(which for the sake of reference will here be called _flagellipodia_, Figure 44) whose pseudopodia stand about midway between typical flagella and typical pseudopods in their activity. In its general characteristics it stands near _A. radiosa_, but quite unlike the stiff, static pseudopods which _radiosa_ very frequently forms, this ameba has usually five or more slender pseudopods of which one or two or more are in slow flagellate motion. The distal third or half of the pseudopod is in the shape of a corkscrew. The free end of the pseudopod travels around in a circle (anti-clockwise in all instances observed), making one revolution in about three seconds. If this motion were very rapid it would act like a propeller and the ameba would swim through the water. The part of the pseudopod back of the mobile portion is usually also thrown into a spiral of gradually decreasing diameter until the spirality disappears. This portion of the pseudopod is not mobile in the same way that the distal portion is. Sometimes the whole of a pseudopod is thrown into a spiral, all of the turns being of equal size and only slightly motile. More than half of all the pseudopods formed become spiralized at one time or another of their existence, the greater number of these being however relatively immotile. Pseudopods frequently fall into spirals while they are being extended. [Illustration: Figure 44. _Amoeba flagellipodia._ _a_, showing nucleus, 4 microns in diameter, and four vacuoles. _b_, a pseudopod of three spiral turns which in a few seconds grew into one of six spiral turns, _c_. _d_, a pseudopod of a number of spiral turns, which a few seconds later took on a shape shown at _e_. The tip of the pseudopod at _f_ turned screw-like anti-clockwise, when looking at the tip and at the main body of the ameba. The tip made one complete revolution in about three seconds.] A better transition form between pseudopods and such flagella as are found, for example, in the peranemas, could hardly be imagined. The difference between crawling and swimming would seem to be merely a matter of speed of movement of the pseudopod.[6] But important as such a transition form is for theoretical purposes in understanding the nature of both flagella and pseudopods, it is of special importance for our present purpose because it shows a strong tendency for pseudopods to fall into spirals and to move in spirals. This tendency is found not only in this species of ameba but is observed also occasionally in _radiosa_ (Figure 7, p. 30) and in several other species. In these latter species the pseudopods are stiff and not capable of waving about in the water, as are those of _flagellipodia_, whether in the spiral shape or not. In _radiosa_ the pseudopods may become spiralized only as a preliminary to withdrawal. It is evident therefore that the spiral urge can express itself best in a plastic pseudopod.