Which of the following groups is of deuterostomes ?

Question

Accepted Answer

The correct option is (3):
Deuterostomes represent one of the two major clades of the Bilateria (bilaterally symmetrical animals), fundamentally defined by their characteristic embryonic development pattern. The defining feature of this group is the fate of the blastopore: in deuterostomes, the blastopore, which is the first opening in the developing embryo (gastrula), develops into the anus, and the mouth forms secondarily (the literal meaning of 'deuterostome' is "second mouth"). Furthermore, they typically exhibit radial and indeterminate cleavage, meaning the plane of cell division is either parallel or perpendicular to the vertical axis of the embryo, and the fate of early blastomeres is not predetermined, allowing for greater developmental flexibility. This clade includes three major phyla: Echinodermata (starfish, sea urchins), Hemichordata (acorn worms), and Chordata (which includes all vertebrates like fishes, amphibians, reptiles, birds, and mammals). Option 3 correctly identifies all three of these major, exclusively deuterostome phyla.

The incorrect option is (1):
The group Annelida (segmented worms) and Mollusca (snails, clams, octopuses) are both classified as Protostomes, not Deuterostomes. Protostomes are defined by having the blastopore develop into the mouth first ("first mouth") and exhibiting spiral and determinate cleavage. Therefore, while Chordata belongs to the Deuterostomes, the inclusion of the protostome phyla Annelida and Mollusca makes this option an incorrect grouping. The fundamental difference in early embryonic development prevents Annelida and Mollusca from being categorized with Chordata.

The incorrect option is (2):
This option lists Annelida, Arthropoda, and Mollusca, all three of which are classic examples of phyla belonging to the Protostomes clade. Arthropoda (insects, crustaceans, spiders) is, in fact, the largest phylum within the entire animal kingdom, and it shares the defining protostome developmental traits, such as the initial formation of the mouth from the blastopore. Since all members of this selection belong to the Protostomia clade, it cannot be the correct group for Deuterostomes. This option represents a pure Protostome grouping, making it fundamentally incorrect for the question asked.

The incorrect option is (4):
This choice includes Arthropoda and Mollusca, both of which are prominent phyla within the Protostomes, as detailed above. While Echinodermata (like starfish and sea cucumbers) is correctly identified as a Deuterostome phylum, the presence of the two protostome phyla, Arthropoda and Mollusca, invalidates the entire grouping as a collection of *only* deuterostomes. For a group to be correctly identified as "groups of deuterostomes," all phyla listed must share the defining deuterostome characteristics. A mixed selection of phyla from both clades does not satisfy the criteria of the question.