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From a morphological point of view, placozoans are among the most simple free-living animals. This enigmatic phylum is
critical for our understanding of the evolution of animals and their cell types. Their millimeter-sized, disc-like bodies consist
of only three cell layers that are shaped by roughly seven major cell types. Placozoans lack muscle cells and neurons but are
able to move using their ciliated lower surface and take up food in a highly coordinated manner. Intriguingly, the genome of
Trichoplax adhaerens, the founding member of the enigmatic phylum, has disclosed a surprising level of genetic complexity.
Moreover, recent molecular and functional investigations have uncovered a much larger, so-far hidden cell-type diversity.
Here, we have extended the microanatomical characterization of a recently described placozoan species—Hoilungia hongkongensis.
In H. hongkongensis, we recognized the established canonical three-layered placozoan body plan but also came
across several morphologically distinct and potentially novel cell types, among them novel gland cells and “shiny spheres”-
bearing cells at the upper epithelium. Thus, the diversity of cell types in placozoans is indeed higher than anticipated
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