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 œsophagus. 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.

Fig. 49.—
Peritromus emmæ,
ventral and lateral aspects.
[ ENLARGE ]

Key to the marine genera of Oxytrichidæ.
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 ([Peritromidæ]). 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 tubiculous Genus Gonostomum
4. Anal cirri present; with or without short lateral bristles Genus *[Epiclintes]
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
6. Right margin of peristome straight as far as the anterior end; 5 rows ventral cirri; 5 anal cirri Genus Oxytricha
Right margin of peristome curved 7
7. Five rows or less of ventral cirri 8
More than 5 rows of ventral cirri Genus Urostyla
8. Membranelles normal; 5-10 anal cirri; no caudal cirri Genus *[Amphisia]
Membranelles normal; 5 to 10 anal cirri; 3 caudal cirri Genus Stylonychia
Membranelles very large and powerful; adoral zone not continued to mouth; 5 anal cirri Genus Actinotricha
* Presence at Woods Hole indicated by asterisk.