128 PHYSIOLOGY OF PROTOZOA 



and it may be supposed that tlieir movement is brought about in a similar 

 manner by changes which occur in the thin cytoplasmic covering, the 

 axial fibre acting as an elastic support. Similarly, the myonemes which 

 occur in gregarines and other Protozoa may not in themselves be contractile, 

 though they may limit the contraction of the cytoplasm itself to definite 

 channels. It is generally supposed, however, that the filaments themselves 

 are contractile. In the case of attached forms like Vorticella the stalk 

 is composed of an axial fibre and a sheath of cytoplasm; when retraction 

 takes place, the axial fibre assumes the form of a compressed spiral. During 

 extension it appears that the elasticity of the axial fibre, which returns to 

 its original condition, is responsible for the extension of the stalk, and it is 

 possible that the sheath of cytoplasm is the sole cause of the retraction. 

 That cytoplasm itself, quite apart from the presence of myonemes or other 

 filaments, is able to perform sudden and rapid movements of contraction 

 is illustrated by the behaviour of contractile vacuoles. 



Another series of internal movements which are common to all cells 

 provided with nuclei are those associated with nuclear division. The 

 complicated process of mitosis, with the formation of the spindle and 

 chromosomes, and the subsequent separation of daughter chromosomes, 

 is in many cases carried out under the influence of the centrosome. In 

 many Protozoa, however, no centrosome is visible, but in neither case 

 has a satisfactory explanation of the phenomenon been given. AVhen a 

 centrosome is present, it appears to be the centre of activity, for it is 

 towards it that the rays of the aster and the spindle fibres are directed. 

 For those who regard the blepharoplasts of flagella as centrosomic in nature, 

 the action of the flagella is supposed to be another illustration of the motor 

 activities of the centrosome. 



The movements of the cytoplasm which have been considered are 

 distinct from the locomotion of the Protozoa themselves. An organism 

 which is in a resting condition and undergoing no changes in shape may 

 still show the streaming movement of the cytoplasm, but it is nevertheless 

 these movements of the cytoplasm which bring about the changes in 

 shape and actual locomotion when these occur. Progressive formation 

 of pseudopodia and changes in shape in amoebse are the result of continued 

 streaming movements in one direction, as explained above. In the case 

 of Mastigophora and Ciliophora it is the result of the continuous action 

 of the special organs of locomotion, which are so arranged that when 

 they are in activity the organism is propelled through the liquid medium. 

 The peculiar gliding or slug-like progression of gregarines has been sup- 

 posed to be due to the rapid secretion of a tenacious fluid from numerous 

 pores in the longitudinal grooves of that portion of the ectoplasm which 

 is in contact with the surface on which the organism is resting. It is 



