Animal kingdom consists of two phylas. Phylum non chordates and chordates. Non chordates consist of Porifera, Colenterates, Ctenophora, Platyhelminthes, Aschelminthes, Annelids, Arthropods, Molluscs, Echinodematas. Hemichordata etc. Platyhelminthes involves flatworms as they do not have body cavity. The body consists of organs but no organ systems. Thus digestive system is also incomplete. It means the system consists of only one opening that serves as both mouth and anus. This is seen in animals like hydra etc.