The Tapeworm's Cardiovascular system
Tapeworms are amongst a population of Platyhelminthes called cestodes, which are simply parasitic flatworms. Tapeworms have no digestive, respiratory, or circulatory systems, and do not own a heart to produce blood on their own. Tapeworms survive by feeding off of their hosts digestive system, and absorb the nutrients that have been processed by their hosts. Independently these worms could not survive, and so the only function they can properly carry out is to reproduce and lay hundreds of eggs at a time, within its host. This is why after they are removed from their host they appear to be like a flattened tube, but while they suck the nutrients of their host, they seem to be a little bit more inflated. Any nutrients processed by the host through the digestive tract is sucked up by tapeworms in order to survive, but this can leave the host weakened and lead to ailments.
Through the design and shape of their mouth, they latch onto the intestines of the host, and in the middle they have a stinger that stays in their mouth, but when latched on, sticks out and stabs into the intestine. The only way tapeworms can enter different organisms is through feeding off of uncooked meets such as beef or pork.