English: Poriferan and Placozoan neuroid systems. (A) Different cell types (different colors) were identified using scRNA-seq in the demosponge Spongilla lacustris (Musser et al., 2021): apnPin—apendopinacocytes; apo—apopylar cells; amb—amoebocytes; arc—archeocytes; cho—choanocytes; mes2 and mes3—mesocytes; myp—myopeptidocytes; nrd—neuroid cells (orange). The neuroid cells are located in the center of the choanocyte chamber, make connections to choanocytes, and might be involved in their control as neuronal-like elements. These neuroid cells contain secretory apparatus and vesicles. However, the transcriptome profiles of these neuroid cells are remarkably different subset from other known neural/neuroid-type cells in metazoans (Nakanishi et al., 2015; Musser et al., 2021), suggesting that these are sponge-specific innovations with no apparent homologs in other animals. The nature of these cellular interactions is unknown. (B) The emerging diversity of cell types in the placozoans. The diagram is based on recent ultrastructural studies (Smith et al., 2014; Smith et al., 2015; Mayorova et al., 2018; Mayorova et al., 2019; Romanova et al., 2021). Several morphologically distinct cell types were identified: cc—crystal cells; fc—fiber cells; gc—gland cells; lc—lipophil cells; le—lower epithelial cells; nlc-neuroid-like cells, which were previously labeled as stellate-like cells (Romanova et al., 2021); ss—shiny spheres; ue—upper epithelial cells. (C1) Scanning electron microscopy of Trichoplax—an animal without an upper cell layer. The photo shows the spatial organization of a complex meshwork formed by elements above the middle layer and the upper layers of the animal. Distributed net-like structures were formed by processes of different subtypes of fiber cells and stellate-like cells, which we also named neuroid-like cells. Some heterogeneity of fiber and neuroid-like cells is anticipated from recent ultrastructural studies (Romanova et al., 2021). (C2) Schematics of the spatial distribution of different subtypes of fiber, neuroid-like cells, and their processes. All these cells might form in a net-like structure above the upper layer with crystal cells as a distributed integrative system. This reconstruction is based on (Romanova et al., 2021; Romanova et al., 2022) and unpublished data. Scale: 20 μm.