Rhynchocoelans

Rhynchocoelans are a group of marine invertebrates belonging to the phylum Hemichordata, which also includes acorn worms. They are characterized by their distinctive proboscis, a muscular, preoral feeding structure that extends from the anterior end of their bodies. Rhynchocoelans are primarily found in shallow marine environments, often buried in sand or mud, where they feed on detritus and small particles.

The body of a rhynchocoelan is divided into three main regions: the proboscis, the collar, and the trunk. The proboscis is used for both feeding and defense, capable of rapid extension and retraction. The collar region is typically ringed with tentacles that aid in capturing food particles. The trunk contains the digestive, circulatory, and reproductive systems. Unlike acorn worms, rhynchocoelans lack a distinct gill slits and instead possess a single, elongated gill slit on each side of the body.

Rhynchocoelans exhibit a unique mode of locomotion, using their proboscis and trunk muscles to burrow through sediment. They reproduce sexually, with external fertilization occurring in the water column. Larvae are free-swimming and resemble those of other hemichordates, eventually settling and metamorphosing into adults.

There are approximately 120 known species of rhynchocoelans, divided into two main classes: Enteropneusta (acorn worms) and Pterobranchia. While Enteropneusta are more commonly studied, Pterobranchia are smaller and often colonial, forming intricate branching structures. Rhynchocoelans play a role in marine ecosystems as detritivores, contributing to nutrient cycling and sediment stabilization.

Their phylogenetic position within Hemichordata remains a subject of study, as they share characteristics with both echinoderms and chordates, providing insights into the evolutionary relationships among these groups. Fossil records indicate that rhynchocoelans have existed for over 500 million years, with early forms resembling modern-day species.