Coelomates. Hemichordata
Among the phylum Hemichordata are the enteropneusts
(acornworms), which are worm-shaped inhabitants of shallow
seas and have a short, conical proboscis, which gives
them their common name. The vascular system of the Enteropneusta
is open, with two main contractile vessels and a system
of sinus channels. The colourless blood passes forward
in the dorsal vessel, which widens at the posterior
of the proboscis into a space, the contractile wall
of which pumps the blood into the glomerulus, an organ
formed from an in-tucking of the hind wall of the proboscis
cavity. From the glomerulus the blood is collected into
two channels that lead backward to the ventral longitudinal
vessel. This vessel supplies the body wall and gut with
a network of sinuses that eventually drain back into
the dorsal vessel.
