Final answer:
Mollusks often have an open circulatory system and a coelom with a complete digestive system, and they may have a calcareous shell, while nemertines generally have a closed circulatory system and a specialized organ called a rhynchocoel. Mollusks use gills or their mantle for gas exchange, and nemertines have a more developed excretory system. Neither nemertines nor mollusks have a segmented body plan or an exoskeleton.
Step-by-step explanation:
The anatomical differences between nemertines (ribbon worms) and mollusks are distinct and pertain to several biological systems. Notably, mollusks have a circulatory system that can be either open or closed. Many mollusks, for instance, utilize an open system, except for some like the cephalopods (e.g., squids), which have evolved a closed circulatory system. In contrast, nemertines usually have a simple but closed circulatory system. Additionally, mollusks typically have a gastrovascular cavity that is part of a complete digestive system, whereas nemertines have a more developed alimentary system with a unique structure called a rhynchocoel that houses the proboscis.
Mollusks commonly possess a coelom and organs like nephridia for excretion, and they often utilize gills (ctenidia) for gas exchange in aquatic environments or exchange gases across the lining of the mantle cavity for terrestrial species. On the other hand, nemertines have a specialized excretory system that is more developed than those of flatworms or rotifers. However, one feature that is often incorrectly attributed to nemertines in multiple-choice questions is an exoskeleton; nemertines do not possess an exoskeleton, while many mollusks secrete a calcareous shell for protection.