48 THE PROTOZOA 



while the posterior end commonly becomes drawn out into processes 

 similar to those seen in a pseudopodium in process of retraction. 

 In other forms, such as A. proteus (Fig. 2), the pseudopodia are sent 

 out on all sides and balance each other, in which case there is very 

 little translation of the body as a whole, and the pseudopodia serve 

 chiefly for food-capture. If, however, the outflow of the pseudo- 

 podia is strongest on one side of the body, the organism moves in 

 that direction as a whole, and the larger, more strongly developed 

 pseudopodia counteract and overcome the pull exerted by those 

 that are weaker. It will be readily understood, therefore, that the 

 most rapid powers of progression are possessed by the slug-like 

 amoebae, in which a single pseudopodium drags the whole body along 

 without opposition from others. 



Rhumbler (34) has drawn attention to the existence of two 

 modes of progression exerted by amoebae of the lobose type. In the 

 more fluid species which creep upon a substratum to which they 

 adhere more or less firmly, like Amoeba proteus, the animal pro- 

 gresses by a flowing movement, such as has been described : this is 

 the commonest type of amoeboid locomotion. On the other hand, 

 in species of the type of A. verrucosa and A. terricola the very 

 slightly fluid body is limited by a thin pellicle, and does not adhere 

 to the substratum. ; then progression is effected by ' rolling ' : 

 movement. The animal throws out a number of pseudopodia on 

 one side, which cause it ultimately to overbalance and roll over to 

 that side ; by continued repetition of this procedure, a slow progres- 

 sion in a particular direction is effected. At other times, however, 

 A. verrucosa may flow along like other amoebae. 



Contrasting with the lobose pseudopodia are the slender, thread- 

 like, so-called " filose " pseudopodia, formed entirely of ectoplasm. 

 Pseudopodia of this type can effect a slow creeping movement, but 

 are not very effective for locomotion, and serve for food-capture 

 principally, or even entirely, as in the radiate floating forms 

 (Heliozoa and Radiolaria) ; food is entangled by them and drawn 

 into the body. The filose pseudopodia may radiate from the 

 body in all directions, remaining separate from one another, or 

 they may anastomose t^o form networks, and are then termed 

 " reticulose." Pseudopodia of the reticulose type are specially 

 characteristic of the Foraminifera (Fig. 21). Radiate pseudopodia 

 which do not form anastomoses, on the other hand, characterize 

 the groups of the Heliozoa and Radiolaria, organisms of floating 

 habit. As noted above, pseudopodia of the radiate type are 

 .generally supported by an axial rod, a secreted structure of firm, 

 elastic nature, and are hence known as axopodia. The actual rod 

 reaches some way into the endoplasm, often to the centre of the 

 bodj-, as in Acanthocystis (Fig. 18), Wagnerella (Fig. 48), etc. ; it 



