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Why are we using the term "incomplete" for digestive system only? We could have also said that for circulatory system that it is either incomplete or present as a simple system. What I mean is that, this doesn't relate to the definition of organ system. In other words, it will mean that the only organ system present in lower animals is digestive system.

Also for other organ systems, it is written that they are absent. What is the reason for this wording? photo of table in textbook

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    $\begingroup$ My advice would be to just learn what these labels are saying about the features the organisms have; the labels placed on them are not really important for "understanding the world", just for communicating with other people (like your biology instructors). Understand the system, then apply the label; don't use the labels to try to understand the system. The biology existed before the label. $\endgroup$
    – Bryan Krause
    Jan 19 at 16:35

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An incomplete digestive system is one in which there is only one opening. In our digestive system food is taken in by one opening (our mouth), the nutrients extracted and the remains are ejected by another opening (our anus).

In these lower organisms the food is taken in by one opening, the nutrients extracted and the remains are ejected through the same opening.

An absent digestive system is not having such a method of ingesting matter to be processed and ejected afterwards. These organism absorb their required nutrients directly from the surrounding media.

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