Diurodrilidae are another of the exclusively interstitial families in Annelida and in general appearance it somewhat similar to Dinophilidae, which have presented before. However, they are smaller with a body length of just about 0.4 mm and hence in that are more similar to gastrotrichs. Together with a dorvilleid species of the genus Noetenotrocha, Diurodrilus minimus is the shortest annelid with a body length of a quarter of a millimeter (or half the tip of a typical pencil). Also they are among the fastest moving interstitial annelid and it that also resemble more a gastrotrich. Their movement is actually quite peculiar with a speedy run followed by a period of rest appearing quite jerky.

Diurodrilids possess a head with stiff sensory cilia and a muscular pharynx for feeding. However, what they feed on is unknown. The head is followed by just one to three indistinct segments without any appendages. Hence, these “polychaetes” also do not possess any chaetae and accordingly lack the name-given character of polychaetes. At the end of the body are four toe-like adhesive pads that allow a strong attachment to the substrate, especially sand grains. This is not an uncommon feature for interstitial taxa as it allows them to attach to a sand grain really quickly and robustly when the sediment is disturbed either due to a strong wave action or due some feeding larger predator. In the latter case, such feeders often sort out sand grains from their diet to avoid filling up their gut with sand. In the former case, due to the higher density the sand grains will settle much faster to the ground again as would the animal. Hence, the chance of being drifted away is much reduced.

While these animals occur globally, their record is still sparse and they are rarely found. In consequence, only seven species have been described to date and their distributions are patchy. Their preferred habitat is intertidal waterlogged sandy beaches often higher up the beaches, but they can also be found in the shallow subtidal. Nonetheless the zone they occupy on a beach is often narrow and species of Diurodrilidae avoid to inhabit the same regions on the beach, possibly indicating a high degree of ecological competition among each other and avoidance of such a competition. The presence of storage chambers (seminal vesicles) in males and the occurrence of clusters of sperm in females indicate a possible direct transfer of sperm and internal fertilization. Given their rareness in records, it is not surprising that we know so little yet about their reproduction.

So why did Diurodrilidae to be their own phylum? As mentioned above, there are some similarities to gastrotrichs and additionally obvious annelid characters are lacking, even the so-called segments are indistinct and hence inconclusive. A first molecular study also could not place them within Annelida. Hence, it was suggested that they could be their own phylum also no other strong links to another phylum could seen. However, following molecular studies firmly place Duirodrilidae in Annelida, which possibly originated by progenetic evolution like, for example, Dinophilidae or some species of Orbiniidae. This means larval or juvenile stages of an ancestor become sexual mature and stopped further development. Especially convincing in this respect was the gene order in the mitochondrial genome, which was almost identical to the one found in third quarters of all annelids. Hence, annelids welcomed the lost Diurodrilidae back with open arms.