370 JULIA ELEANOR MOODY 



of the body is tapering. The difference in width between the 

 anterior and posterior ends is slight, but becomes more marked 

 during division when, after the cell has elongated and the con- 

 striction appears, the future posterior cell is seen to be of some- 

 what smaller diameter than the anterior end (figs. 55 and 56). 



The anterior region of the body is drawn out to form a minute 

 papilla-like structure, at the extreme tip of which lies the mouth. 

 This projection is not always discernible, however, since this 

 region is extended and retracted at intervals, a slight depression' 

 often marking the position of the mouth. 



In the living cell two clearly differentiated regions are dis- 

 tinguishable, a dense central region, the endoplasm, surrounded 

 by more transparent area, the ectoplasm. In common with 

 ciliates generally, the body is covered with an extremely thin 

 membrane, the cuticle, the surface of which is marked by 24 

 or 25 delicate lines extending spirally from the border of the 

 mouth to the posterior end of the body. These lines, which 

 in optical section are seen to be narrow ridges, indicate the 

 places of insertion of the retractile tentacles and the cilia, the 

 latter arranged in small groups at the base of each tentacle. 

 Directly underneath the cuticle is the cortical plasm, a semi- 

 transparent substance in which lie imbedded masses of highly 

 refractive bodies which are in constant circulation. Large 

 vacuoles form a characteristic peripheral border, while strands 

 of endoplasm extend between the vacuoles and grade insensibly 

 into the cortical plasm at the base of the tentacles (fig. 41). 



The retractile tentacles, originating in the cortex and capable 

 of an extension equal to two or three times the diameter of the 

 body, are especially interesting. I have already referred to 

 von Erlanger's work on the tentacles of Actinobolus in which 

 he distinguishes three distinct regions; a spherical basal portion 

 from which extends a rod-like body bearing at its distal end a 

 sharp dart terminated by a minute knob; the basal sphere and 

 middle portion being perfectly transparent, the dart quite opaque. 

 After treating the animal with osmic acid vapor, he noted an 

 extremely fine thread projecting beyond the knobbed tip of 

 the dart. Although a very careful study has been made of 



