STRUCTURAL DIFFERENTIATIONS 159 



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

 the side in forms like Nassiila aurea, or Dallasia frontina (Fig. 168, 

 p. 383), on the ventral anterior surface in Frontonia leucas, or various 

 species of Chilodon (Fig. 34, p. 77), or at the extreme posterior end 

 as in Opisthodon mnemiensis (Fig. 166, p. 379). ^Miereverthe mouth 

 is found the rows of cilia are correspondingly altered from symmet- 

 rically-placed lines as in the generalized forms, to all kinds of asym- 

 metrical arrangements. This has led to the view, first elaborated by 

 Biitschli that the ancestral position of the mouth in ciliates, was ter- 

 minal at the anterior end, and that, in response to requirements of 

 different modes of life, and to various t\7)es of food, the mouth has 

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

 in different t^pes. 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 represented 

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

 oblique mouth, Prorodon and Dalkisio, with subterminal mouths, 

 Amphileptus and Lionotvs with elongated slit-like mouths extending 

 from the anterior end far down the ventral surface, such types lead- 

 ing 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. 157, 166, 167). 



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. 



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 Peritronws, 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 

 separate an anterior frontal field from the remainder of the body, 

 as seems to be the case in Climacostomum virens (Fig. 56, p. 107). 

 With the development of an attaching portion of the body as in 



