48 ILLINOIS BIOLOGICAL MONOGRAPHS [242 



to find in such a situation both a good feeding ground and a safe lurking place. 

 In very many hauls a few specimens appeared, but they were never in such 

 numbers as Stenostoma and were never so boldly free-swimming. They are 

 often seen, however, gliding out from mazes of filament and seem to swim 

 partly by cihar action and partly by muscular contraction. They spend 

 most of the time clinging or crawling among the algae but when removed to 

 a watch glass or aquarium never seemed to creep over the surface as do other 

 species. That is, their organs of attachment are not developed and the loco- 

 motion is entirely free. 



The general appearance much resembles a short, heavy Stenostoma. The 

 length varies from 0.75 to 1 mm. but seems much less owing to the relatively great 

 width. The width is about one-fourth the total length except in the posterior 

 third, which is narrowed as a tail portion The depth varies at about the same 

 rate as the width and is always very nearly the same for any given part. That 

 is, the ventral surface is convex with no flattening. The twisting and con- 

 traction are only sHghtly developed, altho the body is not as rigid as is the 

 case in many forms and the head is often drawn in, the back humped up and 

 the whole made into a nearly perfect sphere. Superficially, the color is 

 an opaque-white with a sHght greenish tinge which is heavy enough to make 

 the animals stand out prominently. There seems to be no pigment whatever, 

 even the eyes are not brilliant or dark as in most members of the genus. 



Both integmnent and parenchyma are somewhat clear. The former is 

 thin and regular in outline, allowing the scattered rhabdites of the internal 

 layer to show thru. The cilia are quite definitely heavy and regular all over 

 the surface except on the tail, where they are about five times as large. The 

 whole end portion of the tail region is evenly clothed with these larger cilia, 

 making a speciaUzed organ of locomotion. The layers of tissue beneath the 

 skin are masses at either end of the body, and also form a thin envelope over 

 the central body organs. This parenchyma is very closely meshed containing 

 space for only a very small amount of fluid material, so that the appearance 

 of both the head and tail is as thick and dark as the middle part. This is 

 somewhat unusual, as the extremities are almost always nearly transparent. 

 Another point to be considered is the fact that these two ends are of much great- 

 er relative size than is ordinarily the case. This seems the result of the storage 

 or concentration there of so much undifferentiated tissue. 



The central body mass is almost entirely composed of the digestive and 

 reproductive organs, the latter during the reproductive season occupying most 

 of the space. The pharynx and intestine are typically simple. The mouth 

 is ventral, lying just posterior to the eyes and opening into the heavily-walled 

 pharynx which is cask-shaped but very narrow in front. The boimdary of this 

 rather muscular structure is sharply defined and reflects the light to such an 

 extent as to appear glistening. It is comparatively small and not more than 

 one-sixth the body diameter. It opens directly into the intestine proper, 

 which is thin-walled and dark. This extends back to the genital pore. During 



