Zoology 10th Edition by Stephen Miller,John Harley

Exercise 1

Chaetopteridae is a family of annelids whose members live in U-shaped parchment tubes. Their bodies are divided into three regions. Highly modified parapodia function in creating water currents and secreting mucus, which entraps food particles brought into their tubes with the circulating water. External fertilization leads to the development of trochophore larvae. Sipunculids burrow in mud and sand of marine habitats. They extend their introvert for feeding, and they lack segmentation. Reproduction is by external fertilization, and development includes trochophore larvae.
In what ways are the Chaetopteridae and sipunculans "unusual annelids?" What features of their biology are annelid-like?