452 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Key to families of Hypotrichida. 
a. Peristome indistinct; cilia on ventral surface uniform and not differentiated into cirri Family Peritromidx 
One genus, *Peritromus p. 452 
b. Peristome more or less indistinct; cilia reduced to a few rows on the ventral surface; anal and frontal cirri 
present Family Oxytrichidx 
c. Cilia entirely reduced; frontal and anal cirri present or reduced; macronucleus band-formed or spherical 
Family Euplotidx 
d. Peristome reduced to left edge and does not reach over the anterior margin Family Aspidiscidx 
One genus, *Aspidisca p. 458 
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 creep- 
ing, 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. Fig. 49 .—Peritromus emmx, ventral and lateral aspects. 
Key to the marine genera of Oxytrichidx. 
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 ( Peritromidx ). 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 tail-like 2 
The posterior end is rounded; not tail-like 5 
2. The front end is pointed 3 
The front end is rounded 4 
3. Frontal and anal cirri absent; often tube-forming Genus Stichotricha 
Eight frontal and 3 caudal cirri; not tubicolous Genus Gonostomum 
4. Anal cirri present; with or without short lateral bristles Genus *Epiclintes, p. 453 
Anal cirri absent; no bristles Genus Uroleptus 
5. With frontal cirri 6 
No frontal cirri; 2 to 3 rows of ventral cirri; anal cirri small Genus Holosticha 
