AP BIOLOGY

Detailed explanation-1: -Members of the phylum Cnidaria have two different forms of individuals or zooids (polyp and medusa). Polyps are the sessile or fixed individuals, while medusae are motile or free-swimming zooids. Polyps appear more or less cylindrical, whereas medusae appear umbrella-shaped.

Detailed explanation-2: -Medusa bodies are shaped like an umbrella with the mouth and tentacles hanging down in the water. The mouth leads upward into the gastrovascular cavity. Medusae (plural; the singular form is medusa) are not sessile, but rather are motile, meaning that they swim freely in the ocean (Fig. 3.25 C).

Detailed explanation-3: -Yes, Cnidaria are generally both sessile and motile. Most species of Cnidaria have two basic forms, a sessile polyp, and a mobile medusa. This allows the animal the ability to move around and find a good space to live and feed, then develop into a sessile form.

Detailed explanation-4: -The medusa is a free-swimming form; it moves by rhythmic muscular contractions of the bell, providing a slow propulsive action against the water. The other principal body type of the adult cnidarian is the polyp, a stalked, sessile (attached) form.

Detailed explanation-5: -There are two basic cnidarian body shapes: a polyp form, which is attached to a surface; and an upside-down free-floating form called a medusa. Some cnidarians change form at different phases of their life cycle, while others remain in one form for their entire life.