CoachThe body plan of a group of organisms is the common arrangement of their parts: the axes of symmetry and the relative positions of the main organs, independently of size, shape or way of life. The vertebrates — fish, amphibians, reptiles, birds, mammals — share one plan: a body symmetric about a vertical plane, with a front end carrying the head and a back end; an internal skeleton whose axis is a column of vertebrae protecting a dorsal nerve cord; the digestive tube running ventrally, mouth in front, and the heart beneath it.
Examples
Example 6.2 (Same plan, different animals)
A trout, a frog, a lizard, a pigeon and a mouse dissected side by side show the plan in every case: the spinal cord in the back, protected by vertebrae; the gut below it with a liver beside the stomach; the heart in front, under the gut; paired limbs (or fins) attached to girdles of bone. The differences — gills or lungs, fins or legs, scales, feathers or fur — are all variations built on the same arrangement.
Example 6.8 (Cytochrome c)
Cytochrome c is a protein of about 104 amino acids used in respiration by every eukaryote. Compared with the human sequence, the chimpanzee’s differs at 0 positions, the mouse’s at 9, the chicken’s at 13, the frog’s at 18, the tuna’s at 21, and yeast’s at 45. Ranked by differences, the species fall into exactly the boxes of the previous figure: mammal, then amniote, then tetrapod, then vertebrate, then eukaryote.
