as Infusoria flagellata. 253 



tion in connexion with the tactile organ. It is a very minute 

 circular body, apparently about as broad as the diameter of the 

 Jlagellum, which lies a short distance behind the end of the 

 head and just in front of the mouth (m). Frequently, from its 

 excessive faintness and light-red colom*, it appears to be ab- 

 sent ; but, under careful scrutiny, it may always be detected. 

 The tendency which prevails to undervalue the importance of 

 this body, because it is present in an apparently similar posi- 

 tion in the zoospores of Algfe, no doubt hinders our advance- 

 ment in the knowledge of its true character and function. 

 Whether it is an organ of vision of any grade, or even a sen- 

 sorial centre of any kind, can only be brought within the range 

 of probability. Its constant presence demands attention, and 

 should excite inquiry on that ground alone ; but when, more- 

 over, we find it in a position which corresponds to that in which 

 the chief sensorial centres are usually situated, no mere resem- 

 blance to something else should divert us into a train of fancies 

 about the homologies of the red oil-globules of the zoospores 

 of Algffi, whilst the main point at issue is left in obscurity. 



If we cannot add anything further that is positive in regard 

 to this organ, it will be well at least to attract attention to it 

 in relation to its homologue in other Flagellata. In Phacus 

 plewonectes, Duj., it is not a uniform red spot, but seems to 

 be divided into two regions, one of which is lunate in shape 

 and of a bright red colour, and projects forward from the upper 

 side of the other like an appendage ; whilst the main part is 

 more dee^jly seated in the dorsum, and consists of a colourless, 

 but quite conspicuous, irregularly circular disk, about as broad 

 as the contractile vesicle, which it partially overlies. In this 

 case one might, with a fair show of reasonableness, suggest 

 that the red portion alone is the true eye-spot, and that the 

 colourless disk is a sensorial centre, not only for the former, 

 but also for the flagellum, which arises close to it, on the ven- 

 tral side. When we recall instances of the presence of a similar 

 disk, which is unaccompanied by a red spot, in certain species 

 of uniflagellate natant Flagellata [Peranemal^ I^^\j-)j ^^^1 mark 

 how long it is persistent after the body has fallen to pieces for 

 the lack of fresh water, one cannot but feel that its superior con- 

 sistency is a fair warrant for the belief that it is at least an 

 important organ, and that, seeing the very faint colour of that 

 of Astasia tricliopliora^ the absence of all tint does not neces- 

 sarily exclude it from the category of visual organs. On the 

 other hand, it might be justly questioned whether even the 

 deepest-colom'ed spots are at all sensitive to light ; and the 

 only answer would be that analogy renders it highly probable 

 that they are. 



