Question

Ctenidia are the respiratory organs of the following:
a. Arthropoda
b. Annelida
c. Mollusca
d. Porifera

Answer 1

Ctenidia is a comb-like construction, especially it is a respiratory organ or gill. It consists of an alliance with a row of bulging filaments. This part of the body normally hangs into the mantle cavity and raises the area accessibility for gas exchange. The word Ctenidia is from Latin however, it is derived from Greek. The Greek word ktenidion which means little comb" gave rise to the term Ctenidia.

Complete answer:
We can say that the Ctenidium is a respiratory organ of Mollusca. This is a comb-like structure present in the mantle cavity of the Mollusca.

Based on the topography, it is divided into five types and they are holobranchiate, merobranchiate, plicate, monopectinate, and bipectinate.

Even though Ctenidia helps in respiration, the organisms which present under the phylum of Mollusca have other specific organs like the mantle cavity and pulmonary sac for sake of respiration.

We can say that the function of Ctenidia is to increase the space inside the body and help in respiration. However, the phylum of Arthropoda, Annelida, and Porifera have specific body parts that help in respiration. Thus, they have no need of this organ in them.

Hence, the correct answer is option (C).

Note: Ctenidium is a respiratory organ or gill which is found in many molluscs. It is found in animals like bivalves, cephalopods, Polyplacophorans (chitons). The aquatic gastropods such as freshwater snails and marine snails also have them. Some aquatic gastropods have only one ctenidium which is known as monopectinate and others have a pair of ctenidia which are known as bipectinate.