11.2 Flatworms, annelids and mollusks

Flatworms (Phylum Platyhelminthes), annelids (Phylum Annelida) and mollusks (Phylum Mollusca), while extraordinarily diverse, share a number of characteristics, which suggests they are more closely related to each other than other invertebrate phyla. Unlike poriferans and cnidarians, all three of these phyla are bilateral and have 3 tissue layers. This means the presence of a head with a concentration of nervous tissues. All of these phyla are also protostomes, or "mouth first" animals.

Phylum platyhelminthes

Phylum Platyhelminthes, also known as flatworms include both predatory and parasitic species, including important parasites of humans. Flatworms have three tissue layers. The flatworms are acoelomates, so their bodies are solid between the outer surface and the cavity of the digestive system.

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.Digestion is extracellular. 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 nervous system consists of a pair of nerve cords running the length of the body with a concentration of nerves at the anterior end of the worm, where there may also be a concentration of sensory cells. These "eyespots" are the first rudimentary vision organs, but only capable of distinguishing dark and light, as well as recognize movement.

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 hermaphroditic, possessing both sets of sex organs, and fertilization is typically internal. Asexual reproduction is common in some groups in which an entire organism can be regenerated from just a part of itself.