The Last Common Ancestor of All Bilaterians Is Thought to Have

question 42

Multiple Choice

The last common ancestor of all bilaterians is thought to have had four Hox genes. Most extant cnidarians have two Hox genes, though some have three Hox genes. On the basis of these observations, some have proposed that the ancestral cnidarian's were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that ________.

• A) all radially symmetric animals should be grouped together in one clade
• B) the radial symmetry of extant cnidarians is secondarily derived, rather than being an ancestral trait
• C) Hox genes play little actual role in coding for an animal's "body plan"
• D) cnidaria may someday replace porifera as the basal bilaterians

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Definitions:

Musculocutaneous

Relating to both muscles and the skin, or a nerve in the arm that innervates the front muscles of the upper arm and the skin of the forearm.

Radial

Pertaining to structures spreading out in all directions from a central point or axis.

Ulnar

Pertaining to the ulna, one of the two long bones in the forearm, located on the side opposite the thumb.

Ulnar

Pertaining to the ulna, one of the two long bones in the forearm, located on the side opposite to the thumb.