Subphylum 2. INTRAMACRONUCLEATA: Class 5. NASSOPHOREA — Diverse, Yet Still Possibly Pivotal

Abstract

Ciliates in the Class NASSOPHOREA have played a pivotal role in phylogenetic schemes of the evolution of diversity of ciliates. Their simplified oral structures were thought to represent the ancestral condition of the more well-developed oral polykinetids of oligohymenophoreans, hetero-trichs, and spirotrichs. They are united by two ultrastructural features: alveolocysts are a presumed synapomorphy of all representatives, although they have not been observed yet in synhymeniids; and the nematodesmata of the nasse bear nematodesmal or X-lamellae, which are not found in the phyllo-pharyngean cytopharyngeal basket. The highly developed nasse is used to ingest various “algae”, typically cyanobacteria such as Anabaena and Oscillatoria, whose natural populations in rare instances nassophoreans may control. The somatic cortex has a highly developed epiplasm. In addition to the nasse, there is a set of “oral” polykinetids that extends often around the body circumference as a linear assemblage called a frange or synhy-menium. This is why stomatogenesis in these forms is considered mixokinetal because both somatic and oral kinetal elements are involved. The genetics of these ciliates is virtually unexplored so details of conjugation, mating type system, and nuclear development remain to be discovered.