a. Peristome indistinct; cilia on Family _Peritromidæ_
ventral surface uniform and not One genus, *_Peritromus_
differentiated into cirri
b. Peristome more or less indistinct; Family _Oxytrichidæ_
cilia reduced to a few rows on the
ventral surface; anal and frontal
cirri present
c. Cilia entirely reduced; frontal Family _Euplotidæ_
and anal cirri present or reduced;
macronucleus band-formed or spherical
d. Peristome reduced to left edge and Family _Aspidiscidæ_
does not reach over the anterior One genus, *_Aspidisca_
margin
* Presence at Woods Hole indicated by asterisk.
Genus PERITROMUS Stein '62.
(Stein '62, '67; Maupas '83.)
The body is flat, colorless or tinged with yellow, and contractile.
It is elliptical in outline, with broadly rounded ends; in some
cases the left edge is slightly incurved, the right edge convex.
The ventral surface is flat, the dorsal surface is arched in the
middle region of the body. The edges being flat are somewhat more
transparent than the remainder of the body. The ventral surface is
striated by longitudinal straight or slightly curved lines, the
dorsal surface is smooth and without cilia. (Maupas describes
bristles on the back, but this is not corroborated.) The adoral zone
is fairly well developed, but not distinctly marked off from the
remaining ventral surface. It begins on the right side and extends
entirely around the frontal margin and down the left side below the
middle of the body, where it turns suddenly to the right, entering
the slightly insunk peristome. The mouth leads into a short,
indistinct oesophagus. One contractile vacuole is situated in the
dorsal swelling at the posterior end of the animal. Macronucleus
double, one in each side of the dorsal swelling. Movement is slow
and creeping, with a peculiar method of contracting the more hyaline
edge, which may turn upward or around a foreign object.
Fresh (?) and salt water.
Peritromus emmæ Stein. Fig. 49.
With the characters of the genus.
[Illustration: Fig. 49.--_Peritromus emmæ_, ventral and lateral
aspects.]