The correct answer is: A. They have an open circulatory system.
Annelids are a group of segmented worms that have a closed circulatory system. This means that their blood is contained within blood vessels, which transport it throughout the body. The blood is pumped by a heart, which is located in the anterior part of the body. The blood carries oxygen and nutrients to the tissues, and removes waste products.
Annelids also have a nerve net, which is a network of nerves that controls their movement and other bodily functions. The nerve net is not as complex as the nervous system of vertebrates, but it is still able to coordinate the activities of the different parts of the body.
Annelids perform gas exchange across their skin. This means that they take in oxygen and release carbon dioxide through their skin. The skin of annelids is thin and moist, which allows for the exchange of gases.
Annelids do not have pseudocoeloms. A pseudocoelom is a fluid-filled cavity that lies between the digestive tract and the body wall. Annelids have a true coelom, which is a fluid-filled cavity that is lined with mesoderm. The coelom provides support and protection for the internal organs.
In conclusion, the correct answer is: A. They have an open circulatory system.