72 ILLINOIS BIOLOGICAL MONOGRAPHS [266 



head, except for shape, can hardly be differentiated from the tail end, as it 

 does not possess eyes or speciaHzed sense organs of any sort. It is ^uite 

 round and nearly as broad as the widest part of the body, tapering not at all. 

 On the whole the shape of these specimens is more like that fo\ind in some of 

 the related genera than it is like this species. The anterior end is much more 

 contractile than the rest of the body, there being almost no contraction possible 

 back of the middle Hne. The amount of contraction, however, is very much 

 less than in most nearly related species, and neither is there the twisting and 

 turning which is characteristic of other types. That is, the body seems stiff 

 and solid. 



Even to the naked eye the color varies. It depends upon the amount of 

 green present and is sometimes deep and briUiant, at other times pale and of- 

 ten almost gray. This gray is the real color of the parenchyma, due to absence 

 of the pure green coloring matter. When there are a number of winter eggs, 

 large and nearly mature, they alter the general appearance, giving a dull 

 brown tinge which sometimes may be quite dark. 



The integmnent is very clear, showing not at all the cellular divisions so 

 plainly to be seen in some forms. It thus seems almost homogeneous. The 

 ciha are not clearly noticeable, as they are very fine and not differentiated. 

 The rhabdites are very small and He in the parenchyma just beneath the 

 epithelium. They are not arranged in definite tracts but are scattered evenly 

 thruout the body. The thickness of the integimient is somewhat surpris- 

 ing, as for such a small form it would hardly seem proportionate for the one 

 layer of outside covering to reach a thickness equal to one-tenth the body dia- 

 meter. 



The parenchyma is unique in every respect. The only cells which are con- 

 spicuous are those which contain the bright green color bodies. These bodies 

 are spherical and all of about the same size. They are arranged in a nearly 

 soUd mass in the cell waU, which is in every instance so filled that the surface 

 shows the outpushings of the solid bodies within. This makes the surface 

 shghtly rough. The wall, however, is fine and delicate, partially elastic so 

 that the boundaries of the green bodies are visible. Most of the cells contain 

 from six to ten bodies but an expecially large one may have as many as eighteen 

 or twenty-five. These zoochlorellae or color-bearing cells are not at all regular 

 in their placing, being scattered a few here, a few there, thruout the whole 

 body. There were never, however, very many in the part anterior to the 

 pharynx, a iict which makes the head much lighter in appearance. Other 

 parenchyma cells are of the common transparent structure and are hardly 

 visible imless the zoochlorellae are so nearly absent that the remaining cells 

 are not obscm-ed. The thinner material which makes up a part of the colored 

 mass is more sharply differentiated than in other types. Its very homogeneous 

 fluid nature is evident. This fluid is a very smooth light green, but whether the 

 color is merely a reflection from the zoochlorellae or is an intrinsic quality de- 

 veloped as a result of their action is hard to ascertain. 



