The most primitive true animal belongs to the phylum

Porifera or Sponges

Explanation:

Porifera is the phylum of the simplest multicellular marine animals. Their body is composed of  two cell layers (something like tissues) and mesophyl between them. The body is in direct contact with surrounding water through the large number of pores and channels. What makes Sponges so simple is that their cells are unspecialized and that they  don't have nervous, digestive or circulatory systems.

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