Sponges are fixed, do not move settle at one place, do not have a reproductive apparatus, respiratory apparatus, or organs, and live in marine environments. Why then zoologists consider that sponges are animals? In the past, zoologists considered them as plants and then considered them animals. What is the principe that zoologists have adopted to classify sponges are animals rather than plants?
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Organisms are classified, not by their behavior, but by their phylogeny (evolutionary relationships). Sponges have unique and complex molecules in their intracellular matrix that developed in a common ancestor of sponges, and are shared with sponges and all other animals. These molecules include collagen, proteoglycans, integrin, and adhesive glycoproteins. These molecules are made by the cells, exist outside the cells, and fill the space between them.
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$\begingroup$ @Ναείστεαισιόδοξοι If you think Karl's answer answered your question, you can click on the check mark next to his answer. $\endgroup$– Remi.bCommented Dec 23, 2017 at 22:05
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Their cells are typical of animal cells: no cellulose cell walls, for one thing. They are also heterotrophic, feeding on small cellular life or bits of organic matter taken into individual cells (collar cells or "choanocytes.")
