Final answer:
The internal organs in coelomates are covered with a mesodermal epithelium called the peritoneum, which functions as the outer covering of abdominal organs and the inner lining of the abdominal cavity.
Step-by-step explanation:
Presence or Absence of a Coelom
In animals with a true coelom (eucoelomates), the internal organs are covered with a mesodermal epithelium called the peritoneum. This includes animals such as annelids, mollusks, arthropods, echinoderms, and chordates. The coelom is a fluid-filled cavity that lies between the digestive system and the body wall, housing organs and containing the circulatory system. The peritoneum functions as the outer covering of the abdominal organs and the inner lining of the abdominal cavity in mammals. Therefore, the correct answer to the question is a. Peritoneum.
Other mesodermal epithelial linings like the mesothelium cover different organs and cavities. For example, the mesothelium includes the pleura, which lines the thoracic cavity and covers the lungs. However, the term mesothelium does not specifically refer to the epithelial lining of the coelomic cavity that encompasses the abdominal organs.