Carina: Keeled Sternum for Bird Flight Muscles

Carina refers to the prominent keel on a bird’s sternum that anchors powerful flight muscles. The correct answer is 3. A keeled sternum for insertion of flight muscles.

Correct Answer

In avian anatomy, the carina forms a prominent median ridge on the sternum (breastbone), providing large surface area for pectoralis muscle insertion essential for downstroke during flight. Flightless birds like ostriches retain reduced carinae; bats show analogous structures.

Option Explanations

1. A free-swimming, ciliated larva (Incorrect)

This describes trochophore larvae of annelids/molluscs or miracidium of trematodes, not carina; ciliated for locomotion in aquatic phases.​

2. A shield-like plate over dorsal part of body (Incorrect)

Carapace (turtle shell) or scute describes dorsal shields; carina specifically denotes ventral keel structures in birds/insects.​

3. A keeled sternum for insertion of flight muscles (Correct)

Bird sternum features deep median carina (keel) maximizing muscle attachment for pectoralis major/minor, enabling sustained flight.

4. A glue-secreting cell (Incorrect)

Mucus or adhesive cells occur in cnidarians (nematocysts) or insect colleterial glands; unrelated to carina terminology.​