Why are we using the term 'incomplete' for the digestive system only? Could we have also said that for the circulatory system it is either incomplete or present as a simple system? What does this mean in relation to the definition of an organ system? In other words, does it mean that the only organ system present in lower animals is the digestive system? Also, for other organ systems, it is written that they are absent. What is the reason for this wording?