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


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 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_