The cuticle of these invertebrates is not ciliate. In addition to a substantial pseudocoel , these animals possess a complete digestive tract, with a mouth and posterior anus. Numerous microvilli greatly increase the surface area available for digestion in the mid-gut. Occasional blood corpuscles of hemerythrin , in addition to digestive phagocytes have been observed within the animals body cavity. The contraction of muscular tissue in the body walls enables circulation of body fluids.
Excretion is carried out by paired nephridia that empty into a duct shared with the gonads.
A typical brain is absent, and is represented instead by a circumpharyngeal ganglion. The primary nerve cord lies mid-ventrally and extends from the pharyngeal ganglion longitudinally down the body. Other nervous tissue lies in close proximity to the epidermis. The smaller species are believed to prey upon micro-organisms such as bacteria living in the interstitial environment. Larger species are voracious carnivores, feeding upon polychaete worms, as well as members of their own phylum.