Which type of circulatory system is present in annelids?
Hint: Annelids are an invertebrate worm phylum. They are the segmented worms, of which there are over 17,000 different species. Earthworms and leeches are well-known species. Annelids can be found in a wide variety of wet environments. Some of these species are parasitic or mutualistic with one another.
Complete answer: Annelids range from earthworms to leeches. They can be found in a variety of environments, including salt water, fresh water, and soil. Annelid bodies are divided into segments. Their circulatory systems differ depending on the organism. Most have a closed circulatory system, which means that blood is contained within vessels throughout the body, whereas others (leeches) have an open circulatory system, which means that blood is not contained within vessels. In an open circulatory system, the blood fluid in the "coelomic cavity" comes into contact with the entire inner organism, exchanging nutrients, particles, and gases. Earthworms can absorb oxygen directly through their skin, but blood is required for nutrient transport. These vessels, like our veins and arteries, transport blood from a series of hearts located near the head. Similar to our single aorta, each heart has a small aortic arch vessel that transports blood to and from the heart. The basic annelid form is made up of several segments. Each segment has the same organs and, in most polychaetes, a pair of parapodia that many species use to move. Many species have septa that separate the segments, but they are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation.
Note: Even though annelids are soft-bodied, their fossils are scarce – mostly jaws and mineralized tubes secreted by some species. Although some late Ediacaran fossils may be annelids, the oldest known fossil that has been identified with certainty dates from the early Cambrian period 518 million years ago.