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.
[ ENLARGE ]

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.