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 emmae Stein. Fig. 49.

With the characters of the genus.

[Illustration: Fig. 49.--_Peritromus emmae_, ventral and lateral
aspects.]

KEY TO THE MARINE GENERA OF OXYTRICHIDAE.

Diagnostic characters: The peristome is not always marked off from
the frontal area. In the most primitive forms the cilia on the
ventral surface are similar to those of the preceding family
(_Peritromidae_). Usually some of the anterior and some of the
posterior cilia are fused into cirri, distinguished as the frontal
and anal cirri, respectively. In the majority of forms all of the
cilia are thus differentiated; strong marginal cirri are formed in
perfect rows, and ventral cirri in imperfect rows. In addition to the
adoral zone there is an undulating membrane on the right side of the
peristome, and in some cases a row of cilia between the membrane
and the adoral zone. These are the par-oral cilia and they form the
par-oral zone.

1. The posterior end is pointed or 2
tail-like

The posterior end is rounded; 5
not tail-like

2. The front end is pointed 3

The front end is rounded 4

3. Frontal and anal cirri absent; Genus _Stichotricha_
often tube-forming

Eight frontal and 3 caudal cirri; Genus _Gonostomum_
not tubiculous

4. Anal cirri present; with or