432 
centre of vity forwards, backwards, or ob- 
liquely. case is different with animals 
moving upon solids, where the weight of the 
body has to be supported as well as urged for- 
wards by the instruments of progression. When 
‘the weight of the water displaced is greater than 
that of the animal, the body floats upon the sur- 
face, as in the Palmipedes ; if, on the contrary, 
the weight of the animal be greater than that 
of the water displaced by its bulk, a verti- 
cal as well as a horizontal force is requisite, 
pe to the difference of the specific gravities 
of the animal and the water, to prevent its sink- 
ing during progression.* 
animal kingdom includes a vast number 
of species which are aquatic and constantly 
reside in ponds, lakes, rivers, and seas, having 
their general structures organized for inhabiting 
in these dense and resisting media, and their 
locomotive organs adapted for swimming. The 
number of these is far beyond the reach of 
calculation. Many of the larve of insects and 
the tadpoles of Amphibia, which in their adult 
state are either entirely or partially terrestrial, 
commence their career in water; in these not 
only the locomotive organs, but their respi- 
ratory systems undergo metamorphosis. 
Ciliograde animals.. -Under this denomina- 
tion are comprehended the polygastric and rota- 
tory animalcules, and many genera of the orders, 
such as the Porifera, Polypifera, and Acale- 
phe, whose locomotive organs are those minute, 
transparent, elastic, and very flexible conical 
filaments well known by the name of Cilia. 
The nature and structure of these organs have 
been fully detailed in the article C111, so as to 
render any further description here superfluous. 
The cilia act as levers, to which the water is 
the fulcrum. 
We may here referto the Volvox, as affording 
a familiar example of ciliary locomotion. The 
figure of this animalcule being spherical, the 
cilia placed on its surface are all equidistant 
MOTION. 
from its centre, but those possess the gre 
mechanical power which are placed at e 
distances from either pole of the animal’s 
of rotation. The volvox is capable of cham 
its axis of revolution, or varying its directic 
and appears to revolve across the field of tl 
microscope like a planet over that of the te 
scope. In the Rotifera, or whee’ 
the cilia are arranged in rows, around t 
margin of one or more circular dises, capab 
of being extended and retracted from the body.* 
When the tail of the animal is free, it moves” 
. 
animaic' 
retraction of the body, the head and tai 
selves by the two posterior exsertile bulbs. 
Porifera and Polypifera.—The comers 
Flustra are for a brief period capable of a cilio- 
grade mode of progression. In the gemmules” 
of sponges the cilia are spread over about two- 
thirds of the body. According to Grant, t 
zoophytes swim in a zigzag course, with the 
bous pre ae directed forwards . their fi 
is pyriform ; their migrations are ry 
duration, for after the lag of a few digs ¥ 
which are spent in seeking for some suit 
locality, they fix themselves during the rem 
der of their lives. ee 
The Actinie are capable of gliding upon 
the discs which form their bases of support. 
Reaumur asserts that they sometimes invert 
their position, and employ their tentacles as 
feet ; they also diminish theit specific gravity 
by augmenting their dimensions through the ab- 
sorption of water; when detaching es 
at the base, they suffer the current of the sea to” 
drift them from place to place. : 
Unlike most of the rhe which are fixed, 
the Hydra viridis is capable of moving in th 
liquid medium which it inhabits (fig. 226). 
226. mt 
tihemsel ve 
Fig. ~— 
ez .. : | Ma 
ATE 4 
Sor SE) | Y 
The Hydra viridis represented in its different stages of terrestrial locomotion, as figured by ' 
has three modes of progression ; the first is ac- 
complished by alternate flexions and extensions 
of the body ; thusthe head being fixed by the oral 
tentaclesat c (fig. 227), thelittle disc terminatin 
the anal extremity is drawn forwards from a, an 
* See theory of Specific Gravities. 
a 
fixed at b ; the head is then raised and carrie 
forwards, by the exension of the body, toward: 
d; these two actions of flexion and extensior 
complete a step, whose length is = ac — be. 
The second mode of progression is perfor 
| 
* See Ehrenberg’s Infus. Berlin, 1830, 
eg 
“a 
