Question

The key evolutionary development of animals seen for the first time in sponges is that of

Answer 1

The key evolutionary development of animals seen for the first time in sponges is that of  Multicellularity.
Multicellularity means having or consisting of many cells or more than one cell to perform all vital functions. Sponge is a member of the phylum Porifera. It is a simple animal with any cells, but no mouth, muscles, heart or brain. It is any primitive multicellular aquatic animal that constitute the phylum stated above.  

Answer 2

The key evolutionary development of animals seen for the first time in sponges is that of Multicellularity.    A Multicellular organism has a longer lifespan than an unicellular organism and since it has multiple cells, it can perform more functions.  Sponges do not have distinct circulatory, respiratory, digestive, and excretory systems – instead the water flow system  supports all these functions.