n CHANGE OF FORM IN MUSCLE DURING ACTIVITY 145 



plasma, the more so since the mobility of the plasma in vegetable 

 cells is on the whole but little developed, and stands at much the 

 same level as that of the free-swimming Amoeba, 



It can, however, be demonstrated that conductivity increases 

 pari passu with increased mobility and sensibility to external 

 stimuli a fact of which we have unmistakable evidence -in 

 Protozoa, on comparing the sluggish Rhizopods with the highly 

 mobile Flagellates and Ciliata. In most Infusoria there is, on 

 excitation, a specific conduction in minute motor organs (cilia, 

 flagellae), which must be regarded as a fibrillar differentiation ; 

 although in these cases the body-plasma itself seems to be the 

 conductor through which excitation is transmitted with extra- 

 ordinary rapidity. 



The ciliary movements in localised excitation of Ciliata belong 

 to this category. If, e.g., Paramsecium aurelia encounters any 

 obstacle in swimming, the cilia of the body collectively make a 

 stroke almost simultaneously in the direction opposed to the 

 normal, thus jerking the animal backwards, after which the original 

 movement begins again. A similar effect on the cilia, without 

 simultaneous co-operation of the myoideum, may also sometimes 

 be observed in this protozoan. 



The contraction of the myoideum itself, the simplest muscle- 

 element known to us, takes place as a rule so rapidly, that analysis 

 of its time-relations in localised excitation is impossible. " If, 

 e.g., a Spirostomum, which owing to its extended form is the best 

 adapted to this kind of experiment, is locally stimulated at one 

 end only, contraction of the whole body will ensue, without any 

 perceptible difference in time between the contraction of the 

 anterior and posterior ends." " Hence it may be concluded that 

 conductivity of excitation within the myoideum is excessively 

 rapid ; the effects of excitation follow immediately on the weakest 

 stimuli without any perceptible latent period, while in the relatively 

 undifferentiated rhizopod plasma there is almost invariably a pro- 

 nounced time of latent excitation between stimulus and visible 

 effects of stimulation " (Verworn). In both cases the myoideum 

 reacts precisely like the most highly differentiated striated muscles, 

 in which, however, notwithstanding the rapidity of transmission, 

 the wave of contraction can be exactly measured. We are 

 indebted to Aeby (1) for the first experiments in this direc- 

 tion ; he used the graphic method to determine the course of the 



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