100 University of California Publications in Zoology [VOL. 13 



lips and the equilibrium is again established, in the one case, when 

 the different portions of the inner adoral lip meet in the epioral 

 region, and in the other case, when the inner dorsal lip meets the 

 dorsal edge of the operculum. 



If it can be assumed that these inner adoral and inner dorsal lips 

 exhibit any degree of inherent elasticity, and such an assumption is 

 entirely within the bounds of probability, then an explanation of 

 the return of these extended inner lips to their normal positions to- 

 gether with the simultaneous protrusion of the oral region is also to 

 be made along mechanical lines as follows; the contraction of the 

 oesophageal fibers is relaxed and the elasticity of the inner lips and 

 possibly of the whole anterior end of the animal, i.e., the tendency 

 to return to the normal position, now exerts a pressure upon the 

 enclosed ectoplasm in a direction opposite to that which caused the 

 protrusion of these inner lips, with the result that as the inner lips 

 return to their original positions the oral region is again protruded 

 and the animal once more presents the normal appearance and re- 

 sumes its activity. 



It is desired in this place to call attention to the fact that previous 

 observers have described the membranelle zones as retractile structures. 

 Both the study of the preparations of fixed material and the observa- 

 tions on the living animals lead to the conclusion that the dorsal mem- 

 branelles, and probably also the adoral membranelles, are not retracted 

 within the body, but that the picture presented by these so-called re- 

 tracted forms (fig. D) is brought about, as described above, by the pro- 

 trusion of the lips. In other words, the evidence goes to show that 

 during the process of oesophageal retraction the membranelles remain 

 stationary and become encased by the protrusion of the inner adoral 

 and dorsal lips respectively. ^ 



In watching these phenomena of retraction and expansion in the 

 living, active animals one cannot help but be impressed with the won- 

 derful co-ordination of parts, the simple and yet efficient mechanism 

 by means of which the encasement and protection of the delicate mem- 

 branelles is effected and withal the probable presence of at least the 

 rudiments of a nervous system. 



