Enteropneusta
English: acorn worms.
Clades: Hemichordata.
Genera: 111 species; 3 families.
Habitat: the sediment on the seabed, subsisting as deposit feeders or suspension feeders.
Families
Harrimaniidae
Ptychoderidae
Tornaria
Spengeliidae
Torquaratoridae: crawling on the surface of the deep ocean bottom and alternatively rising into the water column.
Zoology
Enteropneusta are worm like marine animals. Some can become long, up to 2.5 metres. They live in sediment burrows. It is assumed that the ancestors of acorn worms used to live in tubes like Pterobranchia. They have an iodoform-like smell, by iodine containing secretion.
Acorn worms have an open circulatory system, in which the blood flows through the tissues sinuses. They have a muscular sac acting as a regularly pulsating heart, which is a closed fluid-filled vesicle whose, not directly connected to the blood system.
Acorn worms continually form new gill for respiration slits as they grow.
A plexus of nerves lies underneath the skin, and is concentrated into both dorsal and ventral nerve cords.
Acorn worms have a Y-shaped nuchal skeleton that starts their proboscis and collar on their ventral side.