These organisms are free-living and typically very small, often microscopic, although certain species can reach sizes of up to one centimeter. Their bodies are cylindrical and covered with a hard coat, with both the anterior and posterior ends being as narrow as a needle. This body coat is composed of thick elastin proteins. Beneath the body coat, there are four layers of glands. The mouth opening, located at the front of the body, is surrounded and characterized by lips. Additionally, two unicellular glands called phasmid glands may or may not be present at the periphery of the organism. The end of the animal is elongated.