There are no predatory sipunculans, so you need never worry about finding one or more of these animals in your reef tank. The captured particles are either transferred to the gut directly by retraction the introvert, or are moved via ciliary currents to the mouth and then to the gut. In burrowing species, such as Sipunculus nudus , the animal directly ingests the sand and silt through which it burrows. They have no specialized respiratory system, and simply exchange gases across the surface of the body, particularly on the introvert near the nuchal tentacles where the cuticle is thinnest. Sipunculans are quite good at regeneration, and most species are able to regrow at least parts of the tentacles and introvert, if damaged.
Some species can regenerate portions of the trunk and the digestive system following serious damage.
Both sections then regrow the missing parts, which is especially impressive for the small posterior portion, because it must regenerate most of the body, including all the feeding, reproductive and digestive organs and muscles. If, however, the species you discover in your tank produces pelagosphera larvae, it is very unlikely that the larvae will ever successfully develop in your tank. Well, I think that is probably more than enough detail on Sipunculans for most people. As I mentioned above, annelids are segmented worms. Among the annelids, segmentation is nearly 'perfect,' with appendages, muscles, nerves, blood vessels, excretory and reproductive systems all repeated in virtually every unspecialized segment.