Problem 26 - Objective Biology for NEET

Question

Select the total number of organism from the following which are sessile: Amoeba, Euglena, Adamsia, Spongilla, Hydra, Jelly fish, Earthworm (a) 1 (b) 2 (c) 3 (d) 4

Step-by-Step Solution

Answer

The total number of sessile organisms in the given list is 3.

Step 1: Identify Sessile Organisms

Firstly, identify the organisms from the list which are sessile. Sessile organisms in the list are Adamsia, Spongilla, and Hydra.

Step 2: Count the Number of Sessile Organisms

After identifying the sessile organisms, count them. There are 3 sessile organisms in the list: Adamsia, Spongilla, and Hydra.

Key Concepts

Adamsia

Adamsia is a fascinating example of a sessile organism. This genus includes sea anemones, which are known for their stationary lifestyle. Adamsia attaches itself firmly to rocks or other hard surfaces in marine environments. Because they remain fixed in one place, they rely on water currents to bring food close to their tentacles. These tentacles are equipped with stinging cells called cnidocytes, which help capture prey such as small fish and plankton. This method of feeding is passive and requires minimal energy expenditure.

Despite their simplicity, sea anemones exhibit a mutualistic relationship with other marine animals, such as hermit crabs. In this relationship, the anemone provides the crab protection with its stinging tentacles, while the crab offers locomotion to the anemone—a clever way for Adamsia to get a change in scenery without moving on its own. Sea anemones also reproduce both asexually and sexually, ensuring their presence in diverse marine ecosystems.

Spongilla

Spongilla is a genus of freshwater sponges, recognized as one of the few representatives of sessile organisms within freshwater ecosystems. These sponges adhere to submerged surfaces like rocks or logs in calm environments such as lakes and streams. As filter feeders, Spongilla extracts nutrients from water passing through its porous body.

The structure of Spongilla is quite simple. Its body is full of tiny openings called ostia, through which water flows in. Larger openings, known as oscula, allow the filtered water to exit. Inside, specialized cells called choanocytes drive the water current and trap food particles like bacteria and algae.

Spongilla plays a crucial role in the aquatic ecosystem through this filter-feeding habit, maintaining the water quality by filtering out undesirable particles. Their reproduction can occur both asexually, through budding, and sexually, by producing larvae able to disperse to new locations.

Hydra

The Hydra is a small, freshwater organism belonging to the phylum Cnidaria, much like the sea anemone. These creatures are exceptional in their regenerative abilities, known for their capacity to regenerate entire bodies from small pieces of tissue. Hydras are sessile, typically attaching themselves to substrates underwater using a specialized structure called a "foot" or basal disc.

Hydras exhibit radial symmetry and possess tentacles encircling their mouth. These tentacles harbor specialized cells called cnidocytes, which help in capturing prey. They employ these stinging cells to immobilize small aquatic organisms, which are then carried to their mouth for ingestion.

Remarkably, hydras also reproduce asexually through budding, though they can reproduce sexually under certain environmental conditions. Their ability to anchor themselves in place while capturing food with their tentacles is a clear example of efficient adaptation among sessile organisms.