306 Biology in America 



than doos Jciiiiiiifrs;, ascribing tlio backing and tnrning 

 movement of tlie animal on its approach to au unfavorable 

 environment, to a reversal of the ciliary movement on the side 

 stimulated, and to the asymmetrical shape of the body. The 

 controversial pliase of the snl)ject is one which does not 

 interest tlie general reader; the important point is that a 

 primitive animal like Paramoecinm, lacking any specialized 

 sense organs or nervous system, is nevertheless as sensitive to 

 stimuli as the higlier organism, with its indescribably complex 

 organs of sense, and intricate maze of nervous paths. 



Many of the unicellular organisms, both plant and animal, 

 are exceedingly sensitive to light. This is especially well 

 shown by the ciliate Stentor. This is a gourd-shaped cell, 

 completely covered with cilia, except at the basal end or 

 "foot" by means of which the animal occasionally attaches 

 itself. At one end is a flattened or hollow disk surrounded 

 by a band of strong cilia which guide the food to a depression 

 in the tlisk, the mouth. Close to the outer surface of the 

 animal are a number of delicate contractile fibrils which func- 

 tion as muscles, in which respect Stentor shows a marked 

 advance in structure over Paramnecium. If the water in which 

 this ciliate is swimming be suddenly illuminated, the animal 

 reverses its movements, turns always in the same direction 

 (in respect to the sides of the body) and then goes ahead once 

 more. This reaction may be repeated a number of times, with 

 the final result that the animal, through a series of ' ' trials and 

 errors" is finally brought into a region of less light. 



Many of the unicellular plants and animals are provided 

 with little spots of red pigment which are sensitive to light. 

 In these forms, which belong to the group of flagellates, or 

 forms bearing one or more long, whip-like cilia, and many of 

 which are on the problematical fence between plants and 

 animals, light reactions are well marked. The reactions may 

 be either positive or negative, vigorous or weak, and may vary 

 with the physiological state or condition of the organism at 

 different times; but all serve to bring it into that strength 

 of light which the organism "likes" best, i. e., to which it 

 is best adapted. 



We are accustomed to think of unicellular organisms as 

 expressing life in its simplest terras, but we have seen never- 

 theless that many of them are indeed very complex creatures, 

 possessing organs of locomotion, digestion, excretion, contrac- 

 tion and even in some cases of special sense ("eye spots"). 

 Kecently Kofoid and his students working at the University 

 of California have discovered structures in certain Protozoa 

 which they believe represent a primitive nervous system. 

 These are delicate fibrils which can be clearly brought out 



