Final answer:
Monogeneans are ectoparasites that primarily infest fish and can be found in areas such as under the eyelids. They attach to their host to feed and are capable of cross-fertilization, exhibiting a simple life cycle with one specific host.
Step-by-step explanation:
The Monogeneans are a group of ectoparasites that most commonly infest fish. They have a direct life cycle involving only one host and can localize in various areas, including under the eyelids of some fish. Monogeneans attach to their host using hooks or suc_kers and may feed on host tissue, mucus, or skin particles.
These ectoparasites exhibit hermaphroditism, generally with a preference for cross-fertilization over self-fertilization. While the question does not mention a specific animal host, it is known that many species of fish can carry monogeneans, including those that may inhabit the area under the eyelids.
Since the life cycle of monogeneans is relatively simple and involves no intermediate hosts, they are highly adapted to their specific fish hosts. The fish species that are affected by these parasites would vary, and no specific species is identified in relation to the presence of monogeneans under the eyelids in the reference material provided.