Acoelomates are animals that lack a coelom, which is a fluid-filled body cavity lined with mesoderm. The three main groups of acoelomates are flatworms, ribbon worms, and rotifers. Flatworms are the most common type of acoelomate, and they include tapeworms, flukes, and planarians. Ribbon worms are long, thin worms that live in the ocean. Rotifers are microscopic animals that live in both fresh and salt water.
All acoelomates have a simple body plan. They have a digestive system that consists of a mouth and an anus, and they have a nervous system that consists of a simple network of nerves. Acoelomates also have a reproductive system, but it is usually very simple.
One of the most distinctive features of acoelomates is their lack of a coelom. This means that their body cavity is not lined with mesoderm. Instead, the mesoderm is located between the ectoderm and the endoderm. This arrangement is thought to be primitive, and it is believed that the coelom evolved later in evolutionary history.
Acoelomates are an important group of animals because they represent the most primitive animals. They are also very diverse, and they play a variety of roles in the ecosystem. For example, some acoelomates are parasites, while others are free-living predators.