EDMONDSON — PROTOZOA OF IOWA. 83 



vided with a conspicuous undulating membrane. Dorsal surface 

 spirally striated. A few long, fine setae, extending from the pos- 

 terior border in an oblique direction, one usually longer than the 

 others. Contractile vesicle in th*e posterior region opposite the 

 oral aperture. Nucleus spherical, anterior to the contractile 

 vesicle. 



Ivcngth, 30 microns. (Fig 135, Pi. XVII.) 



The oral membrane of this organism vibrates rapidly and pro- 

 trudes tongue-like, apparently, however, having the power to 

 retract so that it may sometimes be invisible from a lateral point 

 of view. 



The setae developed from the posterior border are directed 

 toward the left and act in the capacity of a rudder, the result 

 being that the animal does not move forward in a straight line 

 but continually swerves to the left, often describing circles, but 

 always turning to the left when the dorsal surface is up. 



Reproduction is by transverse fission. Found everywhere in 

 pond water. 



Family, PARAM^CIID^. 



PARAMECIUM Muller. 



Elongate-oval, entirely clothed with cilia. Mouth ventral, at 

 the posterior end of an oblique oral groove. Nucleus and con- 

 tractile vesicles conspicuous. Trichocysts usually abundant. 



Paramecium caudatum Ehr. 



Body elongate, anterior extremity narrow and bluntly rounded, 

 wider posteriorly. Oral groove extending from the anterior 

 extremity obliquely backward to or beyond the center of the 

 body. Mouth opening into a short, curved, ciliated pharynx. A 

 tuft of longer cilia produced at the posterior tip of the body. 

 Macronucleus and micronucleus central in position. Contractile 

 vesicles two, one in either extremity of the body. 

 Length, 230 microns. (Figs. 136-138, PI. XVIII.) 

 Paramcccmm caudatum is perhaps the most common ciliated pro- 

 tozoon known. Apparently the form differs from ParamcEciufu 

 aurelia, as recorded by Kent, only in the possession of longer cilia 

 at the posterior tip of the body. The "long-tailed Paramo'ciuvi" 

 seems to be the characteristic variety of this country; however, 



