original passage: While humans, as well as other vertebrates, have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, diploblastic animal phyla lack circulatory systems.
original sentence: The most primitive, diploblastic animal phyla lack circulatory systems.
sentence2: From this passage, does it sound like oxygen might be absorbed through an animal's skin, but then passed through a basic circulatory system in even the most primitive of animals?
sentence1: While humans, as well as other vertebrates, have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open circulatory system containing a heart but limited blood vessels. There is no circulatory system inside of the most primitive, diploblastic phyla of the animal kingdom.