I only rarely find Frontonia acuminata. The species is very similar to the much more common species Frontonia angusta, but Frontonia acuminata has a characteristic dark pigment spot located near the anterior end and the shape is slightly asymmetrical (shield-shaped, s. figs. 1 a-c and 2). The pigment spot is localized on the dorsal side and consists of greyish-black grains with a diameter of 1–2 µm (s. fig. 10). The grains are not crystalline and are not bacteria. Their nature is still unknown.

The contractile vacuole is located approximately in the middle of the body on the right side (s. fig. 1 b). It has two (rarely 3) excretory pores on the dorsal side (s. fig. 7). Under the pellicle there is a fringe of dense, spindle-shaped extrusomes. In my population they were between 7–9 µm long (s. fig. 9). The mouth opening is located in the anterior third on the left side. It corresponds to the Frontonia type. The pre-oral and post-oral sutures are clearly visible in the slightly squashed specimens (s. fig. 5). Below the mouth opening there is also a triangular field of framed cilia with unknown function (s. figs. 5 and 6).