e uniform in length.

Habitat similar to that of _L. infusionum_, in zoogloea masses.
Length 26 to 30 mu; diameter 7 to 8 mu.

Although Quennerstedt's description of _L. pusillus_ makes no mention
of a caudal bristle, the size and other characters are so closely
similar that I hesitate to make a new species. The bristle is
extremely delicate, scarcely thicker than a cilium, and easily
overlooked, yet with proper focussing of the condenser I found it on
every specimen examined.

[Illustration: Fig. 43.--_Lembus pusillus_.]

KEY TO MARINE GENERA OF OPALINIDAE.

Diagnostic characters: The form is oval, and the body may be short or
drawn out to resemble a worm. They are characterized mainly by the
absence of mouth and pharynx.

Anterior end not pointed; body Genus *_Anoplophrya_
cylindrical; tapering

Anterior end pointed; body elongate; Genus _Opalinopsis_
cylindrical; tapering

* Presence at Woods Hole indicated by asterisk.

Genus ANOPLOPHRYA Stein '60.

(Stein '60; Claparede '60; Leidy '77; Vejdovsky '79; Kent '81;
Balbiani '85; Buetschli '88; Shevyakov '96.)

The general form is elongate, cylindrical or slightly flattened, with
rounded ends, the posterior end tapering. The body is striated with
clearly defined, often depressed lines, which run longitudinally and
sometimes spirally. The contractile vacuoles are usually placed in
rows upon the edges. The macronucleus is almost always long and
band-formed, rarely oval, and generally extending through the entire
length of the body. Micronuclei have been made out in one case.
Reproduction is effected by simple cross division or by budding at
the posterior end, and is frequently combined with chain formation.
The main characteristic is the entire absence of mouth and
oesophagus, the animals being parasitic in the digestive tract of
various annelids. Parasites, salt-water forms.

Anoplophrya branchiarum. Stein '52. Fig. 44.

_A. circulans_ Balbiani.

The body is cylindrical to pyriform, in the latter case broadened
anteriorly. Cuticle distinctly marked by longitudinal striations
which take the form of depressions and give to the body a
characteristic melon shape. The endoplasm contains a number of large
refringent granules--probably body products. The nucleus is elongate,
somewhat curved, and coarsely granular. A micronucleus lies in the
concavity. The cilia are long, inserted rather widely apart along
the longitudinal markings.