Different species that belong to the same biome, but occur in widely separated geographic regions, often appear similar due to

convergent evolution

Explanation:

The convergent evolution is process through which animals that belong to different families, but live in the same or very similar conditions, exhibit the same or very similar physical and behavioral traits. There are lot of examples in nature of convergent evolution, both in the present and in the past. As a nice example we can take the sabretoothed predators of the past. They all lived in similar environments, hunted larger prey, and developed the same or very similar body shapes, hunting strategies, are sabreteeth. The sabretoothed cats, barbourfelids, nimravids, thylacosmilus, were all mammals, and they all looked incredibly similar, even though they were different species, belonging to different branches of the mammalian family tree.

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.