There is a basic body plan common to most of the animals presented in this book. At its most obvious, they all have a head, a body, and four limbs. Most are four-legged and stand on all fours, and are described as having front limbs and rear limbs. The front limb is anatomically equivalent to the arm and hand in humans and primates, and the rear limb to the human lower limb. The animals in this book are surprisingly similar in many ways. The head is connected to the rib cage by the neck vertebrae and the rib cage is connected to the pelvis by the lumbar vertebrae. The two front limbs are connected to the rib cage, and the two rear limbs are connected to the pelvis. These units move in relation to one another, establishing the stance, or pose, of an animal. Animals differ primarily in the shape and relative proportions of these structural units, in the position of the wrist, heel, and toe bones when standing and walking, and by the number of their toes. An animal can be visualized as being constructed of a series of simplified, three-dimensional, somewhat geometric volumes (head, forearm, thigh). Each of these volumes has one dimension that is longer than the others. A line projected through the center of the mass of this volume on its longest dimension is called its axis (plural, axes). For the most part, especially in the limbs, these axes follow the skeleton, so that a line drawn through the long dimension of a bone is on, or close to, the axis of the volume of that region (for example, the position of the radius is close to the axis of the forearm). One of the more confusing regions of the body is the volume of the upper arm. The humerus (upper arm bone) is mostly deeply buried in muscle, and lies toward the front of this muscle mass, with the massive triceps muscle located at its rear.
The basic body plan of arthropods: insights from evolutionary morphology and developmental biology