The motions of cilia and of flagella are probably also due to changes of surface tension—alternately on one side and the other in the cilium, but passing round in circular succession in the flagellum,[25] giving rise to a conical rotation like that of a weighted string that is whirled round the head. This motion is, however, strongest at the thicker basal part, which assumes a spiral form like a corkscrew of few turns, while the thin lash at the tip may seem even to be quietly extended like the point of the corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any object, the motions induce a jerking motion, which in this case is reciprocating, not rotatory. When the organism is free, the flagellum is usually in advance, and the cell follows, rotating at the same time round its longitudinal axis; such an anterior flagellum, called a "tractellum," is the common form in Protista that possess a single one (Figs. 29, 7, 8; 30, C). In the spermatozoa of Higher Animals (and some Sporozoa) the flagellum is posterior, and is called a "pulsellum." The cilium or flagellum may often be traced a certain distance into the substance of the cytoplasm to end in a dot of denser, {19}readily-staining plasm, which corresponds to a "centrosome" or centre of plasmic forces (see below, pp. 115, 121, 141); it has been termed a "blepharoplast."[26] Again, the cytoplasm may have differentiated in it definite streaks of specially contractile character; such streaks within its substance are called "myonemes"; they are, in fact, muscular _fibrils_. A "muscle-cell," in the Higher Animals, is one whose protoplasm is almost entirely so modified, with the exception of a small portion of granular cytoplasm investing the nucleus, and having mainly a nutritive function. Definite muscular fibrils in action shorten, and at the same time become thicker. It seems probable that they contain elongated vacuoles, and that the contents of these vary, so that when they have an increased osmotic equivalent, the vacuoles absorb water, enlarge, and tend to become more spherical, _i.e._ shorter and thicker, and so the fibril shortens as a whole. The relaxation would be due to the diffusion outwards of the solution of the osmotically active substances which induced expansion.[27] The MOTILE REACTIONS of the Protozoa[28] require study from another point of view: they are either (1) "spontaneous" or "arbitrary," as we may say, or (2) responsive to some stimulus. The latter kind we will take first, as they are characteristic of all free cells. The stimuli that induce movements of a responsive character are as follows:—(i.) MECHANICAL: such as agitation and contact; (ii.) force of GRAVITY, or CENTRIFUGAL FORCE; (iii.) CURRENTS in the water; (iv.) RADIANT ENERGY (LIGHT); (v.) changes in the TEMPERATURE of the medium; (vi.) ELECTRIC CURRENTS through the medium; (vii.) the presence of CHEMICAL SUBSTANCES in the medium.