Class Cestoda

Among the different classes of the flatworms, class cestoda, or better known as tapeworms, are distinctly different. There are two subclasses, Cestodaria and Eucestoda (true tapeworms). The adult tapeworm infection is prevalent in countries such as South America, China, India and Southeast Asia.

It is common to find this endoparasitic tapeworms living in the intestines of vertebrates, including human beings. They can even grow to long length (~5meters) in the living hosts. They are considered parasitic as they not only eat on the food supplied by the hosts; they also produce wastes and obstruct the alimentary canal. The common types of tapeworms are beef and pork tapeworms which are proven to be dangerous to human beings. A person infected with tapeworms may show signs/symptoms of nausea, muscle weakness, diarrhea, weight loss, abdominal pain and passing out worms sections in feces.

The body structure of the tapeworm is unique because it is made up of many segments called proglottids (circled red in diagram below). Each proglottid is a reproductive unit that produces gametes.

Attachment

The tapeworms generally do not move around much but they are capable of having muscular undulations of the body. The tapeworm make use of a distinguishing feature, (yet similar functions) the scolex (diagram shown on the right), to attaches itself firmly to the host’ intestinal wall.

The tip of the scolex is equipped with a retractable hook-bearing rostellum which acts like discs with teeth to hold onto the intestinal wall. Therefore, the tapeworm will just hang on to the intestinal wall and absorb food through their skin via diffusion. Hence, tapeworms do not have any digestive tracts and also lack mouths and digestive enzymes, unlike the other classes of platyhelminthes.

Watch the video here [Scolex] and take a look at the scolex.

Reproduction

Cestodes, or more commonly known as tapeworms, are also hermaphroditic. They are able to self-fertilize among themselves but cross-fertilisation is more preferred when mates are available.

Male System

The male part of the tapeworm consists of many testes, single-coiled sperm duct, cirrus and genital atrium. Look at the red circles on the diagram on the right to see the different parts of the male systems.

Female System

For the female parts, it comprises of the ovaries, oviduct, ootype, the uterus (extend from the ootype) and vagina.

Both the male and female reproductive parts will mature as it moves nearer to the tip of the body. New proglottids will be produced to replace the mature proglottids which is surrounded with an expanded uterus (as shown on the diagram) filled with developing embryos. The mature proglottids which can contain thousands of eggs will eventually break free from the tapeworm and be deposited in the feces to continue its lifecycle.

Life Cycle

The life cycle of the Tapeworms consists of 3 stages: Eggs, larva and adults and infect two hosts. They are intestinal parasites meaning they side in the intestines of some meat-eating mammals including humans.

At the first stage of the life cycle, the adult tapeworms will attach them to the intestinal wall of their hosts (eg, cats) using their attachment appendages, scolex and hooks. It will then start to mature and grow many body segments called proglottids.

Each proglottid (body segments), contains eggs that are released into the intestinal tracts and discharged from the body in the host’s faeces. The segments will then opens and release the eggs passing it into the environment.

Sometimes, the tapeworms’ eggs may be eaten by some animals. These animals are then called the intermediate hosts. Inside these intermediate hosts, the eggs will then hatch and form larvae called cysts. These infected intermediate hosts are then eaten by other hosts (eg. humans). The larvae are then transferred into the primary hosts where the larvae will then develop into adult tapeworms. These tapeworms do not have any digestive tract hence they are considered parasites as they absorb nutrients through diffusion from the host’ intestines

Watch the video here to see the lifecycle of the tapeworms Life Cycle of the Tapeworm or you can watch the animation at this link Animation on the Life Cycle of the Tapeworm. Watch the lifecycle of a pork tapeworm which infects pork and human beings. [Pork Tapeworm Lifecycle]