88 



University of California Publications in Zoology [Vol. 13 



a view helps to explain not only the number and distribution of the 

 opercular fibers, but also the fact that the oral cilia and the mem- 

 branelles of either or of both zones may be set in motion within the 

 body, i.e., before the protrusion of the oral region and the return of 

 the inner adoral and inner dorsal lips to their normal positions, when- 

 ever the animal is again surrounded by a favorable medium. 



Such a hypothesis would also help to explain the sometimes sudden 

 retraction of the membranelle zones when the animal bumps into an 

 obstruction or swims into an irritating medium, and upon no other 

 hypothesis can all these phenomena of retraction, protrusion and won- 

 derful co-ordination of membranelles and membranelle zones be so 

 easily and so satisfactorily explained. 



retr.or.cil. 1 I r- 



1 I 



t. ador. lip , ' , I r 



I I I ' r 



ador. m. --) i i I I i J 



■ I. ador. Up 



-- ador. m. 



I o. ador. Up 



cil. r. 



--op. 



o. ador. Up - - 



sk. Uim. 



oes. retr. sir. 



mw. 



Fig. D. Diplodinium ecaudatum. Eetracted form constructed from camera 

 lueida drawings. X 1150. ador. m., adoral membranelles; ant. c. v., anterior 

 contractile vacuole; bd. I., boundary layer; circ. oes. r., circumoesophageal ring; 

 B., dorsal surface; d. m., dorsal membranelles; i. ador. lip, inner adoral lip; i. d. 

 lip, inner dorsal lip; I. sic. a., left skeletal area; mac, macronucleus ; mic, micro- 

 nucleus; 0. ador. lip, outer adoral lip; o. d. lip, outer dorsal lip; oes., oesophagus; 

 oes. f., oesophageal fibers (neural in nature) ; oes. retr. str., oesophageal retractor 

 strands (contractile in nature) ; op., operculum; op. f., opercular fibers (nervous 

 in nature); or. disk, oral disk (retracted); post. cil. r., posterior ciliary roots; 

 retr. or. cil., retracted oral cilia; sic. lam., skeletal laminae; V., ventral surface. 



