274 WILLIAM A. KEENER. 



with a possible shifting of the mesoderm at the exact time of 

 discharge may reasonably account for this single exception that 

 is represented in Fig. 1 1 . 



There is a second type of nematocyst found in Hydra. These 

 are the so-called "grappling" nematocysts. By means of them 

 the Hydra lays hold of or entwines objects of prey; when the prey 

 is thus seized the tentacles of Hydra sweep it into the mouth of 

 the polyp. This seems to be the chief function of these nema- 

 tocysts; but the poison sac at the base of this type of nemato- 

 cysts suggests that they are something more than mere prehensile 

 structures. If they are not more than this, I wonder at Micro- 

 stoma taking them into its tissues and passing them out to its 

 surface where they are, so far as my observations are concerned, 

 always oriented as are the barbed nematocysts. Frequent 

 nematocysts of this type have been found by me within sections 

 of Microstoma. In some cases these grappling nematocysts 

 lie within vacuoles in \vhich no attending cell or cnidophage can 

 be recognized. Fig. 12 shows such a nematocyst lying near a 

 vacuole from which it may have been discharged. These nema- 

 tocysts are small and their vacuoles likewise small so that it is 

 difficult to determine whether cells, associated with them, lie 

 within their vacuoles or not; but frequent nematocysts of this 

 type have been found that have attending cnidophages within 

 their vacuoles (Fig. 13). Moreover all the exotic "grappling" 

 nematocysts were so oriented that their discharging poles were 

 directed towards the surface. 



I have come to look upon the nematocysts of Microstoma as 

 being derived from Hydra. This conclusion is based upon the 

 following facts: (i) that there is a great variation in the number 

 of nematocysts of Microstome from very few to many; (2) the 

 close resemblance between the nematocysts of Hydra and Mi- 

 crostoma; (3) the absence of cnidocils in Microstoma; and (4) 

 the absence of cnidoblasts about the nematocysts when found 

 within the endoderm and deeper mesoderm of Microstoma. 



Toward what end is all this careful handling of the exotic 

 nematocysts of Microstoma directed? 



Glaser (1909) has determined by experiment that nematocysts 

 will not be discharged in the digestive fluids of seolids. 



