Worms are primarily identified by their lack of limbs, but the specific body shape of worms can vary. The most common body shapes are tubular/cylindrical, string-like and flat. Although worms lack appendages, they may have hooks, fins, bristles or other small protrusions that aid in locomotion and attachment. Most worms have elongated, snake-like bodies, but they lack skeletal systems. The smallest worms are less than 1 millimeter in length, whereas the largest worms on land can be nearly 100 feet long, and the largest sea worms can be up to 180 feet long.
Some worms reproduce asexually by dividing or by budding. However, many worms reproduce sexually. Most worms do not have an individual sex, but instead are hermaphroditic, having both working male and female reproductive organs. Although hermaphroditism is common, worms cannot sexually reproduce without a mate.
Most worms have some form of musculature that allows them to move of their own will, but some worms lack muscles and rely on other animals or forces of nature to move them about.