No. 3-] MORPHOLOGY OF THE ST EN TORS. 487 



Its invisibility here is possibly due to the proximity of the 

 underlying myoneme. 



The occurrence "tactile spines" (" Tastborsten ") in 5. 

 cceruleus and ^. roeselii has been mentioned by Stein and 

 others. To account for the suddenness of their appearance in 

 places where none could be seen before, Stein expressed the 

 view that they could be withdrawn into the body and again 

 suddenly protruded. I have frequently observed these so-called 

 "spines" in S. asnileus and occasionally in S. roeselii (Fig. 4, 

 right-hand outline). They are more frequently seen about the 

 anterior end of the animal, when one is looking directly down 

 upon the frontal field. I have often noted their sudden appear- 

 ance and disappearance. It is my strong belief that these 

 "spines" are simply cilia that have become momentarily rigid, 

 and therefore visible, while their disappearance is due to the 

 fact that they have resumed their rapid swinging motion, in 

 which condition they are not individually discernable. I have 

 not seen any "spines" so large as many figured by Stein, and 

 their length was certainly not greater than that of cilia. In 

 drawing such structures, however, it is an easy matter to 

 exaggerate unconsciously their size, and this is probably the 

 case with Stein's figures. 



While it is perhaps idle to speculate on the function of tem- 

 porarily rigid cilia, the view that they are improvised tactile 

 organs seems the most reasonable. The anterior border of the 

 animal, which is brought most frequently into contact with 

 foreign objects, is naturally the most in need of tactile organs, 

 and it is just here that I have most frequently observed the 

 "spines." It is well-known that permanently rigid spines or 

 bristles occur in many Infusoria, notably the Hypotricha (e. g., 

 the three caudal spines of Stylonichia mytiliis), and there is 

 every reason to believe that these possess a tactile function. 

 All such spines are undoubtedly modified cilia. It would seem, 

 then, that we have in the temporarily rigid cilium of Stentor 

 an organ which is virtually a cilium, but having acquired 

 enhanced tactile sense (probably possessed in some degree by 

 all cilia), and the power of becoming rigid, is already on its 

 way to become a specialized organ of touch. 



