NEMATOCYSTS OF MICROSTOMA. 269 



these fixing fluids gave good results. The sections were mounted 

 serially. Some individuals were cut 3 microns, others at 4, 5, 6 

 and 7 microns. All the material was stained with Haidenhain's 

 iron heematoxylin. Some sections were counter stained with 

 Bordeaux red. From these many individuals a large list of 

 sectioned nematocysts has been made. 



As the specimens were being collected it was not an unusual 

 experience to find greatly gorged specimens, which upon the 

 addition of the fixing fluid would burst and thus liberate or expose 

 a dead Ch<ztogaster such as is represented in Fig. 3. In passing, 

 this observation is of interest, for Martin says "one of the com- 

 monest enemies of Microstoma appears to be Chcetogaster, which 

 devours it greedily" (Martin, 1908, p. 268). 



Within the enteron of the sectioned Microstoma, one frequently 

 finds the large unbroken setae of these ingested oligochaetes (Fig. 

 4). If according to Martin, as I interpret him, the passage 

 of the nematocysts from the enteron through endoderm and 

 mesoderm to ectoderm is passive, I wonder why at times these 

 harsh setae of Chcetogaster do not passively find their way into 

 the tissues of the body of the flatworm. Other solid bodies 

 such as diatom shells, and round or spheroidal objects such as 

 the shells of Arcella lie within the enteron. These too, if the 

 migration of the nematocysts be passive, should find their way 

 to the surface as do the nematocysts; but not a single instance 

 of such objects lying outside the lumen of the enteron was 

 observed. 



If, therefore, nematocysts do migrate from the cavity of the 

 enteron to the general surface of the body, their migration seems 

 to be accomplished through some active agency. 



The first tissue of course to be concerned in this selective 

 function is the endoderm. The cells of the endoderm must dis- 

 tinguish between the nematocysts and other solid or rounded 

 objects within the enteron, just as an Amceba distinguishes be- 

 tween food and non-food. So far as the present series of ob- 

 servations is concerned twenty endodermal cells were found 

 in which a nematocyst was enclosed (Fig. 5). In each case the 

 nematocyst lies within a vacuole near the base of the endodermal 

 cell. 



