Genus LEMBUS Cohn '66.

(Cohn '66; Quennerstedt '69; Kent '81; Fabre-Domergue '85;
Gourret & Roeser '88; Bütschli '88; Shevyakov '96.)

Free-swimming animals of elongate form, more or less elastic, and flexible, bending readily to avoid obstacles, etc. The anterior half is usually drawn out into a slightly curved neck-like portion. The peristome is a small groove leading from the anterior end to the mouth about midway down the ventral side of the body. Bütschli, following Quennerstedt, describes an undulating membrane on each side of the peristome groove. Other observers, however, usually describe but one, the left, which is clearly defined and stretches out some distance from the body, while the right border is described as having smaller but very active cilia. The general body surface is clothed with fine, uniform cilia, and body striæ are usually absent. One or more caudal bristles may be present. The contractile vacuole is posterior and terminal, and may be multiple. The macronucleus is spherical and perhaps double (Kent). Food is chiefly bacteria, and the animals are frequently found with the anterior end embedded in zooglœa masses. Salt water, usually in infusions.

Lembus infusionum, n. sp. Fig. 42.

The body is elongate, lancet-shaped, with a tapering anterior extremity. The dorsal outline is concave through the bending of the anterior end, while the ventral outline presents an even, convex curve. The mouth lies slightly above the center of the body and marks the posterior limit of the ventral peristomial groove, which curves slightly from the anterior extremity. Each side of this groove bears an undulating membrane, the left being much larger and conspicuously striated. The general form of this left membrane is triangular, the widest part is anterior, the narrowest at the mouth. The right membrane is similar in form, but smaller and more active. The endoplasm is colorless and finely granular, not regionally differentiated. The ectoplasm consists of a relatively thick cortical plasm specially noticeable in the posterior half of the body and a delicate cuticle which bears almost imperceptible longitudinal markings—the insertion points of the fine cilia. The body is covered with uniform cilia except at the anterior extremity. Here they are much larger and bristle-like. I was unable to find any cilia in the peristome. One long caudal bristle, one-quarter of the length of the body, trails out behind. The macronucleus is spheroidal and placed near the center of the body; a conspicuous micronucleus lies near it. A row of contractile vacuoles extends from the posterior end. I have seen as many as six of nearly equal size and one or two smaller ones. The intervals of contraction are quite long. Length 70 to 75µ; greatest diameter 10 to 12µ.


Fig. 42.—Lembus infusionum.
[ ENLARGE ]

L. infusionum resembles L. elongatus in its general form and in its mode of life, for it excavates a retreat in zooglœa masses and lies there for considerable periods perfectly quiet. It differs from L. elongatus and from L. velifer (probably the same as L. elongatus of Claparède & Lachmann) in the presence of the caudal bristle, in the absence of annular markings, number of contractile vacuoles, and in the slightly smaller size. It resembles Lembus verminus (Müller) as described by Kent (Proboscella vermina), and L. intermedius as described by Gourret & Roeser (Lembus verminus syn.)in the absence of annular markings and in the presence of a caudal bristle. It differs from the former, however, in the absence of a tentacle-like process, and from both in the absence of a double nucleus and in the presence of many vacuoles. These features are so characteristic of all the specimens examined that I have concluded, somewhat reluctantly, to give it a specific name. It is common in old infusions of algæ, especially after decomposition is well advanced. Its food consists of bacteria.

Lembus pusillus Quennerstedt 1869. Fig. 43.

Synonym: L. subulatus Kent 81.

This species is much smaller than the preceding, and might easily be mistaken for [Uronema marina]. It is subcylindrical in form, the anterior end bluntly pointed, the posterior end rounded. The oral apparatus is quite different from [Uronema]. The mouth, as in the preceding species, is at the end of a long peristomial groove extending from the anterior end to the middle of the body. The edges of the peristome bear undulating membranes as in L. infusionum. Like the latter, there is one caudal bristle, but unlike it there is only one posterior contractile vacuole, while the endoplasm is filled with large granules or food balls. The cuticle is distinctly striated with longitudinal markings, and the cilia are uniform in length.

Habitat similar to that of L. infusionum, in zooglœa masses. Length 26 to 30µ; diameter 7 to 8µ.


Fig. 43.—Lembus pusillus.
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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.

Key to marine genera of Opalinidæ.
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 cylindrical; tapering Genus *[Anoplophrya]
Anterior end pointed; body elongate; cylindrical; tapering Genus Opalinopsis
* Presence at Woods Hole indicated by asterisk.