212 University of California Publications in Zoology [VOL. 19 



are themselves slightly amoeboid and may slowly change position and 

 form, though this is unusual. 



On many occasions I have noticed a change of form of a different 

 character and at first thought true pseudopodia were being extended 

 from the whole body surface, but by continued observations it was 

 discovered that some undigested plant, as Scenedesmus or Lagerheimia, 

 was escaping or being extruded (text fig. B, 1-4). The pellicle and 

 cytoplasm extended out around the rays or processes and undoubtedly 

 part of the cytoplasm was lost when the object was set free. The 

 cytoplasm immediately contracted, the ruptured edges or margins drew 

 closer together until they were rounded up and fused (text fig. C, 1-7). 

 This seemed not so much a healing process as simply protoplasmic 

 contraction and rounding up. I have noticed on two occasions, how- 

 ever, that the ruptured edges met and fused, enclosing a water vacuole 

 in the healing or coalescing process (text fig. C, 5). These phenomena 

 make it evident that there is no cuticle and that the pellicle has 

 developed little beyond the state of a firm protoplasmic gel. Changes 

 in shape in the sulcal region are amoeboid, most other changes involv- 

 ing the whole body are metabolic or euglenoid. There is no differ- 

 entiation of ectoplasm and endoplasm. The surface is exceptionally 

 smooth and well rounded, in spite of the fact that the cytoplasm is 

 highly vacuolated. 



The function of the sulcal region is not hard to determine, for un- 

 doubtedly it is through this region almost altogether that food is 

 ingested ; but to decide its true status and homology is a more difficult 

 problem. It is a restricted area of the body, which has either retained 

 or evolved an amoeboid and miscible surface. It is always a unit, 

 a persistent though variable character, and of constant function. 

 Amoeboid pseudopodia are almost entirely restricted to this area. 

 Many flagellates, such as Euglena and Astasia, are exceedingly meta- 

 bolic, constantly varying in shape, but they have no such differentiated 

 surface area as this found in Collodictyon. Mastigamoeba is classified 

 as a polarized rhizopod, the entire surface of which is amoeboid. The 

 exceeding voraciousness of Collodictyon is indicative of a fairly ad- 

 vanced organism and in view of this fact it seems best to regard the 

 whole sulcal region as a modified cytostome. It must be considered 

 more homologous to the amoeboid surface of Mastigamoeba, however, 

 than to the more restricted gullet of Euglena, which is probably 

 homologous with only the anterior end of the sulcus. This structure 

 indicates a possible origin of the more specialized cytostomes as found 



