Bonelliidae

Characteristics

Members of the class Echiura are plump, unsegmented worms, commonly known as spoonworms. The mouth is at the anterior end of the trunk and a flattened proboscis extends forward in front of the mouth. The ventral side of the proboscis has a ciliated channel along which food particles and mucus are moved towards the mouth. Close behind the mouth are two hooked chaetae and one or two nephridial pores. The gut is much longer than the body and is folded and coiled inside the coelom (body cavity). The anus is at the posterior end of the body and two anal vesicles with ciliated funnels open into the cloaca.

In the family Bonelliidae, the females are very much larger than the dwarf males, and in most, if not all, instances the males live on or inside the female. The anal vesicles of the females are sac-like and bear the ciliated funnels on tubules, which branch in most genera. Some genera lack the hooked chaetae behind the mouth, and none of the genera have chaetae at the posterior end of the trunk. Most genera contain the pigment bonellin which gives them a green colour and may have antibiotic or defensive properties.

Males are quite different in appearance; the minute, ciliated body consists of little except a gonad, a seminal vesicle and two protonephridia. In most species, the male is found inside the genital sac of the female. The sex of a bonelliid spoonworm depends where the planktonic larva settles. If it settles on an adult worm of its own species, it develops into a dwarf male, but if it settles anywhere else, it develops into a full-size female.