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THE AMERICAN NATURALIST [Vol. XL 



space left between the ring muscle and the esophagus. On violent 

 stimulation (dissecting an animal that is not well narcotized) this 

 process may be so extensive as to double the whole proboscis 

 within the ring muscle, just as if one were to push in a finger of a 

 glove down to its base. In other words the proboscis may be 

 doubled in on itself down to the place marked x in Fig. 2. 



This "Einstiilpung" must be brought about by the extensive 

 contraction of the retractors of the proboscis and their continua- 

 tion, the inner longitudinal muscular layer of the proboscis wall. 

 The proboscis cannot be turned wrong side out throughout its 

 entire length, as could a finger of a glove; for in the anterior end 

 the esophagus, which is closely bound to the dorsal wall of the 

 proboscis, and the buccal mass are united. Besides, the protractors 

 of the cartilage prevent such a complete doul)ling (Fig. 17). 



As to the protraction of the proboscis, I have onlv theory to 

 offer, as the experiments which I had contemplated ui)oii this i)oint 



tudinal fibers, as well as the contraction of the rin uhu- lilxM-s (»t' the 

 prol)Oscis wall will account for its .•loiiirntioii, (.ut how is the pro- 

 boscis forced out of the head fold? It i.m.v Ur that tin. is<|on<- by 



be filled with blood, and then the lun.rl,.. of i\u- li'ra.l L.l c ontract, 

 the eft'ect would be to expel the proboscis. The region of the 

 head fold that is covered by the collar of the mantle is cavernous, 



