Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Todd, Robert Bentley
centre of gravity forwards, backwards, or ob- from its centre, but those possess the greatest 
liquely. The case is different with animals mechanical power which are placed at equal 
moving upon solids, where the weight of the distances from either pole of the animal’s axis 
body has to be supported as well as urged for- of rotation. The volvox is capable of changing 
wards by the instruments of progression. When its axis of revolution, or varying its direction, 
the weight of the water displaced is greater than and appears to revolve across the field of the 
that of the animal, the body floats upon the sur- microscope like a planet over that of the tele¬ 
face, as in the Palmipedes ; if, on the contrary, scope. In the Rotifera, or wheel-animalcules, 
the weight of the animal be greater than that the cilia are arranged in rows, around the 
of the water displaced by its bulk, a verti- margin of one or more circular discs, capable 
cal as well as a horizontal force is requisite, of being extended and retracted from the body.* 
equal to the difference of the specific gravities When the tail of the animal is free, it moves 
of the animal and the water, to prevent its sink- by its cilia, pursues, and darts upon its prey in 
ing during progression.* every direction. The Rotifera are also capable 
The animal kingdom includes a vast number of crawling upon solids, by the extension and 
of species which are aquatic and constantly retraction of the body, the head and tail being 
reside in ponds, lakes, rivers, and seas, having alternately fixed points : they are also capable 
their general structures organized for inhabiting of revolving with great velocity on fixing them- 
in these dense and resisting media, and their selves by the two posterior exsertile bulbs, 
locomotive organs adapted for swimming. The Porifera and Polypi fera.—The Gorgona and 
number of these is far beyond the reach of Flustra are for a brief period capable of a cilio- 
calculation. Many of the larvæ of insects and grade mode of progression. In the gemmules 
the tadpoles of Amphibia, which in their adult of sponges the cilia are spread over about two- 
state are either entirely or partially terrestrial, thirds of the body. According to Grant, these 
commence their career in water; in these not zoophytes swim in a zigzag course, with the bul- 
only the locomotive organs, but their respi- bous extremities directed forwards ; their figure 
ratory systems undergo metamorphosis. is pyriform ; their migrations are of very brief 
Ciliograde animals. -Under this denomina- duration, for after the lapse of a few days only, 
tion are comprehended the polygastric and rota- which are spent in seeking for some suitable 
toiy animalcules, and many genera of the orders, locality, they fix themselves during the remain- 
such as the Porifera, Polypifera, and Acale- der of their lives. 
phæ, whose locomotive organs are those minute, The Actiniae are capable of gliding upon 
transparent, elastic, and very flexible conical the discs which form their bases of support, 
filaments well known by the name of Cilia. Reaumur asserts that they sometimes invert 
The nature and structure of these organs have their position, and employ their tentacles as 
been fully detailed in the article Cilia, so as to feet; they also diminish their specific gravity 
render any further description here superfluous, by augmenting their dimensions through the ab- 
The cilia act as levers, to which the water is sorption of water; when detaching themselves 
the fulcrum. at the base, they suffer the current of the sea to 
We may here referto the Volvox, as affording drift them from place to place, 
a familiar example of ciliary locomotion. The Unlike most of the Polypes which are fixed, 
figure of this animalcule being spherical, the the Hydra viridis is capable of moving in the 
cilia placed on its surface are all equidistant liquid medium which it inhabits (fig-226). It 
Fig. 226. 
Thë Hydra viridis represented in its different stages of terrestrial locomotion, as figured by Trembley. 
has three modes of progression ; the first is ac¬ 
complished by alternate flexions and extensions 
of the body ; thus the head being fixed by the oral 
tentacles at c (fig. 227), the little disc terminating 
the anal extremity is drawn forwards from a, and 
* See theory of Specific Gravities. 
fixed at b ; the head is then raised and carried 
forwards, by the exension of the body, towards 
d; these two actions of flexion and extension 
complete a step, whose length is = ac — be. 
The second mode of progression is performed 
* See Ehrenberg’s Infus. Berlin, 1830.


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