All of the class cestoda are endoparasitic tapeworms. This class consists of around 5,000 species and most of these worms need two hosts to live. The adult worms require a vertebrate. Also, there are a number of species of tapeworm that inhabit humans. Tapeworms are like trematodes and monogeneans in that they are all accustomed to the parasitic lifestyle. They are without cilia but have an outer tegument. The scolex is a knob-shaped head which contains hooks and suckers which help to attach onto a host. Tapeworms will grow by making body sections called proglottids which follow right behind the scolex, which would mean that the oldest proglottids are at the rear end. These worms can grow up to 12 meters.
Tapeworms are without mouths, digestive tracts, and sense organs. The way they obtain their nutrients is by absorbing them through their folded teguments. These folds in the tegument allow more surface area for absorbtion. The tapeworm excretory system rids the proglottid of wastes.
In every proglottid there are both male and female reproductive systems. Although self-fertilization can occur, cross-fertilization is typical. The eggs from the tapeworm develop into embryos which have a hard outer shell. The eggs do not hatch until they have been consumed by a suitable intermediate host.
Cestoda are common in people as adults. The larval stages are mostly found in swine, but have also been found in mammals and birds.
Photo courtesy of Univ. of California Museum of Paleontology
The class turbellaria is made up mostly of marine flatworms that are free living. One of the most well known turbellarian species is the planarian. It is not marine, but can be found in ponds and streams. There are twelve orders and more than 4,500 species of turbellaria. The five general groups of turbellaria are primarily based on the differences of how the digestive cavity is formed. The acoels are the most primitive turbellarians and lack a digestive cavity. Acoelomates make up most of the planarians. They are three-layered and have an outside ectoderm layer, a middle mesoderm layer, and an inside endoderm layer.
Planarians are able to sense the intensity and direction of light with two anterior eyespots tha thave photosensitive cells. There are two anterior clusters of nerve cells called ganglia that form a simple brain. The ganglia attain information from sensory cells and send impulses to the rest of the body. Also, planarians seem to have the ability to learn. Some learned responses are able to be stored chemically.
Some planarians can be carnivorous although most are scavengers. Their feeding mechanism is rather simple. It extends to a muscular tube, the pharynx, out of its mouth. The pharynx leads into a cavity with only one opening that branches throughout the entire body. This type of cavity is called a gastrovascular cavity. This cavity is used for digestion and to transport food to the entire body.
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