GENERAL HISTOLOGY

Detailed explanation-1: -Echinoderms are bilaterians, meaning that their ancestors were mirror-symmetric. Among the bilaterians, they belong to the deuterostome division, meaning that the blastopore, the first opening to form during embryo development, becomes the anus instead of the mouth.

Detailed explanation-2: -In echinoderms, the primary body cavities form the haemal system, whereas the secondary body cavities develop to constitute the coelom and are thus also termed coelomic cavities.

Detailed explanation-3: -Echinoderms are coelomates that have a deuterostome pattern of embryo development. The same pattern of development occurs in the chordates.

Detailed explanation-4: -The echinoderms and the chordates are deuterostomes, while all other invertebrates are protostomes.

Detailed explanation-5: -In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system.