The Nematoda which are parasitic during their whole life may similarly be divided into two classes - those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts.
Indeed Mechnikov, to whom we owe much of our knowledge of these forms, calls them "creeping Nematoda."
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.
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.