§2] EFFECT ON STRUCTURE AND FUNCTIONS 137 



These are representatives of the groups Crustacea and Ver- 

 tebrata. 



Among all the species studied by Nagbl there was only one 

 which gave results not easily assignable to either of the two 

 types. This organism is the larva of a dragon-fly, ^schurea. 

 Its formula is excitation = excitation ; rest, rest. Nagbl says, 

 however, that these results were uncertain and variable. 



In some other groups studied — Coelenterata and Echinoder- 

 mata — the current used provoked no response at either pole ; 

 while with Amphioxus the current employed produced excita- 

 tion at both poles on both making and breaking the circuit. 

 Such variations as these, are, however, easily accounted for on 

 the ground that different species require currents of different 

 strengths to call forth what maj"- be termed the typical response. 



Not merely between different groups do we find a difference 

 in the type of response, but even inside the group of Protozoa 

 dissimilarity has been shown to occur. Thus Verwoen ('89*, 

 p. 301) has given reasons for believing that three flagellate 

 species, the ciliate Opalina, and some bacteria belong to the 

 katex type, although as just stated (p. 133) other Protozoa 

 exhibit the anex type of response. 



Finally it appears that individuals of one and the same species 

 subjected to different intensities of current may give rise to 

 responses belonging to the opposite types. Thus when a 

 medium current is " made " through Triton cristatus the exci- 

 tation is greater at the kathode than at the anode ; but when 

 the weakest current is employed a making response occurs at 

 the anode only, and when a slightly greater intensity is used the 

 continuance of the current provokes a continuance of the exci- 

 tation at the anode. (Nagel, '92% p. 341.) Likewise the 

 reaction of Vertebrate muscle varies with its internal condition. 

 Thus degenerated or over-stimulated muscle shows predominat- 

 ing anode stimulation on making the current ; and transverse 

 stimulation of the muscle fibre gives the same result.* 



To sum up, two principal types of response to the electric 

 current may be distinguished : the first or anex type character- 

 izing most Protozoa, MoUusca, Vertebrates (slightly stimulated), 



* For a discussion of these cases see Vekwokn ('89°, p. 24). 



