in AXIAL FIBRE 273 



The coiling of the stalk leads us to the consideration 

 of the particular form of contractility called muscular, 

 which is met with in multicellular animals, e.g., the frog 

 (p. 60). It was mentioned above that while the stalk in 

 its fully expanded condition is straight, the axial fibre is 

 not straight, but forms a very open spiral, i.e., it does 

 not lie in the centre of the stalk, but at any transverse 

 section is nearer the surface at one spot than elsewhere, 

 and this point as we ascend the stalk is directed succes- 

 sively to all points of the compass. 



Now suppose that the axial fibre undergoes a sudden 

 contraction, that is to say, a decrease in length accom- 

 panied by an increase in diameter, since, as we have 

 already seen, there is no decrease in volume in proto- 

 plasmic contraction. There will naturally follow a corre- 

 sponding shortening of the elastic cuticular substance 

 which forms the outer layer of the stalk. If the axial 

 fibre were entirely towards one side of the stalk, the 

 result of the contraction would be a flexure of the stalk 

 towards that side, but, as its direction is spiral, the 

 stalk is bent successively in every direction, that is, is 

 thrown into a close, spiral coil. 



The axial fibre is therefore a portion of the protoplasm 

 which possesses the property of contractility in a special 

 degree ; in which, moreover, contraction takes place in a 

 definite direction the direction of the length of the 

 fibre so that its inevitable result is to shorten the fibre 

 ! and consequently to bring its two ends nearer together. 

 This is the essential characteristic of a muscular con- 

 traction, and the axial fibre in the stalk of Vorticella is 

 therefore to be looked upon as the first instance of a 

 clearly differentiated muscle which has come under our 

 notice amongst unicellular animals. 



There are some interesting features in the repro- 



T 



