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: Could someone with a basic knowledge of biology conclude, from this passage, that determining whether or not an organism should be classified as an animal would be more complicated than simply checking to see if it has a circulatory system?
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.