonecta. 



Decapods. In the Crustaceous Macrourous 

 Decapods, such as the Lobster, Prawn, and 

 Shrimp, the tail is prolonged, and equals 

 the length of the body. It is the princi- 

 pal organ of locomotion, and the seven seg- 

 ments which compose it are nearly of a 

 semi-elliptical form, the terminal being fur- 

 nished on each side with laminae, which the 

 animal spreads out transversely like a fan, in 

 order to produce a greater surface in striking the 

 water. At the dorsal aspect, the segments of the 

 tail are locked before its extension is com- 

 pleted, but on the abdominal aspect there is 

 greater freedom of motion. The segments of 

 the tail are articulated with each other on both 

 sides by ginglymoid joints, of which the axes of 

 rotation are directed perpendicularly to the plane 

 of the mesial section, consequently their mo- 

 tions are restricted to one plane. A slight 

 eccentricity, however, in the direction of the 

 articulating axes of the joints permits a li- 

 mited obliquity of motion in the tail, but at 

 the expense of muscular power. In swim- 

 ming, the convex surface of the tail is pre- 

 sented to the water during the back stroke, 

 and the concave ventral surface in the effective 

 stroke, by which the force translated to the 

 centre of gravity in flexion is to that of exten- 

 sion nearly as two to one.* During each 

 flexion of the tail the animal is propelled 

 backwards, and its velocity is accelerated, but 

 during each extension it is retarded, so that its 

 movement is retrograde and accomplished by 

 a succession of impulses. In the swimming 

 Decapods, the thoracic stemmata are laminated 

 to assist in progression. 



The Cepfiulopods. The Cephalopods swim 

 according to the same principles as the Holo- 

 thuria, by admitting water into the interior of 

 the body and jetting it through the funnel with 

 sufficient velocity to communicate a locomotive 

 retrograde impulse to the animal, which enables 

 it to traverse the sea with considerable speed. 

 From the time of Aristotle to that of Cuvier, 

 the Argonaut, or Paper-Nautilus, has been sup- 

 posed to have its slight and delicate mono- 

 thalamus shell designed for a boat, and the 

 broad expanded membranes terminating the 

 two dorsal feet organized for sails; but, what- 

 ever poetry may have been associated with this 

 view, must be abandoned by the Zoologist for 



the more modern and more physiological con- 

 clusions to be deduced from the researches of 

 M. Sander Rang* and Madame Power, who 

 have discovered and assigned the true function 

 of these expansions, the fabrication of the shell. 



Pteropoda. Amongst Pteropods, the Clio 

 Borealis presents a conical shaped body about 

 an inch long ; its locomotive organs consist of 

 two uniform expansions attached on each side 

 of the neck, the planes of which lie parallel to 

 the axis of the body. According to Eschricht, 

 the tins are composed of one muscular fasci- 

 culus, which passes through the neck, and 

 this muscle acts in a manner resembling the 

 principle of the double-paddled oar with which 

 the Greenlander steers his course on the 

 surface of the same seas wherein the Clio is 

 found. The inclination of the planes of the 

 fins to that of the axis of the body determines 

 the direction of the animal. The Clio, how- 

 ever, is destitute of organs of prehension, and 

 consequently incapable of fixing itself to solids ; 

 it must therefore either remain at the bottom 

 of the sea or paddle its course upon the dense 

 medium which it inhabits. 



Pisces. Amongst the great multitude of ani- 

 mals moving in seas, rivers, and lakes, Fishes 

 next claim our attention. The medium in which 

 fishes move being nearly of the same specific 

 gravity as themselves, they are sustained by such 

 an amount of hydrostatic pressure as almost to 

 neutralize the force of gravity upon their mass, 

 so that organs of progression, calculated to 

 support nearly their whole weight, such as occur 

 in terrestrial animals moving on solids and in a 

 rarer medium, are unnecessary. We observe also 

 that as they are sustained on all sides by great 

 hydrostatic pressure, they do not require their 

 organs of support to be of that magnitude and 

 density which are requisite to terrestrial 

 Mammalia for resisting the shocks of external 

 forces. In the osseous fishes the bones are, 

 therefore, light and elastic, and in the cartila- 

 ginous fishes the organs of support are still 

 more light and flexible. The specific gravity 

 of fishes, although small, is greater than unity, 

 consequently we know, by hydrostatic principles, 

 that without continued muscular effort, or some 

 provision for rendering themselves of equal or less 

 specific gravity than ^the water, they must sink 

 to the bottom and remain there ;f but the eco- 

 nomy of a great number of fishes requires that 

 they should sustain themselves permanently far 

 above the solids forming the beds of rivers, 

 lakes, and seas, and that they should be enabled 

 to rise to the surface, or sink into the depths of 

 the ocean in pursuit of their prey. As this, 

 however, would otherwise require a vast and 

 never-ceasing play of muscular action during life, 

 Nature has provided them with an apparatus 

 which prevents this waste of muscular energy by 

 the introduction into their system of the air- 

 bladder. This hydrostatic apparatus is of va- 

 rious shapes, but always of sufficient dimensions 

 to contain, when it is distended, as many cubic 

 inches of air as will render the fish specifically 



* See Principles of Resistance of Curved Sur- 

 faces moving ia Fluids. 



* Vide Guerin's Magazin de Z -ologie. 



t See Theory of Specific Gravities, sect. 1, p. 412. 



