The large opening at the base of the skull through which the spinal cord passes is an important clue to evolutionary relationships. it is called the

It is called The Foramen Magnum

Explanation: It is a large oval opening linking the Spinal cord and the Brain

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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.