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The spermatozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion.

Now it is evident that the cochlea must act either as a whole, all the nerve fibres being affected by any variations of pressure, or the nerve fibres may have a selective action, each fibre being excited by a wave of a definite period, or there may exist small vibratile bodies between the nerve filaments and the pressures sent into the organ.

Other lowly plants propel themselves by means of a pair of filamentary protoplasmic threads, which vibrate actively, and are therefore called vibratile cilia.

This larva swims by means of minute vibratile hairs, or ciliæ.

They are flat rounded-oblong creatures, with a distinct integument or skin, "through which numerous vibratile cilia pass in regular rows."