64 THE CELL 



former may be distinguished from cilia by their greater thickness 

 and length, and by the fact that they are somewhat wide at the 

 base, whilst they taper off to a fine point (Fig. 48). Further, like 

 other special contractile tissues (muscular fibres), they exhibit a 

 fibrillar differentiation, so that they may be split up into many 

 delicate fibrils (Biitschli). These cirri occur with especial 

 frequency in hypotrichous Infusoria, being situated chiefly around 

 the mouth. The undulating membranes also terminate at the 

 mouth cavity. They are locomotive organs which have been 

 developed superficially; they may frequently be seen to be dis- 

 tinctly marked with delicate striae extending from the base to the 

 free edge, and hence they, like the cirri, must possess a fibrillar 

 structure. 



Infusoria have various methods of locomotion. As a rule the 

 body, when it moves freely through the water, revolves about its 

 longitudinal axis. It has the power of changing the direction in 

 which it travels ; the rate at which the cilia move may suddenly 

 be altered, being either slowed or quickened ; the body may even 

 keep still for a short time, without any apparent external cause. 

 Hence various kinds of movements take place, suggesting the 

 idea of volition. In addition, it is remarkable that the cilia, often 

 thousands in number, of one and the same individual, always act 

 together in a strictly co-ordinate fashion. " They do not only 

 always oscillate at the same rate, and with the same amplitude of 

 t>eat (rhythm), but they always strike the water in the same direc- 

 tion, and in the same order" (Yerworn). This co-ordination is 

 carried out to such an extent, that two individuals which have 

 been produced by the division of a parent cell always exhibit 

 uniform and synchronous movements as long as they are united 

 by a bridge of protoplasm. Hence it follows, that although the 

 cilia possess the power of spontaneous contraction, yet their work- 

 ing together is regulated by stimuli from the protoplasmic body 

 itself. 



The ectoplasm seems to play an especially important part in 

 the transference of these stimuli, as is shown by an experiment 

 made by Verworn (IV. 40). He made a slight incision with a 

 lancet in Spirostomum ambiguum (Fig. 49) and in Stentor 

 cmruleus in the ectoplasm supporting the rows of cilia. " Under 

 these circumstances it could be plainly seen that the ciliary 

 waves did not cross the area of the incision, but were confined to 

 the one side, and could not be seen on the other." Occasionally 



