Summary1The structure of the eye‐spot inEuglena viridie, E.Ehrenbergii, and Phacuspyrayormeis not as Franzé haa described it. The so‐called ‘crystal body “and” lens bodies “were not found in any of the individuals of the species examined by me. As earlier observers have stated, the eye‐spot consists simply of a mass of pigment granules arranged in a single layer, and probably embedded in a protoplasmic framework, but the latter could not be seen, and its presence was only inferred from the fact that the pigment granules must be held together in some way, t h d the eye‐spot as a whole is capable of expansion, and that the pigment granules can be separated from each other.2The gullet at the anterior end of the body does not end blindly, as was formerly supposed, near the principal vacuole, but actually opens into it, thus forming one continuous cavity open to the exterior.3The eye‐spot is in close contact with the gullet at the point where it opens into the excretory reservoir, and curves around it slightly.4The flagellum is not attached to the wall of the gullet, but arises from near the posterior or basal side of the excretory reservoir, to which it is attached by a bifurcate base. The bifurcation begins at about the level of the eye‐spot, and in this region an enlargement of one of the basal supports takes place. This, which is oval in shape, is in close contact with the eye‐spot, on its inner concave side. It is in the same position as that described by Frame for his “crystal body,” and may have been mistaken for this by him.5An explanation of this structure suggested by the author is that the light which is absorbed by the eye‐spot in some way stimulates this enlargement, which in turn reacts upon the flagellum and cause8 its movements to become modified. This is supported by the facts which are stated in the three following paragraphs.6The rays of light which are absorbed by the eyespot‐those in the region of the blue of the spectrum‐are the same as those which are active in modifying the direction of movement of the Euglena‐cell.7These rays can only influence the movements of the cell by acting upon or controlling in some way the mechanism by which the flagellum moves.8All those chlorophyll‐containing motile cells which are very sensitive to light contain an eyespot. Many other organism8 which do not possess an eye‐spot are sensitive to light, but not to 80 high a degree.9If the explanation given in par. 5 is correct, we have inEuglenaan extremely simple form of light‐organ, consisting of B specialized sensitive portion of the cell‐the enlargement on the flagellum‐and a light‐abs