1914] Sharp: Diplodinium ecaudatum 89 



RETRACTED FORM 



A detailed description of the mechanism of the retraction of the 

 oral zone and the resulting encasement of the adoral and dorsal mem- 

 branelles will be given later in the account of observations on the liv- 

 ing material. At this point will be given only a brief description of 

 figure D, which is a reconstruction of three camera lucida drawings 

 made from three paramedian sagittal sections each five microns thick. 

 The noteworthy points may be summed up as follows: (1) The whole 

 oral region is retracted within the body. (2) The oral cilia come to 

 lie within the oesophagus (retr. or. cil.). (3) The attachment of the 

 boundary layer to the oesophagus is pulled posteriorly to a considerable 

 extent, thus showing that both the point of .attachment of the oseopha- 

 geal retractor strands (oes. retr. sir.) and the region of actual con- 

 traction of these strands is below the point of junction of boundary 

 layer with oesophageal wall (compare with fig. B). (4) The inner 

 adoral lips (i. ador. lips) and the inner dorsal lip (i. d. lip) are ex- 

 tended in such a manner as to become direct continuations of the outer 

 and more rigid adoral and dorsal lips (o. ador. lip and o. d. lip) re- 

 spectively and at the same time to meet in the epioral line in the case 

 of the adoral lips and to meet the dorsal edge of the operculum (op.) 

 in the case of the dorsal lip, thus forming a complete protective 

 encasement for the delicate membranelles. (5) The circumoesopha- 

 geal ring (dr. oes. r.) appears to be somewhat enlarged and stands 

 out even more clearly than in the extended animals. (6) The indi- 

 vidual fibers (oes. /'.) and individual retractor strands (oes. retr. str.) 

 are also very distinct indicating shortening and thickening. This 

 figure does not show the motorium and its connections as that struc- 

 ture lies to the left of the plane here depicted. The cut ends of the 

 opercular fibers (op. /.), however, show very clearly. In the descrip- 

 tion of the neuromotor apparatus it will be remembered, strands were 

 described which passed to and ran along in the inner adoral and inner 

 dorsal lips respectively. These strands showed fairly well in those 

 sections stained with the modified Mallory's connective tissue stain. 

 No sections of animals in the retracted condition, however, have been 

 prepared with the Mallory stain. But in the sections of the retracted 

 animals stained with Heidenhain's iron-alum haematoxylin no trace 

 of such strands can be distinguished within the extended inner lips. 

 Whether this fact is due to the extended condition of the lip with a 

 corresponding separation of the fibers so as to make them too minute 

 for identification, or whether it is due to a lack of "affinitv" for the 



