as Infusoria flagellata. 253 
tion in connexion with the tactile organ, It is a very minute 
 eircular body, apparently about as broad as the diameter of the 
flagellum, 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 colour, 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 Algw, 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 Algee, 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 
pleuronectes, 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 deeply 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 ik 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 (Peranema?, Duj.), and 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 
mportant organ, and that, seeing the very faint colour of that 
of Astasia trichophora, 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-coloured spots are at all sensitive to light; and the 
only answer would be that analogy renders it highly probable 
that they are. 
