Jennings, Behavior of Paraniechini. 



457 



a return of the oral cilia to the backward stroke, the others re- 

 maining reversed ; (3) then causes the body cilia of the left side to 

 strike toward the oral groove (whereas before they struck in the 

 opposite direction), while the forward stroke of the body cilia 

 becomes converted into a backward one. There is a co-ordi- 

 nated system of movements, producible in many ways, a sys- 

 tem that is variable in many respects, yet as a rule varies in 

 such a way as to retain throughout its co-ordination. 



The change in the stroke of the cilia is correlated in many 

 cases with certain other phenomena. Paramecium still retains 

 to a very slight degree the power of contraction that is so 

 marked in many other ciliates. The anterior end especially 



Fig. 7. Relation of reversal of the ciliary stroke to contraction, a, usual 

 condition : over the entire surface of the slender body the cilia strike backward ; 

 b, the body is contracted, becoming short and thick : all the cilia are reversed ; 

 r, anterior end alone contracted, and cilia reversed in this region alone.; d, con- 

 traction on the aboral side, curving the body : cilia reversed in the contracted 

 region. 



may be shortened and thickened, or narrowed and lengthened, 

 or bent to one side, to an appreciable degree. These move- 

 ments are hardly to be observed in specimens swimming freely 

 through the water. But if the movements are impeded and 

 the animals partly flattened out between the slide and cover, 

 partial contractions are very evident. It is then to be observed 

 that whenever contraction takes place, the cilia of the contracted 

 region become partly or entirely reversed (Fig. 7, b), beating 

 no longer forward, but backward or transversely. At times the 

 whole body contracts, becoming shorter and thicker ; at the 



