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: Although they may not be as complex as a vertebrate's system of veins and arteries, would you still expect to find some kind of blood vessel in even the simplest 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. The most primitive, diploblastic animal phyla lack hearts, but do nonetheless have simple circulatory systems with vessels simply passing from one side of the animal to the other.