If whales evolved from land-dwelling mammals, one would expect the fossil record to include intermediate forms, creatures more whale-like than land-dwelling mammals yet still possessing hind limbs and a pelvis lacking in modern whales. such intermediates are termed forms
vestigial features
hybrids
homologous structures

transitional forms

Explanation:

the form of organism  that exhibit the shape which is intermediate between the  their ancestors and the   descendants  (just like a father between grandfather and grandson) is known to be the transitional form of respective organism.

The transitional form organisms or fossils have some traits and similarities which are common to both ancestors and the   descendants.

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.

this transfer is call conjunction