The Saccocirridae are small interstitial polychaete

Polychaete

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...

s common in coarse sand, reflective, surf beaches, usually within the zone of retention. The Saccociridae are members of the clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Protodrilida, which is in turn part of the clade Canalipalpata

Canalipalpata

Canalipalpata, also known as bristle-footed annelids or fan-head worms, is an order of polychaete worms, with 31 families in it including the Sabellida and the Alvinellidae, a family of deep-sea worms associated with hydrothermal vents.The Canalipalpata have no teeth or jaws. Most are filter feeders...

. Saccocirridae have a world wide distribution and it is likely that many more species remain to be described. These polychates are usually between 2 and 10 mm in length and 500 μm wide. They have reduced parapodia

Parapodium

Parapodia , singular parapodium, are paired, un-jointed lateral outgrowths from the bodies of two different invertebrate groups, which are primarily marine in habitat...

 and are considered a true interstitial species, incapable of burrowing through finer sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s.

Morphology

Based on Saccocirrus sonomacus from the Pacific Coast of the Americas. The prostomium supports a pair of groved palps that have, primarily, a sensory purpose. A pair of eyes are also present. The peristomium is probably reduced to a circumoral ring. Pygidium

Pygidium

The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...

is usually bi-lobed. It contains a duo-gland adhesive system.