Answer: Their body structure becomes increasingly more complex.
Explanation:
In the classification of living things, all animals belongs to Kingdom Animalia. This is contains two main groups namely:
--> invertebrates and
--> Vertebrates.
The vertebrates are complex animals which are characterized with the presence of a backbone.
The invertebrates are less complex animals with no backbone. The phyla that exists in this group includes: porifera, coelenterata, platyhelminthes, nematodes,annelida, mollusca, arthropoda,echinodermata and many among others.
Their body structure becomes increasingly more complex from the phylum porifera to the phylum echinodermata. The Sponges are simple aquatic invertebrates that have ASYMMETRICAL BODIES (this makes them to lack specialised tissues that could prevent them from moving about). While the echinodermata have RADIALLY SYMMETRICAL body with body parts arranged around a central mouth and anus. They have a unique system of water canals for transport inside the body and numerous tube feet moving about.
