Final answer:
Annelids, including earthworms and leeches, are eucoelomates since they possess a true coelom, a body cavity fully lined with mesodermal tissue. Unlike acoelomates or pseudocoelomates, annelids have well-developed body systems and segmentation.
Step-by-step explanation:
Animals that possess a true coelom, which is a body cavity completely lined with tissue derived from the mesoderm, are referred to as eucoelomates. Annelids, which include earthworms and leeches, belong to the phylum Annelida and are eucoelomates. The coelom in annelids is fully lined by an epithelial membrane, which also extends to and lines the organs within, providing structure and the ability to move these organs independently.
This categorization contrasts with acoelomates, such as flatworms, and pseudocoelomates, like roundworms, where the body cavity is either not present or not fully lined with mesoderm-derived tissue.
As members of the annelid phylum, these organisms exhibit a well-developed circulatory, digestive, and neuronal system, marked by their body's segmentation. With these characteristics aligning with the definition of a eucoelomate, we can confidently classify annelids as possessing a true coelom, thereby being eucoelomates.