555-545 million years ago
Hemichordates possess a contractile heart and primitive blood vessels. Although blood vessels do service the pharynx, they are not extensive enough to allow the pharyngeal slits to function as gills.
A number of invertebrates possess contractile vessels throughout the
body including hemichordates, Amphioxus, and annelids in which blood is
pumped through peristalsis (Burighel, from Harrison, 1997, p. 232; Ruppert,
from Harrison, 1997, p. 445-52; Prosser, 1973). Hemichordates possess
a heart which includes a venous sinus which receives venous blood (Hickamn;
Benito, from Harrison, 1997). Hemichordates possess contractile blood
vessels (in addition to the heart) to pump blood through their open circulatory
system and sinuses which drain into the heart (Benito, form Harrison 1997;
Hickman). In hemichordates, there is a concentration of blood vessels
around the pharnygeal slits but most of the gas exchange is still performed
by the skin. The gill bars are more important in the filtration of food.
Thus, the pharyngeal blood vessels in tunicates may not be involved in
respiration but merely function in servicing the gill bars (Benito, form
Harrison 1997, p. 59).