The Nematoda possess an elongated and thread-like form.
Those that are usually dwelt on are treated with the Rotifers and Nematoda and Turbellaria.
The Nematomorpha form an isolated group; at first sight they seem to be connected with the Nematoda, but in reality their only common feature is the tubular genitalia opening into a cloaca, and it seems at present impossible to connect them with the Annelida.
The muscles are striated and arranged in four quadrants, two dorso-lateral and two ventro-lateral, an arrangement which recalls that of the Nematoda, whilst in their histology they somewhat resemble the muscles of the Oligochaeta.
It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these.