Question

Sponges exchange \(\text{O}_2\) with \(\text{CO}_2\) by

• A. gills
• B. simple diffusion over their entire body surfaces
• C. moist cuticle
• D. tracheal tubes

Hint:

• Simple diffusion across the body surface is the primary mode of respiration for lower invertebrates, including sponges (Porifera), coelenterates (Cnidaria), and flatworms (Platyhelminthes).

• The water canal system in sponges serves a triple purpose: nutrition (food gathering), respiration (gas exchange), and excretion (removal of metabolic wastes).

Answer 1

The Correct Option is B

Concept:

• Phylum Porifera: Sponges belong to the phylum Porifera, characterized by a multicellular body with a cellular level of organization. They lack true tissues, organs, or specialized organ systems.

• Water Canal System: Water enters through microscopic pores (ostia) in the body wall, passes into a central cavity (spongocoel), and exits through a large opening (osculum).

• Gas Exchange: Without specialized respiratory structures, metabolic exchanges occur directly between the individual cells and the surrounding water.

Step 1: Analyze the cellular level of organization in sponges
Because sponges do not possess specialized respiratory tissues or organs (such as gills or lungs), every cell must interact directly with the aqueous environment to carry out physiological processes.

Step 2: Evaluate the mechanism of gas exchange
The water current flowing through the canal system brings dissolved oxygen close to the cells. Since the concentration of oxygen is higher in the incoming water than inside the cells, and the concentration of carbon dioxide is higher inside the cells than in the water, gases move along their concentration gradient. This movement occurs via simple diffusion across the cell membranes over the entire body surface exposed to water.

Step 3: Analyze the incorrect options

• (A) Gills: These are specialized vascularized structures found in more complex aquatic organisms, such as fish, molluscs, and crustaceans.

• (C) Moist cuticle: This respiratory surface is characteristic of annelids, such as earthworms, which use cutaneous respiration.

• (D) Tracheal tubes: This is a network of tubes used for gas exchange in terrestrial insects and some other arthropods.
Thus, simple diffusion over the body surface is the process utilized by sponges.