What causes multipcellualr organizms to grow?

The cell proliferates to produce many more cells that result in the multicellular organism. ... The process starts with a single fertilized cell that increasingly divides to form many more cells. In the process, the genome causes the cells specialize through selective gene expression.

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organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.