[1] Today, its circumscription is controversial: Bicudo and Menezes[2] consider twenty-one species as Cryptoglena, of which, nine are uncertain.

[4] After being first described in 1831, little work was done on the genus until the late 1970s and early 1980s, after the scanning electron microscope completed development and was implemented into laboratories.

[4] "Cryptoglena" derives from Latinized Greek and means "hidden-eyeball" or "secret-eyeball", crypto (κρυπτός) being the combining form of "secret" and glene (γλήνη) being "eyeball".

[7] Even today, C. pigra and C. skujae often fail to be noticed in field samples due to their small size.

In the early 2000s, DNA and RNA sequencing became much more accessible to laboratories around the world, which lead to more species being classified in the genus[3] In nature, Cryptoglena is free-living in both freshwater and marine environments.

[7] Cryptoglena are consumed by species that are larger in size or that have the ability to increase oral cavity volume or break down cells prior to ingestion, such as those in the phylum amoebozoa.

The majority of species found to be a part of the genus are green-blue in colour (Triemer and Zakryś, 2015).

[3] Microtubules connect to the interstrip regions of the pellicle and act in a skeleton-like function protection the cell from external pressure and giving it shape (Rosowski and Lee, 1978).

Another band of supportive microtubules forms in the dorsal region and is therefore called the dorsal band of microtubules[3] The stigma (eyespot) is located on the right side of the organism when it is looked at in ventral view and is orange in colour from the carotenoids that are present in a plate-like conglomeration.

In the posterior region of the cells lies the nucleus, which contains the chromatin that remains permanently condensed and attached to the inner nuclear membrane.