Why does it extend and contract?

This ciliate repeatedly extends and contracts its long, thin body, reaching a length of more than 1000 microns when extended, and 500 microns when contracted.

The main part of the body including its mouth is narrow and dark brown, but the back end is transparent and a little pudgy.

Commentary by Prof. Yuji Tsukii, Hosei University
This organism belongs to genus Spirostomum (as shown in video No.2). From its large size and cytostome (mouth) in the middle of the body, it is likely to be Spirostomum intermedium.

http://protist.i.hosei.ac.jp/PDB/Im.....

Sampling Date : 27 May 2009

Sampling Site : Hirose River A  Google Map