i ORGANISATION AND STRUCTURE OF MUSCLE 5 



lateral contact. The cortical layer seems at first sight to consist of 

 a labyrinth of crumpled fibres. When, after slowly retracting, the 

 animals are almost globular, there is still a possibility of con- 

 traction. Ei-cry fibril suddenly becomes short, thick, and straight. 

 Instead of the labyrinth of little waves there is an instantaneous 

 reappearance of thick, straight, shining longitudinal stripes lying- 

 parallel with one another." 



Stentor can apparently contract spontaneously without 

 any assignable external stimulus. Engelmann (2, p. 447) 

 found on applying dilute acetic acid (O'l %), HC1 (O'l /), 

 H. 2 S0 4 (4 ^/), etc., that single fibrils at first contracted 

 frequently, even including those which the shrinking of 

 the endoplasm had separated from the pellicula. Ether, 

 chloroform, and the electric current also produce a sudden con- 

 traction of the muscular layer in the first instance. The lower 

 threshold of stimulation differs in different forms. Stentor, e.g., 

 reacts to much weaker currents than Carchesium. While the 

 current is passing, the protozoans usually remain in a state of con- 

 traction, but when it is weak they relax completely after a time, 

 even during its passage (Stentor, cf. Verworn, 4, p. 114). 



All the evidence, therefore, goes to prove that there are true 

 excitable and contractile fibrils in the myoid layer (myonema) of the 

 Infusoria in question, and that it is the rapid shortening of these 

 which produces the body twitches of Stentor and other Infusors. 

 Along with these, however, there are slower contractions (as 

 already stated), which indicate that the remaining protoplasm, 

 which is comparatively undifferentiated, also possesses contractility in 

 a definite direction. In these contractions the muscle-fibrils are 

 bent up in a wavy form, and are therefore relaxed. The endo- 

 plasm cannot here play an active part, since, although contractile, 

 it streams about in the most contrary directions, even while the 

 animal is slowly contracting. The fact of there being many 

 highly contractile Ciliata (Hypotricha^), in which, nevertheless, no 

 fibrils can be detected, proves that the differentiation of the 

 latter is causally connected with a definite kind of movement, i.e. 

 that muscle-fibrils subserve only rapid and energetic contractions. 

 Of this the most salient example is afforded by the so-called 



" stalk-muscle " of Vorticella. 



i 



As we have seen, the fibrils at the posterior end of the body 

 of Vorticella converge towards the neck of the stalk. In Polyps 



