Stated by Geza Entz and Raoul Francé to be due to the spiral twisting of a plasmic membrane, and to be like a cone formed by twisting paper, with the free edges overlapping.
Discovered by Leidy. For the most recent description of this group see Grassi and Sandias in Quart. Journ. Micr. Sci. xxxix. (figures) and xl. p. 1 (text), 1897.
Bezzenberger has given an analytical table of the eleven known species of the genus Opalina in Arch. Protist. iii. 1903, p. 138.
Such movements, permissible by the perfectly flexible but firm pellicle, are termed "metabolic" or "euglenoid" in contradistinction to "amoeboid." They also occur in many Sporozoa.
Within which is often harboured the Rotifer, Proales parasita, Vol. II. p. 227.