72 University of California Publications in Zoology [VOL. 13 



attempt at an approximation of their number. Changes in the micro- 

 nucleus in preparation for and during division will also be described 

 in a subsequent paper. 



ORGANS OF LOCOMOTION 



It has already been pointed out that one of the main characteristics 

 of the genus Diplodinium is the presence of what Schuberg (1888 and 

 1891), Eberlein (1895), and Giinther (1899 and 1900) have termed a 

 second, or dorsal, membranelle zone, and what Fiorentini (1889) calls 

 a transverse crown of cilia. In Diplodinium ecaudatum the cilia of 

 both the dorsal and adoral zones are grouped to form clumps or tufts 

 of cilia. Normally all of the cilia composing each tuft adhere closely, 

 just as do the hairs of an ordinary camel's-hair paint-brush when 

 moistened so as to form a flexible pencil. The composition of these 

 brush-like tufts of cilia will be more thoroughly considered below, but 

 here attention is called to the structural difference between these ciliary 

 brushes and true membranelles, i.e., "flapping or swinging membranes 

 formed by fusion of two or more transverse rows of cilia implanted 

 side by side and adhering to form a flat membrane" (Minchin, 1912, 

 p. 55). Each ciliary brush is a perfectly definite unit, both struc- 

 turally and functionally, and although structurally these ciliary 

 brushes resemble cirri more closely than they do membranelles, still 

 from the point of view of homology it seems best to retain the designa- 

 tion membranelle, and hence in this paper each such tuft of cilia is 

 referred to as a membranelle. Throughout the Ophryoscolecidae, as a 

 matter of fact, these membranelles have the form of brushes and may 

 be designated as brush or penicillate membranelles in contradistinction 

 to those found elsewhere, as for example in the Vorticellidae, in which 

 the cilia of the membranelle are arranged in the form of a plat$ of one 

 or two lines of cilia fused in one locomotor unit of flattened type. 



Those of the dorsal region are termed dorsal membranelles and 

 those of the adoral region, adoral membranelles. The complete row 

 of dorsal membranelles, together with the inner and outer dorsal 

 lips and intervening furrows, is termed the dorsal membranelle zone 

 and likewise the row of adoral membranelles with its corresponding 

 inner and outer adoral lips and furrows is designated as the adoral 

 membranelle zone. The dorsal and adoral membranelle zones form 

 the locomotor apparatus of the animal and since these two zones are 

 not continuous the locomotor apparatus may be said to consist of two 

 component parts, a dorsal locomotor apparatus or dorsal membranelle 



