34 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Since contractility is possessed by all forms of muscle-tissue, it is evident that 

 it is independent of superficial structural differences. Nor is muscle the only 

 substance possessing this property. Even isolated microscopic particles of liv- 

 ing matter are capable of making movements, both spontaneously and when 

 excited by external influences. As far back as 1755, Rosel von Rosenhof 

 described the apparently spontaneous changes in form of a living organism 

 composed of a single cell, a fresh-water amoeba. Moreover, he noted that, if 

 quiet, it could be excited to action by mechanical shocks. 



The amoeba (Fig. 1) is a little animal, of microscopic size, which is found 

 in the ooze at the bottom of pools, or in the slime which clings to some of our 

 fresh-water plants. Under the microscope it is seen to be composed of jelly- 

 like, almost transparent matter, in which are a vast number of fine granules, a 

 delicate tracery of finest fibrils, a small round body, called the nucleus or 

 endoplast, a round hollow space termed the contractile vesicle, which is seen to 

 change in size, appearing or disappearing from time to time, and small parti- 

 cles, which are bits of food or foreign bodies. In the resting state the body 

 has a somewhat flattened, irregular form, which, if the slide on which it rests be 

 kept warm, is found to alter from minute to minute. Little tongue-like projec- 

 tions, pseudopods (false feet), are protruded from the surface like feelers, and 

 are then withdrawn, while others appear in new places. Evidently the little 

 creature, though composed of a single cell, is endowed with life and has the 

 power of making movements. Moreover, it may be seen to change its place, 



the method of locomotion being a peculiar 

 one. One of the processes, or pseudopods, 

 may be extended a considerable distance, and 

 then, instead of being withdrawn, grow in size, 

 while the body of the animal becomes corre- 

 spondingly smaller ; thus a transfer of material 

 takes place, and this continues until the whole 

 of the material of the cell has flowed over to 

 the new place. This power of movement per- 

 mits the animal to eat. If when moving over 

 the slide it encounters suitable food material, a 

 diatom for instance, it flows round it, engulf- 

 ing it in its semifluid mass ; and in a similar 

 manner the animal gets rid of the useless sub- 

 stances which it may have surrounded, by flow- 

 ing away from them. These movements may 

 result from changes which have occurred within 

 its own substance, and apparently independently 

 of any external influence. On the other hand, 

 if its body be disturbed by being touched, by 

 an unusual temperature, by certain chemicals, 

 or by an electric shock, it replies by drawing in all of its pseudopods and 

 assuming a contracted, ball form. 



FIG. 2.-Vorticella nebulifera, X 600: 

 a. cilia of ciliated disk ; 6, ciliated disk ; 

 c, peristome ; d, vestibule ; e, oesophagus ; 

 f, contractile vesicle ; g, food-vacuoles ; 

 h, endoplast ; i, endosarc ; k, ectosarc ; I, 

 cuticle ; m, axis of stem (after Brooks : 

 Handbook of Invertebrate Zoology). 



