In our discussion of the bilateria, we begin with those organisms that do not have a body cavity (the acoelomates). These organisms do not have a fluid-filled internal body cavity, but instead, have a relatively solid body mass. Members of the Platyhelminthes include the carnivorous flatworms (e.g., planarians). Cephalization occurs in the form of eyespots and paired ganglia, as well as an actual nervous system. Many planarians are capable of regenerating a complete body from a very small fragment if they are cut into pieces.
Tapeworms are parasites that consist of a scolex (head), which has hooks for attaching to their host and suckers for extracting food. This Taenia serialis was recovered from a dog; you can see the hooks in the center of its head, surrounded by four suckers.
The flukes are also parasitic. Some flukes exhibit very complex life cycles. Examples include the various species of blood flukes in the family Schistosoma, and the liver fluke depicted C. sinensis.
The life cycle ofSchistosoma mansoni, a fluke in the Phylum Platyhelminthes, is complicated, involving multiple symbioses. This figure shows a copulating pair of male and female Schistosoma. Sexual reproduction occurs inside a vertebrate host (e.g., a human).
The life cycle of Schistosoma mansoni is complex. Fertilized eggs are eliminated in the feces of the first host. The larvae that emerge parasitize a second host, a snail.
The flukes reproduce asexually within the snails, and their second-stage larvae emerge to infect yet another vertebrate host. The larva exhibit chemotaxis, attracted to vertebrates by the chemicals on their skin surface. After attaching to the skin, the larvae break down skin protein and enter the bloodstream through a venule (located between a capillary and its vein). Once in the bloodstream, the parasite migrates to the lungs, and then the liver. The entire process takes about ten days.
People who suffer from schistosomiasis exhibit various symptoms, including a distended abdomen. Other symptoms include pain, extreme diarrhea, and developmental impairment in children. Though the disease has a low mortality rate, its morbidity is such that schistosomiasis ranks as the second most socioeconomically-devastating disease in the world, after malaria.
People who work in or around, freshwater habitats of snails contaminated with human feces are at risk for harboring Schistosoma and contracting schistosomiasis. The infection can be treated with an annual dose of praziquantel at a cost of about a dollar per person.