Final answer:
The very small, flat worms are called trematodes, a type of flat-bodied worm within the phylum Platyhelminthes, and distinguished from round-bodied worms (nematodes) and segmented flat-bodied worms (cestodes). The correct answer to the question is B) Trematodes.
Step-by-step explanation:
Parasites that are very small, flat worms are called trematodes. These flat-bodied worms belong to one of the classes within the phylum Platyhelminthes, which contains different groups of flatworms. The trematodes are collectively referred to as flukes and are recognized for their non-segmented bodies. Helminths, which include these flatworms, are divided broadly into two groups: the round-bodied nematodes and the flat-bodied worms, which include trematodes and cestodes. Cestodes are segmented flat-bodied worms, whereas nematodes are round-bodied worms such as roundworms and hookworms.
Among these, trematodes are usually associated with specific body sites in their host, where the adult forms reside. For instance, we have the liver flukes that mainly infect the liver of the host. In contrast, cestodes, such as tapeworms, are known to inhabit the digestive system of their hosts. Nematodes, on the other hand, can be found in various parts of the host's body. Understanding these parasites is essential as they can cause serious health consequences, though some may be so well adapted to their human host that they cause little to no apparent disease.
The correct option for very small, flat worms is:
B) Trematodes