168 BIOLOGY OF THE PROTOZOA 



to the posterior end (Fig. 84, p. 154). In the majority of cases, 

 however, the mouth is not terminal but may be found at various 

 points on the side or upon the ventral surface. Thus it may be on 

 the side in forms like Nassula aurea, or Glaucoma (Dallasia) frontata 

 (Fig. 8, p. 29), on the ventral anterior surface in Frontonia leucas 

 (Fig. 93, B), or various species of Chilodon, or at the extreme pos- 

 terior end as in Opisthodon mnemiensis (Fig. 191, p. 472). Where- 

 ever the mouth is found the rows of cilia are correspondingly altered 

 from symmetrically placed lines as in the generalized forms, to all 

 kinds of asymmetrical arrangements. This has led to the view, 

 first elaborated by Biitschli, that the ancestral position of the mouth 

 in ciliates was terminal at the anterior end, and that by adaptation 

 to different modes of life, and to various types of food, the mouth 

 has shifted from the anterior end to the various positions as now 

 found in different types. With this shifting the focal points of the 

 ciliary rows have similarly shifted, and the positions of the lines of 

 cilia in some forms are used as evidence to indicate the path of this 

 shifting and the mode of evolution of the present-day cytostomes. 

 A familiar illustration of such shifting is the series of forms repre- 

 sented by the genera Holophrya, with terminal mouth, Spathidium, 

 with oblique mouth, Colpidium, Glaucoma (Dallasia) and many 

 others, with subterminal mouths, Amphileptus and Lionotus with 

 elongated slit-like mouths extending from the anterior end far down 

 the ventral surface, such types leading to the various proboscis- 

 bearing genera like Dileptus in which the mouth is limited to the 

 posterior end of such an ancestral slit-like aperture, now represented 

 for the most part by a row of trichocysts (Figs. 6, 13, 203). 



In Chilodon there is an oblique line of cilia running from the 

 anterior left-hand margin of the ventral surface to the circular 

 mouth which in some species may be shifted well over on the right 

 side. The lines of ventral cilia begin at this line and not at the 

 mouth, while an oblique row of specialized cilia suggests the begin- 

 nings of adoral zone formations characteristic of the majority of 

 Trichostomina, while the line itself may well represent the positions 

 held by the mouth in ancestral forms. 



In many types of ciliates, a special region of the body, not found 

 in the more generalized forms, is developed as a feeding surface. 

 Such regions, known as frontal fields, are characteristic of ciliates 

 which live permanently or temporarily as attached forms. There 

 is some evidence to indicate that such frontal fields as occur in 

 Stentor, and the Peritrichida, are derived from the anterior ventral 

 surface of more actively moving forms. In Pcritromus, for example, 

 the line of the peristome cuts out a definitely limited frontal region 

 of the ventral surface, which is provided with special motile organs, 

 the frontal cilia. Biitschli (1888) suggested that such a peristome, 

 if continued around the right side of the organism, would completely 



