surface.  The depression becomes much deeper at about the center of the body, and is especially marked on the left side of the peristome.  In this deeper portion is the mouth, with an almost imperceptible oesophagus.  Upon the left edge of the peristome is a high, undulating membrane, sail-like in appearance when extended.  This may stretch around the posterior edge of the peristome and upon the right aide, thus forming a pocket by means of which the food particles are directed into the mouth.  The rest of the right edge of the peristome is occupied by closely approximated powerful cilia (Buetschli) or a second undulating membrane (Stein).  The body cilia are relatively long.  Trichocysts and caudal bristles may be present.  The contractile vacuole is subterminal and dorsal; it is questionable whether there are canals leading to it.  A round macronucleus with one micronucleus is in the anterior half of the body.  The anus is terminal.  Food is chiefly bacteria.  Movement combines springing with swimming and rotation.  Fresh and salt water.

Pleuronema chrysalis Ehr., sp.  Fig. 40.

Synonyms:  Pleuronema crassa Dujardin ’41; P. marina Duj. ’41;
Fabre-Domergue ’85; P. coronata Kent ’81; Paramoecium chrysalis
Ehr. ’38; Lembadion orale G. & R. ’88; Histiobalantium agile
Stokes ’85, ’88.

The body is ovoid, slightly flattened, rounded at both ends, the anterior end sharper than the posterior.  The ventral surface is almost entirely taken up by a peristome which extends from the anterior end posteriorly three-quarters of the body length.  The posterior end of the peristome is straight, the left curved, following the depressed portion.  The body is covered with fine cilia in longitudinal lines, except on the peristome.  The mouth is small and situated in the hollow of the peristome near the left border.  On the left peristome edge is a large undulating membrane.  It begins near the anterior end of the body and increases in height posteriorly following the peristome edge around on the right side.  This posterior bend of the membrane causes the appearance of a full sail, so often seen.  It can be entirely withdrawn and folded together in the peristome.  On the right edge of the peristome are large, powerful cilia.  The contractile vacuole is central and dorsal; the macronucleus is in the anterior half of the body, with one attached micronucleus.  Food consists of bacteria.  Not very common.  Fresh and salt water.  It often remains quiet, with membrane and cilia outstretched, as though dead, but suddenly gives a spring and is gone.

[Illustration:  Fig. 40.—­Pleuronema chrysalis.]

Pleuronema setigera, n. sp.  Fig. 41.