plakinid

Plakinids are a group of extinct marine invertebrates belonging to the phylum Placozoa. They are characterized by their flat, simple, gelatinous body plan and a lack of distinct tissues and organs. Plakinids are found in marine environments worldwide, often inhabiting the surface of algae, rocks, and other substrates in shallow to moderately deep waters. Their exact evolutionary relationships are still debated, but they are considered one of the simplest multicellular animals known.

These organisms typically measure a few millimeters across and possess a bilaterally symmetrical, amoeboid structure. The body consists of a few different cell types, including ciliated epithelial cells on the upper surface, gland cells that secrete mucus, and a single layer of amoeboid cells on the underside that are responsible for movement and feeding. Plakinids move by gliding using their ventral cilia and by extending pseudopods. They are microphagous feeders, consuming algae and other organic detritus by engulfing it with their ventral cells. Reproduction in plakinids can occur both asexually through fission and sexually, although details of their sexual reproduction are less understood. Their simple body plan and unique cellular organization make them a subject of interest in developmental biology and evolutionary studies, offering insights into the origins of multicellularity.