The body of a centipede is slightly flattened, with only one pair of legs on each segment (joint). The body of a millipede is rounded, and each segment has two pairs of legs.
Centipedes and millipedes have a lot in common. As their names suggest, they both have a lot of legs. (The terms centipede and millipede mean “hundred feet or legs” and “thousand feet or legs,” respectively.) Depending on the type, the number can range from 15 to 200 pairs. The animals in both classes have long, slender bodies with many segments. Only a few people are born with all of their segments and legs, but the majority of people grow new ones as they get older. The last segment splits in half, and the new last segment splits in half again. This process of segmentation continues until the animal has reached its adult number of segments.
Centipedes and millipedes have antennae-like feelers on their heads, similar to insects. Some have simple eyes, while others are completely blind. They have mouths that resemble those of insects.
The muscles and body organs of centipedes and millipedes are protected by hard outer shells. These shells serve as skeletons for the animals. The outer layer of the shell is shed from time to time, allowing the shell to grow in size. Many body segments have air pores that connect to a system of tiny tubes inside the body. Air is carried through these tubes to all parts of the body.
A nerve center in the head of centipedes and millipedes functions as a simple brain. The lower part of the body has a nerve cord that runs along it, with smaller nerve centers in each body segment.
The digestive system is made up of a tube that runs from the mouth to the intestines. Wastes are expelled through a hole near the body’s end.
The circulatory system is straightforward. It is made up of a tube similar to a heart that pumps blood from one end of the body to the other.