STRUCTURAL DIFFERENTIATIONS 



137 



flagellates represent the furrow — which separates an epitheca or 

 upper, from the hypotheca or lower part of the shell — with a fringe 

 of cilia, and the name Cilioflagellata given to the group, indicates 

 the extent to which this view prevailed. In some aberrant types 

 like ExnvioeUa marina, in which there is no transverse groove, the 

 "transverse" flagellum makes a sharp bend near the anterior end of 

 the body and vibrates in a circle exactly as though it were still 

 confined within a transverse groove (Fig. 71). 



A B 



Fig. 71. — Exuvicella marina (A) and E. lima (B). (After Calkins.) 



Delage and Herouard have attempted to explain the dynamics of 

 flagellum action whereby the comparatively heavy body is moved 

 forward by reason of the vibrations of the exceedingh' delicate 

 thread. In the usual type the extremity of the flagellum describes 

 a rather wide circle so that it is in a certain focus of the microscope 

 for only an instant of time. With this circular movement, which 

 varies in dift'erent species, constant undulations pass from the base 

 to the tip. A forward pull results from the combination of such 

 movements and the cell either glides smoothly after its active pro- 

 peller, as in Peranema trichophora, or Euglena o.vi/uris, etc., or 

 rotates more or less rapidly on its long axis while freely swimming 



