436 
magnitude, swims in an inverted position; it 
1s propelled exclusively by its posterior legs, 
which are lengthened, and move in a plaue 
lel to the axis of the body. The thighs, 
egs, 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, arid 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 laminz, 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 
rotationare 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 Cephalopods.—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- 
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 
* See Principles of Resistance of Curved Sur- 
faces moving in Fluids. 
MOTION. 
ug 
the more iodern and more physiological con- 
clusions to be deduced from the researches of 
M. Sander Rang* and Madame Power, ‘ho 
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 al 
an inch long; its locomotive organs cons 
two uniform expansions attached on each 
of the neck, the planes of which lie paral 
the axis of the body. According to Esch 
the fins are composed of one muscular fasei- 
culus, which passes through the neck, a 
this muscle acts in a manner resembling t 
principle of the double-paddled oar with wh 
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 y determines 
the direction of the animal. The Clio, how- 
ever, is destitute of organs of prehension, 
consequently incapable of fixing itself to so 
it must therefore either remain at the botton 
of the sea or paddle its course upon the dense 
medium which it inhabits. 7” 
Pisces.—Amongst the great multitude of ani- 
mals moving in seas, rivers, and lakes, F 
next claim our attention. The medium in whit 
fishes move being nearly of the same speci! 
gravity as themselves, they are sustained by suc 
an amount of hydrostatic pressure as almost to 
neutralize the force of gravity upon their ma 
so that organs of progression, calculate ¥ 
support nearly their whole weight, such as occu 
in terrestrial animals moving on solids and in 
rarer medium, are unn . Weobservea 
that as they are sustained on all sides by gr 
hydrostatic pressure, they do not require the 
organs of support to be of that magnitude ami 
density which are requisite to  terrestri 
Mammalia for resisting the shocks of externa 
forces. In the osseous fishes the bones an 
therefore, light and elastic, and in the cart 
ginous fishes the organs of support are s 
more light and flexible. The specific gray 
of fishes, although small, is greater than un 
consequently we know, by hydrostatic prineig 
that without continued muscular effort, or s¢ 
provision for rendering themselves of equal or! 
specific gravity than the water, they musts 
to the bottom and remain there ;f but the: 
nomy of a great number of fishes requires 1 
they should sustain themselves pane 
above the solids forming the of ri 
lakes, and seas, and that they should be ena 
to rise to the surface, or sink into the depi 
the ocean in pursuit of their prey. As 
however, would otherwise require a vast” 
never-ceasing play of muscular action during 
Nature has provided them with an é 
which prevents this waste of muscular | 
the introduction into their system of t 
bladder. This hydrostatic apparatus 18 Of 
rious shapes, but always of sufficient dimensio! 
to contain, when it is distended, as mi 
inches of air as will render the fish s 
hy = 
* Vide Guerin’s Magazin de Zoologie. _ 4 , 
t See Theory of Specific Gravities, sect. du 
