Archive image from page 445 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana03todd Year: 1847 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 it
Archive image from page 445 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana03todd Year: 1847 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 well 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.
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