BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

The arthropods and the vertebrates are by far the most mobile of the multicellular animals. Both groups possess paired locomotory appendages—legs and sometimes wings—and both have evolved a hard, jointed skeleton. In these animals, skeletal muscles are attached by one end to one section of the skeleton and by the other end to a different skeletal section. When the muscle contracts, it causes the skeletal joint between its two points of attachment to bend or straighten, depending on the placement of the muscle at the joint. In many ways, then, the skeletal and muscular systems of arthropods and vertebrates show striking functional similarities, even though these two groups of animals evolved from entirely different ancestral lineages. Of course, the differences between them are equally as significant.

The most obvious difference between the skeletal systems of arthropods and of vertebrates is that the arthropods have an exoskeleton—a hard body covering with all muscles and organs located inside it—whereas vertebrates have an endoskeleton—an internal framework to which muscles attach. Besides functioning as structures against which muscles can pull, both types of skeleton are important in providing shape and structural support for animals, particularly land animals, which do not rely upon the buoyancy of water for support. Exoskeletons, which are composed of noncellular materials secreted by the epidermis, function not only in support but also as a protective armor for all the softer body parts, and as protection against desiccation.

From the standpoint of mechanics, a hollow tube can support more weight than a solid rod of equal weight, so if weight is a consideration, exoskeletons would seem to have an advantage over endoskeletons. The exoskeletal system does, however, limit the possible size of the animal. Since the weight of an animal is a function of its volume, a doubling of an animal’s linear dimensions increases its weight by a factor of 8 (that is, 2 x 2 x 2). To support this added weight, the cylindrical exoskeletons of arthropods must become disproportionately thicker and heavier with increasing length. The immense bulk that would be required in an exoskeleton strong enough to support an insect as large as a human being would pose insurmountable mechanical problems. This is certainly one reason why arthropods, as varied and successful a group as they are, have never even approached the size of most vertebrates. Further, rigid exoskeletons impose difficulties in overall growth, such as the periodic molting and deposition of a new covering that are necessary to permit increases in size.

While not as drastic, endoskeletons also impose limits on size. Endoskeletons, which provide support and protection for an animal’s internal organs, must be strong enough for the disproportionate increase of weight in larger animals. Horses must have thicker bones than antelope, and elephants must have thicker bones than horses. The structural support and muscular mass required for large animals imposes constraints on size. It should come as no surprise that the largest vertebrates, the whales, are aquatic. The buoyancy of the water in which they live provides much of the support their great weight requires.

Endoskeletons have two advantages over exoskeletons: they are made of stronger materials, and they are located inside the attached muscles instead of enclosing them. Without the confining walls of an exoskeleton, the muscles of vertebrates can be large enough to support a body of relatively large volume. But in small animals, exoskeletons and endoskeletons are about equally effective, and in very small ones exoskeletons are probably superior.