EUPLOTES WORCESTERI: II. 325 



ing the mieromicleus in only a few instances. Tlie invagination extends 

 a little farther forward in E. worcesteri than in E. harpa. At the same 

 time it pushes a little backward and mediae! and also increases consider- 

 ably in width and depth. Before the invagination has attained its full 

 length the opening to the exterior xisually becomes elongated, its edges 

 approach and finally fuse, and the new peristome exists for a time as a 

 completely closed cavity inside the body of the Euplotes. The ventral 

 wall of the new peristome now lies -1 or 5 /x dorsad to the old peristome. 



The position of the new peristome is shown very clearly by sections. Figure 

 7, Plate IV, is a drawing of a transverse section of an animal in about the 

 same stage as figure 4. The section passes just in front of the tip of the 

 pharynx. The new peristome (P) lies below the old adoral membranellie; inside 

 of it the membranella; of the new adoral zone are already well developed; as 

 Wallengren also has observed, they move actively insid.e the new peristome almost 

 from the first. Figure 16 (Plate V) was drawn from a longitudinal vertical 

 section of a stage similar to figure 10. It passes through the old adoral zone 

 (az) parallel to the axis of the body. Below this lies the new peristome which 

 reaches the surface at O, where the lips of the external opening are still in 

 contact. The membranellse of the new adoral zone (AZ) lie outside of part of 

 the section; at the posterior end of the cavity the membranellse which there lie 

 upon the outer wall are shown in transverse section. The individual cilia of 

 the adoral membranelloe are particularly noticeable in a section like this. After 

 remaining closed for a time the external aperture of the new peristome reopens 

 and rapidly increases in size. (Plate V, figures 10, 13, 14; Plate VI, figures 

 18, 20, 22, 25; Plate VIII, figures 33, 34, 35.) 



The figures shoAV that the time at which the final opening occurs 

 varies somewhat, using the condition of the meganucleus as a standard 

 for comparison, and so does the rate at which the enlargement of the 

 aperture increases. Wallengren did not observe any closure of the 

 peristomial aperture in Euplotes harpa. 



After the new peristome has become permanently opened, the pos- 

 terior end of the cavity increases considerably in length and also bends 

 toward the center of the body. The portion of the posterior end of the 

 invagination which will become the pharynx is shown by Plate V, figure 

 10, where the rudiment of the suboral group of membranella (SO) lies 

 in a slight expansion of the cavity. Since the suboral membranellse of 

 the adult Euplotes lie just within the mouth, it is clear that the invagina- 

 tion includes the rudiment of the pharynx as well as that of the peristome, 

 and that the pharynx is formed some distance from its final position. 

 The same peculiar bulging of the medial wall of the invagination is shown 

 in Plate V, figure 13, although the suboral membranellaa were not visible 

 in this specimen. 



As figures 10 and 13, Plate V, were among the last drawings made, it 

 is almost certain that the slight enlargement of the pharynx was present 

 in other specimens (Plate V, figure 14; Plate VI, figures 18, 20, 22, 25; 

 Plate VIII, figure 33) but not noticed at the time the sketches were made, 



