434 
organs with sufficient force to propel itself 
through the surrounding medium. e Ya 
gression of the Salpa cristata is effected by 
drawing water into its body, at an opening si- 
tuated in the posterior segment of the mantle, 
where a valve (fig. 228 c) is placed to pre- 
Salpa cristata. 
vent its returning by the same aperture; the 
mantle having been distended by the water, 
contracts upon it, by which action it is ex- 
pelled at an opening situated at the side of the 
mouth (a); its progression is retrograde, or in 
the direction of a 6, opposite to that of the 
fluid in 6 a. 
The larve of some Dragon Flies, such as the 
Ashna and Libellula, draw in and expel the 
water alternately at the anus, which they occa- 
sionally lift out of the water, and project a 
small stream at a distance above the level of 
its surface: the hydrodynamic effect of these 
actions is to give a locomotive impulse to the 
centre of gravity in a direction opposite to that 
of the ejected fluid. The reaction of the water, 
which is equal to the action of the ejected 
stream, is not only sufficient to overcome the 
resistance of the surrounding medium and the 
inertia of the body of the animal, but also to 
drive it along. The velocity of the Syringo- 
des is accelerated during the expulsion of 
the water, and retarded during its reception ; 
consequently the motion is never uniform. 
The Vermiform animals are for the most part 
destitute of distinct locomotive organs, yet,owing 
to the flexibility of their lengthened and usually 
cylindrical bodies, they swim with great faci- 
lity. They glide by a series of lateral undu- 
latory movements of the body, with which the 
strike the water obliquely backwards, and wit 
equal force on each side of the axis of motion, so 
- that the force impressed on the water is trans- 
lated to the centre of gravity of the animal in 
an opposite direction forwards; the com 
sition of all the forces giving a resultant the di- 
MOTION, 
rection of which coincides with the axis of motion. 
Many serpents whose habits are chiefly terres- 
trial, swim with the head elevated wk 
surface of the water; others glide enti “ e- 
neath it. Some of the Entozoa, as the Tenia, 
and, among the Annelides, the Planarie, when 
immersed in warm water, swim by similar un 
dulations of the body; the latter, however, wil 
the ventral aspect upwards. In the Ophi 
Hydrophyli, or water-snakes, the tail i 
tened ; and its planes being directed vertically. 
give it the properties of a powerful oar, im 
striking the water by lateral oscillations. TI 
many chetopod Annelides, the sete cirt 
form numerous and complex external or 
progression. The Terebella has four rows ¢ 
sete in tufts; the dorsal row projecting in 
horizontal, the ventral in the vertical plane. 
They extend along the whole of the elongate 
subquadrangular-shaped body. In the Eunice 
Gigantea, which grows to the length of mor 
than ten feet, each ring is furnished with tw 
lateral packets of bristles, and two cirrhi. I 
the Nereis nuntia the locomotive ns @ 
complex and greatly multiplied. @ cirr 
and sete may propel the y forwards, 
dependently of those undulatory moveme 
which are indispensable for the progress 
of the apodous Annelides. wa 
Aquatic insects —The perfect insects sw 
like quadrupeds and birds, by the a 
flexion and extension of their legs. A 
the aquatic insects the Dytiscus is one 
best organized for swimming ; its figure res¢ 
bling that of a boat, being calculated to gli 
through the water with little resistance. T 
posterior legs aregreatly developed, an 
moved by powerful muscles perfor 
office of oars, and are the principal in 
used in swimming. Their movements fo 
are made in a plane nearly horizontal. TI 
haunches being xed to the thorax, give firm 
ness and precision to these legs, which do t 
differ materially from those of other Co 
with the exception of the tarsi, which are m 
flattened, and present their broad surf 
the water. They are furnished with 1 
stiff hairs, which bend when the leg is 
forwards, and become straight when its mor 
ment is vigorously reversed, thus in 
Fig. 229. 
The Dytiscus, from Straus-Durchheim, sho 
various positions which the posterior legs t 
swimming, J pe 
