I find Amphileptus pleurosigma very frequently, especially in the Purren pond, a body of water with lots of fallen leaves. Amphileptus pleurosigma can be found here practically all year round.

The specimens in my population were between 200–350 µm long and swim quite fast. In the free-swimming state, the body is very slender and slightly sigmoid. The bundle of curved extrusomes at the apical end is clearly visible. Only in slightly squashed specimens can one recognize that further extrusomes are distributed in the cytoplasm (s. fig. 2 a-c).

Amphileptus pleurosigma is a pleurostomatid ciliate. Some genera of this order (e.g. Litonotus or Loxophyllum) have a bilateral structure with a ciliated right side and a rudimentarily ciliated left side. Often the left side is also naked. In the case of Amphileptus pleurosigma, there are only 4–6 rows of very short, bristle-like cilia on the left side (s. fig. 5).

The mouth opening of Amphileptus pleurosigma is not located where the apical bundle of extrusomes is, but is a ventral slit in the anterior third, which can only be seen when feeding. Amphileptus pleurosigma prefers to feed on peritrichous ciliates.

The nuclear apparatus of Amphileptus pleurosigma consists of two macronuclei and a micronucleus in between. This is enclosed in a clearly visible membranous sac called the funiculus (s. fig. 6). Together with the curved extrusomes, which have the shape of a tusk, this funiculus is an important identifying feature.