Ecological succession a) The first organisms that appear in an area after it has been disturbed
b) The variety of life in an ecosystem
c) A biodiverse community of organisms in an ecosystem many years after a
disturbance
d) Gradual changes in the types of species that live in an area

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explanation:

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.