550 



THE AMERICAN NATURALIST [Vol. XLI 



fications by Engelmann/ Piitter, Parker and Gunvitsch.^ Ac- 

 cording to this theory, the ciliuni consists of an axial snpport and a 

 contra(.'tile protoplasmic sheath, 'i'he natnre ol* the axis has heen 

 less the subject of discussion than that of the slu^ath which Mngel- 

 mann regards as fibrillar, Piitter as j)r()toplasni with temporary 

 fibrillar arrangements, and (iurwitsch as ])n)t()plasm of changing 

 surface tension. As stated above, \'(M-W()rn thinks that the ciiium 

 of ctenophores is formed of two cohmms of contractiU' protoplasm 

 whose differential contraction moves the ciiium, and Kiigelmann 

 seems to lean toward this view. 



Less generally accepted theories are those of Benda and Schiifer. 

 Benda ' believes that the ciiium is passive and is operated by a 

 mechanism at its l)ase, but the cases cited al)()ve of the movement of 

 cilia entirely separated from the basal body and the cytoplasm, 

 and the failure with few exceptions to find such a mechanism, 

 make this view unacceptable. The velar cilia above described, 

 the existence in some cells of the liypobasal layer wliich seems to 



Vcruorn's obM'rvatioii> n},oii (•t.Mioj)horc cilia Mign,-r. however, 

 that ahhoiigh this theory will not apply to all cilia, it is the only 

 theory which explains the action and stnictun> of certain cilia. 

 ^Schafer"" re-ard. the cilinm a. an ciaMic lube (Fig. L'. r., one 



forced can>iiig tli<- ciiium to Lend o\cr touani it. Ic- ria^ic ^idr. 

 This the..rv is plausible and the stnictniv is nuvhanically 



tozoan flagella. It seems, however, that the action of the .nctorian 

 tentacles which are evauinated and invaginated like the finger of a 

 glove can only be explained by this tlieory. 



