THE ORGANIZATION OF THE PROTOZOA 47 



be followed. In the growing pseudopodium a strong current can 

 be observed flowing down the axis to the tip, and there spreading 

 out and breaking up into weaker currents which turn round and 

 flow backwards along the surface of the pseudopodium. In amoebae 

 with a very viscid surface layer the back-currents are very feeble, 

 ceasing a short way from the tip of the pseudopodium, and often 

 scarcely discernible, or even absent altogether ; in species with a 

 fluid ectoplasm, however, the back - currents are distinctly seen, 

 and may even pass back and bend round again to join the forward 

 axial current, as described by Rhumbler (34) in Amoeba blattce. 



While the extrusion of the pseudopodium is an active process, 

 the retraction requires nothing but the action of purely physical 

 forces of surface-tension to explain it. The protoplasm then flows 

 back into the body of the animal, and may present some character- 

 istic appearances in doing so. If one surface is in contact with the 

 substratum on which the animal is creeping, the adhesion of the 

 pseudopodium often causes the tip to be drawn out into slender 

 processes like spikes or hairs. At the same time the surface of the 



FIG. 20. Diagram to show the protoplasmic currents in a Umax- 

 amoeba which is moving forward in the direction indicated by 

 the large arrow on the left. The smaller arrows indicate the 

 direction, and their length the intensity, of the currents in 

 different parts of the body. A forwardly-directed " fountain 

 current " starts from near the hinder end, and passes along 

 the axis of the body to the extremity anterior in movement ; 

 there it turns outwards and passes back along the sides of the 

 body, diminishing rapidly in intensity, and finally dying out in 

 the regions where the two dots are placed. After Rhumbler 

 (34). 



pseudopodium may present a wrinkled appearance, as the viscid 

 ectoplasm shrinks in consequence of the rapid withdrawal of the 

 fluid endoplasm. 



The pseudopodia of different species of organisms, or even of 

 the same species at different periods of the life-cycle, vary greatly 

 in form, appearance, and structural characters, and the more im- 

 portant variations require a special terminology. In the first 

 place, the pseudopodia may be broad and thick relatively to their 

 length, as in Amoeba proteus (Fig. 2) ; they are then termed " lobose " 

 (" lobopodia "), and usually have a core of endoplasm. A typical 

 lobose pseudopodium is, in fact, nothing more than an outgrowth 

 of the body-protoplasm as a whole. In the most extreme cases of 

 this t}^pe, the whole body flows forward in one direction, forming, 

 as it were, a single pseudopodium. Such a mode of progression is 

 characteristic of Amoeba Umax (Fig. 20) and other similar forms, 

 in which the body glides forward like a slug as the animal creeps 

 over substratum ; the end which is anterior in movement is rounded, 



