68 University of California Publications in Zoology [Vol. 13 



forms a sort of sack enclosing the entoplasm, into which opens the 

 oesophagus {oes., pi. 4, fig. 3) and out of which leads the rectum 

 {red., pi. 4, fig 3). This "boundary layer," as above described, 

 refers only to the definite structureless membrane separating the 

 ectoplasm from the entoplasm and bounded on either side by a layer 

 of alveoli. Eberlein (1895, p. 244, 245) describes for this boundary 

 layer ("Grenzschicht") a definite fibrillar structure and is inclined 

 to assign to it also two or more alveolar layers. Giinther (1900, p. 

 643) states emphatically that, notwithstanding Eberlein 's description, 

 he is unable to find any alveolar structure in the above-named layer. 

 It must be noted, however, that the oesophagus contains within its 

 walls, as will be described later, many fibrillae which are in this paper 

 designated as oesophageal retractor strands {oes. retr. str., figs. B, D ; 

 pi. 4, fig. 3) and that in certain portions of the body the oesophagus 

 comes to lie in such close contact with the boundary layer as to defy 

 microscopic separation of the two. Hence, in these places, it is easy 

 to see how a fibrillar structure might be assigned to the boundary 

 layer. Careful investigation of this point, however, indicates an essen- 

 tially non-fibrillar structure for the true boundary layer. 



The micronucleus and macronucleus, and the two contractile 

 vacuoles, lie in depressions on the outside of this sack. At the anterior 

 end of the body this boundary layer dips down, approaches the oeso- 

 phagus as a funnel-shaped depression, accompanies it, and finally 

 blends with the incoming cuticle (pi. 4, fig. 3). In the same way it 

 approaches the rectum, and ascends with it to the caecum, on the sides 

 of which it blends with the cuticle and becomes lost upon it. That 

 this layer belongs to the ectoplasm rather than to the entoplasm is 

 determined from the following facts. (1) When these Protozoa are 

 surrounded by an irritating chemical medium, or by disturbing physi- 

 cal conditions, the entoplasm frequently flows out through the gullet, 

 but in these cases the boundary layer alwaj^s remains with the ecto- 

 plasm. (2) While D. ecaudatum is feeding the entoplasm is in con- 

 stant, definite motion. The boundary layer takes no part in this 

 movement but remains stationary with the ectoplasm. (3) After feed- 

 ing, the entoplasm shows many changes, which, however, are not dis- 

 tinguishable in either the boundary layer or in the ectoplasm. 



Entoplasm 

 The entoplasm {ent., pi. 4, figs. 3-5; pi. 7, figs. 25-33), shows 

 under the most powerful lenses (2600-3400 magnifications) no definite 



