Which of these best describes Earth's mantle?
Most dense layer, consists of the outer and inner parts
Thinnest under the oceans and thickest under continents
Middle layer, density increases with depth as pressure increases
Least dense layer overall, consists of two layers of different thickness

The best description of Earth's mantle is it is made up of rock so hot that it flows. This layer is also called the asthenosphere. This Earth's mantle has a thickness that reaches 2,900 kilometers to wrap up the Earth's core. It is a rocky layer that covers 84% of Earth's volume.

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.