PHYSIOLOGICAL 231 



plasmic threads, anticipations of muscle-fibres, but intracellular. 

 A familiar example is the axial filament that runs inside the non- 

 contractile sheath of the stalk of the Bell animalcule (Vorticella). 

 There are also good instances among the larger Sporozoa, which 

 move sluggishly, probably through a contraction of the internal 

 threads, which alters the shape of the cell and presses the firm cuticle 

 against the substratum. To amoeboid movement, probably the most 

 primitive mode of animal locomotion, special reference will be made 

 presently. In spite of its primitiveness, it is very intricate and 

 difficult to understand. 



CONTRACTILITY AND MOVEMENT 



In the single-celled animals movement is effected (a) by protrudmg 

 and retracting lobes or threads of living matter, as does the Amoeba ; 

 {b) by the contraction of internal threads (myonemes), which are 

 analogues of muscle-fibres, but within the cell ; and (c) by the flexing 

 and straightening of cilia, or by the undulations of flagella. Perhaps 

 a fourth method must be distinguished in the sluggish movements 

 of some of the corticate Sporozoa, which alter the shape of their 

 cell and probably press the firm cuticle against the substratum. 

 This may be sub-amoeboid; yet in some of the larger forms, such 

 as Gregarines, internal myonemes are clearly seen. 



Amceboid Movement. — How many eyes must have watched the 

 movements of Amoebse since Roesel von Rosenhof studied them in 

 1755. He was by profession a painter of miniatures and, therefore, 

 naturally interested in the minute. He tells us that he "frequently 

 spent two or three hours in observing" what he called the "kleine 

 Proteus" — the "Proteus animalcule", as it is still sometimes called — 

 and, with a sharply pointed quill, he performed micro-dissection on 

 the just visible animal, which is often about a hundredth of an inch 

 in diameter. Large for a Protozoon, but small for a quill! 



The Amoeba's flowing along is the most primitive mode of living 

 locomotion, yet uncommonly difficult to understand! It has been 

 studied and pondered over by many naturalists, not only for its 

 own sake, as movement before muscle, but because it persists in 

 higher animals, as, for instance, in the mobile phagocytes that form 

 a literal bodyguard, and in the microscopic tip of the fibre that 

 grows out from an embryonic nerve-cell, as if feeling its way into 

 the adjacent tissue. Here is a fine instance of conservatism in evolu- 

 tion — the same mode of cellular movement from Amoeba to man ! 



To the ordinary observer an Amoeba appears as a naked blob of 

 living matter that protrudes blunt finger-like "pseudopodia", and 

 draws them in again, altering its shape but not its volume, and 

 flowing along the substratum at a variable rate that can be measured. 



