Which coelenterate is not polymorphic

The correct answer is Adamsia.

Understanding Polymorphism in Coelenterates:
Polymorphism refers to the occurrence of more than one structurally and functionally distinct individual (zooids) within the same organism or colony. In the phylum Coelenterata (Cnidaria), polymorphism is highly common and typically involves two main body forms:
1. Polyp: A sessile, cylindrical form that reproduces asexually (e.g., Hydra, sea anemone).
2. Medusa: A free-swimming, umbrella-shaped form that reproduces sexually (e.g., jellyfish).

Analyzing the Options:
1. Physalia (Portuguese man-of-war), Velella (by-the-wind sailor), and Porpita (blue button) are colonial coelenterates belonging to the class Hydrozoa. They exhibit extreme polymorphism with several types of specialized zooids in a single colony, such as gastrozooids (feeding), dactylozooids (defense/sensory), and gonozooids (reproduction).
2. Adamsia (commonly known as the sea anemone, which lives in a symbiotic relationship with hermit crabs) belongs to the class Anthozoa. Anthozoans exist only in the polyp form. Because Adamsia exhibits only a single structure (the polyp form) throughout its life cycle and does not form polymorphic colonies with differing zooids, it is not polymorphic.