Rhombozoans: Rhombozoa

Rhombozoans (RAHM-boh-ZOH-uhns) are parasites (PAIR-uh-sites) that live in the kidneys of bottom-dwelling octopuses, cuttlefish, and sometimes squid. Parasites are animals or plants that live on or in another animal or plant, called a host, without helping it and usually harming it. The body of rhombozoans is made up of only eight to forty cells in a simple arrangement. They have the fewest cells of any animal.

Rhombozoans have two types of organization. One type is wormlike embryos and adults. This form consists of a central cell shaped like a cylinder and a layer of eight to thirty outer cells that have hairlike fibers. At the front of the animal, four to ten of the outer cells form a cap, the hairlike fibers of which are shorter and denser than on the outer cells toward the rear of the animal. The shape of the cap varies among species of rhombozoans. The second type of organization is an embryo that consists of thirty-seven or thirty-nine cells that are more specialized than the cells of wormlike rhombozoans. Inside these embryos are four large cells, each containing another cell that may give rise to the next generation. It is these specialized embryos that invade the host animal.