why sponges ( porifera) are considered as a transition between unicellular and multicellular organisms

Sponges are aquatic, invertebrate animals that make up the phylum Porifera. The word Porifera means “pore-bearing,” and a highly porous body is one of the most striking features of sponges. Sponges are the simplest multicellular animals found in the fossil record. It is difficult to imagine that an organism as complicated as a human being could be related to such a remarkably simple animal. However, sponges represent an essential step in the evolution of complex animals: the transition from simple protists to multi-celled, complex animals. In this concept we will consider the characteristics and classification of sponges, their specific structural features, how they reproduce, and the environments that they inhabit.

Sponges are not unicellular, but they also don’t have tissues that multicellular organisms have

The type of rock she find is a sediment rock