Parasites are highly specialised organisms that derive food from their host, at its expense. Examples include the tapeworm (Taenia) as well as lice (Pediculus), both body and hair lice.
Tapeworms reside in the small intestine where they can conveniently tap into the host’s nutrients, and so interfere with the normal absorption of the host’s nutrients into its bloodstream, thereby depriving the host to potentially dangerous levels of malnutrition and other side effects such as anaemia and fatigue.
Lice live on the body, such as pubic lice or on eyelashes, or the scalp in the case of head lice, and feed on the host’s blood. Sensitivity to lice saliva causes itchiness, and some lice can be vectors (carriers) of dangerous infectious agents such as Epidemic typhus and Trench fever.
Tapeworms such as Taenia solium, the pork tapeworm, can be more than a couple meters long, and has a specialised head that helps it attach to the small intestine of the host. It is hermaphroditic as its segments each have full reproductive systems. These segments of its body are called proglottids.
Specimens and slides of the tapeworm reveal these segments, as well as the head called the scolex where the attachment hooks are. Moving away from the head, we see a gradual change in the proglottids, from immature, to mature, and finally gravid (pregnant) proglottids.
Each proglottid can house tens of thousands of eggs upon fertilisation within the proglottid, and these eggs end up in faeces once the proglottid ruptures. They can survive in the environment up to two months.