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Physiological processes of flatworms

The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts. Most flatworms, such as the planarian shown in [link] , have a gastrovascular cavity rather than a complete digestive system. In such animals, the “mouth” is also used to expel waste materials from the digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.

There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms.

Most flatworm species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups.

Illustration shows the digestive, nervous and excretory systems in a flat, worm-like planaria. The digestive system starts at the ventral mouth opening in the middle of the animal, and then extends to the head through the middle of the body, and toward the along the sides of the body. Many lateral branches occur along the digestive system. The nervous system has 2 cerebral ganglia at the eyes in the head, and 2 ventral nerve cords with transverse connections along the length of the body to the tail. The excretory system is arranged in 2 long mesh-like structures down each side of the body.
The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus. The excretory system is made up of tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.

Diversity of flatworms

Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda ( [link] ). As discussed above, the relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group, a group that does not have a single common ancestor.

Photo A shows a Bedford’s flatworm from the class Turbellaria. The worm has the appearance of a fringed ribbon, black with pink stripes, swimming above the sand. Photo B shows a Dactylogyrus from the class Monogenea. The worm’s body is a long, thin translucent oval with bulges at one end that give the appearance of a head. Three dark spots appear in the head, and four more dark spots three-quarters appear of the way down the body. Anchors that enable the worm to latch onto gills are located near these spots. Photo C shows a foot- shaped brown worm. Photo D shows a long, thin ribbon-like white worm.
Phylum Platyhelminthes is divided into four classes. (a) Class Turbellaria includes the Bedford’s flatworm ( Pseudobiceros bedfordi ), which is about 8–10 cm in length. (b) The parasitic class Monogenea includes Dactylogyrus spp. Dactylogyrus , commonly called a gill fluke, is about 0.2 mm in length and has two anchors, indicated by arrows, that it uses to latch onto the gills of host fish. (c) The Trematoda class includes Fascioloides magna (right) and Fasciaola hepatica (two specimens of left, also known as the common liver fluke). (d) Class Cestoda includes tapeworms such as this Taenia saginata . T. saginata , which infects both cattle and humans, can reach 4–10 meters in length; the specimen shown here is about 4 meters. (credit a: modification of work by Jan Derk; credit d: modification of work by CDC)

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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