The circulatory system in a phylogenetic context for non-vertebrate deuterostomes

Tunicates (Urochordata) do not have a closed circulatory system, only craniates have the first epithelial ducts that transport oxygen to the locomotion and digestive system-related muscles. However, a small “heart” spreads via peristaltic movements oxygen dissolved in water that circulates through the body. The case in Cephalochordata is even more bizarre since there is no real organ that spreads oxygen through the body. By the contrary the isosmotic fluids run through the body and will eventually reach each cell. In craniates, both the evolutionary novelty of hemoglobin and the heart significantly increase the efficiency of gaseous transactions in the body, which is linked to the characteristic high mobility and levels of activity in craniates.