356 Dr. H. M. Woodcock and Mr. G. Lapage. 



and reminds us somewhat of the posterior extremity of Cercobodo (Cerco- 

 monas), with its long cytoplasmic " tail," though in Helkesimastix it is never, 

 in normal conditions, drawn out to anything like the same extent. The hody 

 is usually about 6/a to 7/a long by or 2/x broad. Under other conditions 

 the shape of the body may be oval or slightly pyriform (figs. 9, 18) ; this is 

 frequently the case in smaller individuals, and also when the flagellates are 

 sluggish, in a rather denser medium than usual. 



There is a single flagellum, generally about two and a half to three times 

 as long as the body, or even longer, inserted at, or very near to, the anterior 

 end. This new flagellate is remarkable in having its flagellum always directed 

 backwards, i.e. it possesses only a trailing flagellum, whence the generic name. 

 In life, the flagellum is usually contiguous to the body for practically the 

 entire length of the latter, and the proximal portion is always in very close 

 contact with about the anterior third or so of the body, from which it never 

 becomes free. This is seen very clearly when a steadily progressing individual 

 makes a sharp turn and goes off in another direction. The proximal portion 

 of the flagellum also turns immediately, along with the body, but. the remain- 

 ing part is for the moment free from the latter and turns more gradually into 

 the new direction (cf. fig. 14). The flagellum lies along the middle of the 

 upper (dorsal) side of the creature ; in ordinary conditions it is never on the 

 under side. Sometimes the flagellate turns on its side, when the close 

 adherence of the flagellum is well shown (fig. 10). Yet, in spite of this 

 close contact, there is certainly no attaching membrane developed, for, in 

 individuals killed by osmic acid, the flagellum is sometimes seen to stand 

 off completely from the body (cf. fig. 12). Frequently there is a definite 

 row of three or four conspicuous granules along the line of contact of the 

 body with the flagellum (figs. 10, 11). The nucleus can generally be seen 

 as a clear, spherical area near the anterior end ; in individuals in motion it 

 is sometimes difficult to see the karysome, but in those quiescent, or in 

 individuals killed by osmic acid vapour, this body can also be made out 

 ■(cf. fig. 12). Helkesimastix is not a binucleate ; it has no kinetonucleus.* 



There is certainly no cytostome or definite mouth-aperture present. We 

 are not quite certain, however, whether the creature does or does not ingest 

 solid particles, such as small cocci, etc. ; we are strongly inclined to think it 

 does not. In this connection, a peculiar mode of food-ingestion which we 

 have observed in Cercomonas (or Cercobodo) longicauda is of interest. While 



* We may add that, as seen in stained preparations, the nucleus is of the usual 

 flagellate character, consisting of a nuclear membrane, a clear zone (the nuclear sap), 

 and a large central karyosome, connected with the membrane by delicate radiating 

 fibrils. 



