Kinorhynchs are tiny marine worms that almost no one except invertebrate zoologists have even heard of, much less seen. Their common names are 'mud dragons' and spiny-headed worms. The name Kinorhynch means movable snout, which refers to the way they can pull in their heads into their bodies and then push them back out when they move.
There are many types of worms that appear in the fossil record during the Cambrian Period, some 500 million years ago. A number of them were once lumped together in a Phylum called the Aschelminthes, which my invertebrate zoology teacher scathingly called the 'ash-can' phylum because it contained a lot of different worm-like animals that were probably unrelated. Sure enough, this phylum was eventually dumped and many of the animals in it got their own phyla, including the mysterious Kinorhynchs. There are two orders of Kinorhynchs in the phylum and about 150 described species.
Kinorhynchs are very small animals, being from 0.2 millimetres to a maximum of about 1.2 millimetres in length. They are entirely marine, living in the benthos in sand or mud. They are tolerant of wide ranges in pressure, temperature and salinity so they live in bottom sediments from intertidal zones to deep benthic communities, and from polar regions to the tropics. They need a good oxygen content in the substrate so they are usually found in the top few centimeters of the sediments.
What do they look like? They have a head with rows of spines around a proboscis. Behind the head are 11 segments, some of which have spines. They are generally white or transparent although a few algae eaters have a bit of colour in their segments. They eat diatoms (algae with shells) or bacteria or detritus. They move by contracting their muscles and pushing their heads forward. They protect themselves by secreting mucous which cements the surrounding mud and sand particles together in a loose tube. They feed by using the bristles around their heads as forceps to catch and pry open the diatoms and then they suck out the contents.
The sexes are separate in this group and they copulate and have internal fertilisation, where the male places a sperm bag in the female. The female then attaches the fertilised eggs in a sac to sand grains. After about ten days they hatch into free-living larvae. They have an exoskeleton so must moult five or six times before reaching adult size.
Kinorhynchs have no known uses by humans and were only discovered in the mid 1800s. They are not endangered or threatened by humans either. Their closest relatives are other pseudocoelomate invertebrates such as the Phyla Priapulida and Loricifera. Pseudocoelomates are animals with a body cavity that develops in a different way embryologically than true coelomates like ourselves.
They may not be important or obvious but they are an example of a group that found a successful life style and stuck with it for hundreds of millions of years. Who knows? They may still be around long after we have disappeared.