The Animal's Main Body Axis

Zoologists and lay persons alike would not hesitate in identifying the main body axis in animals such as an earthworm, a leech, a grasshopper, a fish, or a bird. However, difficulties will appear if one is asked to point to the main body axis of a sea star, a sea squirt, or a hydra.

What is an animal's main body axis? It is the longitudinal axis uniting the animal's fore and rear ends. The fore end coincides with the animal's head, where the mouth, brain, eyes, and other important sensory organs are located. The rear end, to the contrary, is reasonably identified with the site of the animal's anus and, perhaps, excretory and genital openings. In turn, the anterior position of both mouth and eyes, and the posterior position of the anus, correspond to the animal's polarity with respect to locomotion. The animal explores the environment with the aid of the anterior sensory organs and is ready to use its anterior mouth to exploit any food items its sensory organs might discover.