130 University of California Publications. [Zoology 



ilhiiiiinaiion. This could be accomplished by the mode of loco- 

 motion above described, provided that the organisms were or- 

 iented with their heads uppermost. The form of the midbody, 

 especially in the pendant type, and of the antapicals, is strongly 

 suggestive that they may serve as organs of orientation. If they 

 have this function it may result either from a greater specitie 

 gravity posteriorly, or from a smaller specific surface in the same 

 region, or it may be from both. The lists suffice to give the an- 

 terior part a greater specific surface than is found posteriorly, 

 and in the passive sinking of the organism this difference would 

 tend to hold the anterior end uppermost. Treatment with osmic 

 acid reveals an accumulation in the head and neck of fatty or 

 oily substances which turn a darker brown than the cell contents 

 elsewhere in the body. The presence of these lighter substances 

 here Mould also tend to keep the anterior end uppermost. Fur- 

 thermore, it may be that these organisms exhibit a positive helio- 

 tropism for certain intensities of liiiht, as do some other Diuofia- 

 gellates. and that this reaction would result habitually in their 

 active orientation with the anterior end uppermost in levels of 

 optimum illumination. The orientation of the body may thus 

 be passive, as a result, when the organism is sinking, of the mole- 

 cular friction of the Avater on the greater specific surface of the 

 anterior end, or as a result of its smaller specific gravity, or it 

 may be active, resulting from positive heliotropism. In any case, 

 given this orientation, locomotion will result in movement toward 

 the surface away from the ceaseless pull of gravity. 



There is one disadvantage to the organism in this orientation 

 in that its form resistance to the pull of gravity is small. Indeed 

 the area of the water displaced by Tnposolenia in sinking while 

 in a vertical position is the least possible, being less than with the 

 dorsal or ventral edge below, and very much less than it would 

 be if the organism lay on either side. In the case of T. depressa 

 the displacement area in the vertical position is about 390 square 

 micromillimeters, with the ventral edge below it is 540 square 

 micromillimeters, while on one of the lateral faces it is about 

 1,200, or nearly three times as great. 



It is obvious that passive sinking as a result of the pull of 

 aravity, if long continued, would carry the organism below the 



