GENERAL HISTOLOGY

Detailed explanation-1: -Echinoids live on the sea floor, an area known as the benthic zone. In some of the deepest waters (>1000m), sea cucumbers and brittle stars make up the vast majority of sea floor biota, sometimes as high as 90%. There are no freshwater or terrestrial (land) echinoderms.

Detailed explanation-2: -Answer and Explanation: The correct answer is (b). Echinoderms do not have gills.

Detailed explanation-3: -Echinoderms are not bilaterally symmetrical but they are thought to be evolved from bilateral symmetry possessing organisms. Echinoderms have radial symmetry. They exhibit bilateral symmetry usually in their larval stage but adult echinoderms have radial symmetry. So, statement (ii) is incorrect.

Detailed explanation-4: -Echinoderms are named for their spiny skin, which is covered by a thin layer of calcium carbonate plates and spines. Adult echinoderms have radial symmetry, but larvae have bilateral symmetry. Echinoderms have a unique water vascular system, with canals that extend along each body part.