Both Penard (1922) and Kahl (1932) found Halteria oblonga exclusively in the gelatinous mass of the green alga Chaetophora. I can confirm this finding (s. fig. 1). If freely swimming specimens are found, they were released from the jelly by the sample preparation. So far I have only been able to find Halteria oblonga in the Simmelried, where the species is rarely found.

Halteria oblonga has spring bristles, which are bifurcated at the distal end (s. figs. 3 c and 5 a). However, these appear to be soft and flexible, as they are kept attached to the body during swimming. About 4–6 trailing cilia, which often have a characteristic S-shaped form, protrude beyond the posterior (s. figs. 1 a and 4 b). The outer adoral zone of membranelles (s. fig. 5 c) is not as strongly developed as in the species Halteria grandinella. The inner zone of membranelles (s. figs. 3 b and 4 a), which runs to the mouth opening, does not reach a fourth of the body length and runs slightly to the right. The body shape seems to be quite variable from broadly oval (s. fig. 5 a-d) to almost ellipsoid (s. fig. 4 a-c). I have not yet found specimens with symbiotic algae in the cytoplasm, as described by Kahl.