Which of the following is an example of a sessile animal?

Concept: A sessile animal is permanently attached to a substratum (rock, seafloor, or another surface) for its entire adult life and does not actively move from place to place; it typically obtains food by filtering the surrounding water or waiting for it to arrive, rather than by locomotion.

Application: Euplectella, commonly called Venus's Flower Basket, is a genus of glass sponge belonging to the phylum Porifera, class Hexactinellida. Sponges lack any organ of locomotion and remain permanently fixed to rocks or the sea floor throughout their adult life, filtering water through their body wall to obtain food. This makes Euplectella a classic example of a sessile animal.

Cross-check — contrast with the other options:

• Chiton has a broad muscular foot and can creep slowly over rocky surfaces, so it is motile, not permanently fixed.

• A leech swims through water and crawls using its two suckers to reach a host, so it is a highly mobile organism.

• Echinus (sea urchin) moves about the sea floor using rows of tiny tube feet, so despite being slow it is still motile, not sessile.

Among the given options, only the glass sponge genus is permanently anchored and never relocates, so it is the example of a sessile animal.