zoo 
this is the progressive motion which does not wholly depend 
upon the motion of the water, whether it arises from the 
winds or the tides, but where the animal itself is impelled 
by the wind, and has parts peculiar to it fitted for that pur* 
pose. 
“ Some of the mollusca float upon the surface of the sea, 
and have their motion assisted by means of something re¬ 
sembling a sail. The most beautiful contrivance of this 
kind is in the Portuguese man-of-war (Holothuria Physalis). 
The lower part of the animal consists of innumerable tenta- 
cula hauging in the water; the upper is an oval bag dis¬ 
tended with air: when examined in the moist state, there is 
found a small valvular aperture, the size of a hog’s bristle, 
near the pointed end, and another of a smaller size near the 
opposite end, with a thickened edge, which looks like a 
dark spot transparent in the centre. There are preparations 
in the collection made by Mr. Hunter to shew both these 
openings. Dr. Telesius, of St. Petersburg!), who had the 
opportunity of seeing these animals alive, has given elegant 
engravings of them, and particularly describes the two 
orifices, of which no account had been before published. 
Some of the species he has delineated had a crest attached 
to the upper part of the bag, in a concave form, with trans¬ 
verse gelatinous bands, by the contraction of which it could 
be occasionally depressed, and afterwards raised by its own 
elasticity.” 
The next mode in which progression is effected in water, 
is by swimming. 
“ A fish in swimming, after having bent the tail, extends 
the dorsal, anal, and caudal fins, so as to increase the surface, 
and steady its body in the water. Having extended the tail, 
it contracts the fins until the tail is again bent. The stroke 
of a fish’s tail does not propel the fish straight forwards, but 
obliquely: to move forward, it must bend the tail first a 
little to the right side, and after making this stroke, bend it 
to the left so as to be propelled a little to that side; and thus 
the fish’s motion is in a zigzag direction. As fishes breathe 
through the medium of water, and are specifically heavier 
than the medium in which they live, they require some as¬ 
sistance to keep them from sinking to the bottom while 
swimming; this purpose is answered by air bladders, which 
the fish can at pleasure supply with air, so as to vary its 
specific gravity, as occasion requires. There are fishes, how¬ 
ever, which have no air bladders, and therefore cannot rise 
far from the bottom of the waters in which they live; they 
are broad and flat in their form; their mode of swimming is 
different from that of common fishes; it consists in raising 
themselves by the motion of the lateral fins, and pressing 
perpendicularly against the water.” 
The whales strike the water from below downwards, in¬ 
stead of from side to side. 
Water-birds, turtles, and frogs, may be said to row, since 
they use their legs as oars. 
The last and the most difficile mode of using fluid media 
for progression is jlying. The communications that exist 
between the lungs and bones of birds render them so light, 
that they require less muscular exertion than would other¬ 
wise have been necessary. Still this is exerted to a great 
degree. 
“ In swimming, the lower extremities, of whatever form, 
are the principal parts employed—but in flying, the upper 
extremities are the parts used; and those which correspond 
to the lower extremities of other animals take no part in this 
mode of progressive motion, but are destined for one of ano¬ 
ther kind; and the tail is added to direct their movements. 
The act of flying is performed in the following manner: 
the bird launches itself into the air, either by dropping from 
a height, or leaping from the ground: it raises up at the 
same time the wings, the bones of which correspond very 
closely with those of the human arm; but instead of the 
hand there is properly only one finger; it then spreads out 
the wings to their full extent, in a horizontal direction, and 
presses them down upon the air; and by a succession of 
these strokes, the bird rises up in the air with a velocity pro¬ 
portioned to the quickness with which they succeed each 
Vol. XXIV. No. 1675. 
LOGY. 826 
other. If the intervals between them are lengthened, the 
bird remains on the same level, and when made still greater 
it descends. This vertical movement can only be performed 
by birds whose wings are horizontal, as is probably the case 
with the lark and quail. In general the wings are oblique: 
this is principally owing to the length of the feathers, the 
fixed point of which is at the root. When birds fly hori¬ 
zontally, their motion is not in a straight line, but obliquely 
upwards, and they allow the body to come down to a lower 
level before a second stroke is made by their wings, so that 
they move in a succession of curves. To ascend obliquely, 
the wings must repeat their strokes upon the air in quick 
succession; and in descending obliquely these actions are 
proportionally slower. 
The tail, in its expanded state, supports the hind part of 
the body; when it is depressed while the bird is flying with 
great velocity, it retards the motion; and by raising the 
hinder part of the body it depresses the head. When the tail 
is turned up, it produces a contrary effect, and raises the 
head. Some birds employ the tail to direct their course, by 
turning it to one side or the other, in the same manner as a 
helm is used in steering a ship. Birds and flying insects in¬ 
clude all the animals that have regularly formed wings, and 
whose usual mode of progressive motion is flying, which 
they perform in the most perfect manner. There are, how¬ 
ever, some particular species of fish, and the lizard, which, 
although they are not endowed with regularly formed wings, 
yet have a substitute for them, by means of which they sup¬ 
port themselves in the air, and fly, for short distances, tole¬ 
rably well. What is still more extraordinary, there is an 
animal, the bat, belonging to the class mammalia, which, in 
its iuterna! structure, bears a resemblance to the quadruped, 
that is supplied with a kind of wing peculiar to itself, which 
may be considered as an intermediate link in the gradation 
between the wings of birds and those of the different ani¬ 
mals above mentioned.” 
The animals that use the earth and its products as fixed 
points for progression, adopt a multitude of very different 
methods. Man, and the majority of quadrupeds, walk — 
some animals climb —others creep —others again cling, 
either by hands, as monkeys ; by suckers, as the seal; or by 
a glutinous exudation, as some insects that run along ceilings. 
Most brutes jump; but this mode of progression is seen in 
its greatest perfection in monkeys, the kangaroo, and squir¬ 
rels. The worm moves by elongating and retracting its 
rings, of which the rough processes standing against the 
ground, afford fixed points. When we consider the great 
number of these points, we are not at a loss to estimate the 
mode in which, when they are acted against by correspon¬ 
dent muscular force, this creature is enabled to penetrate so 
readily the solid earth. The serpents move by various mo¬ 
difications of the locomotive apparatus; some move by the 
force of their tails; some by making the scutae of their bel¬ 
lies points of attachment to the earth, and then contracting 
and elongating their bodies like worms. The colubra, as 
we have noticed in the article Serpentes, perform their 
quick glidings by a process which is something like that 
our clowns use to draw themselves along the stage on the 
belly, when they have placed on their backs their hands 
and feet. These make the cartilages of the ribs the fixed 
points, alternately resting on those of one side, while they 
draw up the other. The serpent draws both sides forwards 
at once, but makes the ribs, by means of the scutae in which 
they terminate, the points of attachment. 
However various may be the modes of progression, it is ob¬ 
served in all cases, that muscles, when they have contracted, 
have a tendency to elongate themselves to their original length, 
or, when stretched, to retract themselves to their original short¬ 
ness. These phenomena are produced not by any vital power, 
but by the common elasticity that is met with in inanimate 
matter. All muscular fibres are enclosed in cells of an elastic 
nature; they terminate, also, in tendons or fasciae, having a 
similar property; and thus not only is the necessity of many 
antagonizing muscles obviated, but a celerity of movement ia 
acquired, which muscular contraction cannot effect. A fine 
8 Y example 
