260 University of California Publications in Zoology [ V L. 20 



at its middle as to form a short V (fig. I), and its width averages about 

 0.4 to 0.5 of its length. In. the slender types the macronucleus is rela- 

 tively long and slender, being approximately % of the entire length 

 of the organism. It is ordinarily sausage-shaped, but it may also be 

 in the form of a straight rod slightly enlarged toward the ends, or 

 it may be so curved as to form an almost complete ring. In contrast 

 to the form described above, the width in this case is about 0.2 to 0.3 

 of its length. Differences so great as these, viz., a length 2 to 3 times 

 the breadth in one case and 4 to 5 times the breadth in the other are 

 easily recognizable without actual measurement. These figures rep- 

 resent the average and do not mean that the limits of the two never 

 overlap. This difference in nuclei serves as one of the easiest and 

 surest ways of distinguishing the two types, for though the organism 

 during locomotion may modify its proportions tremendously this does 

 not noticeably affect the nucleus. It has been impossible to determine 

 any difference between the micronuclei of the two types. 



A very noticeable difference concerns the relative position of the 

 cytostome. In the ovoid type the cytostome is almost, though never 

 quite, terminal (see figs. F, G, and H). As mentioned previously, the 

 anterior end is ordinarily drawn out to form a fairly decided point 

 which lies within the area enclosed by the adoral cilia. In the slender 

 type the cytostome is more laterally placed. The posterior limit of the 

 right lip of the cytostome may extend ventrally to a point % of the 

 length of the animal, in which case the dorsal portion of the adoral 

 circlet of cilia may pass approximately through the terminal point 

 of the body, but in normal form this point will never be within the 

 adoral area. The parts which make up the adoral region of the 

 animals show no fundamental differences except variation in the rela- 

 tive position of parts due to the lateral displacement of the cytostome. 

 For example, the plane of demarcation between ectoplasm and endo- 

 plasm, which is approximately vertical to the long axis of the animal 

 in the ovoid form, is at a decided angle, the ventral edge lying farther 

 posterior in the elongate forms (see figs. D and E). 



In the opinion of some authors the ventral displacement of the 

 mouth is very significant (Delage and Herouard, 1896; Minchin, 

 1912). These authors believe that the ventral displacement of the 

 cytostome is progressive with evolution of the organism; i.e., that in 

 the more primitive types the cytostome is terminal while in advanced 

 groups it is successively displaced farther ventrally. Viewed in this 

 light, the position of the mouth is here of considerable significance as 



