432 



MOTION. 



centre of gravity forwards, backwards, or ob- 

 liquely. The 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 (he 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, 

 equal to the difference of the specific gravities 

 of the animal and the water, to prevent its sink- 

 ing during progression.* 



The 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 larvae 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- 

 phse, whose locomotive organs are those minute, 

 transparent, elastic, and very flexible conical 

 filaments w^ell known by the name of Cilia. 

 The nature and structure of these organs have 

 been fully detailed in the article CILIA, so as to 

 render any further description here superfluous. 

 The cilia act as levers, to which the water is 

 the fulcrum. 



We may here refer to 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 



from its centre, but those possess the greatest 

 mechanical power which are placed at equal 

 distances from either pole of the animal's axis 

 of rotation. The volvox is capable of changing 

 its axis of revolution, or varying its direction, 

 and appears to revolve across the field of the 

 microscope like a planet over that of the tele- 

 scope. In the Rotifera, or wheel-animalcules, 

 the cilia are arranged in rows, around the 

 margin of one or more circular discs, capable 

 of being extended and retracted from the body.* 

 When the tail of the animal is free, it moves 

 by its cilia, pursues, and darts upon its prey in 

 every direction. The Rotifera are also capable 

 of crawling upon solids, by the extension and 

 retraction of the body, the head and tail being 

 alternately fixed points : they are also capable 

 of revolving with great velocity on fixing them- 

 selves by the two posterior exsertile bulbs. 



Porifera and Polypifera. The Gorgona and 

 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, these 

 zoophytes swim in a zigzag course, with the bul- 

 bous extremities directed forwards ; their figure 

 is pyriform ; their migrations are of very brief 

 duration, for after the lapse of a few days only, 

 which are spent in seeking for some suitable 

 locality, they fix themselves during the remain- 

 der of their lives. 



The Actinise 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 their specific gravity 

 by augmenting their dimensions through vhe ab- 

 sorption of water; when detaching themselves 

 at the base, they suffer the current of the sea to 

 drift them from place to place. 



Unlike most of the Polypes which are fixed, 

 the Hydra viridis is capable of moving in the 

 liquid medium which it inhabits ("/g.226). It 



Fig. 226. 



The 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 ; thusthe head being fixed by the oral 

 tentacles at c (Jig. 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 /. 

 The second mode of progression is performed 



* See Ehrenberg's Infus. Berlin, 1830. 



