Flatworm
The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas. Some planarians exhibit an extraordinary ability to regenerate lost body parts. The body size of planarians along the anterior-posterior axis ranges from 3 to 20 millimeters. The planarian CNS is composed of a brain in the head region and a pair of ventral nerve cords (VNCs) that extend the length of the body (Fig.1A) (Agata et al., 1998; Tazaki et al., 1999). In the first study on the organization of the planarian brain, a homolog of the homeobox-containing gene Orthopedia (Otp) from planaria, Djotp, was isolated. Djotp is specifically expressed in the branch structures of the brain (Umesono et al., 1997). Figure 1: Structure of planarian CNS (A) Ventral view of whole-mount in situ hybridization with a DjPC2 (Dugesia Japonica Pro-hormone Convertase 2) RNA probe as a pan-neural marker. Anterior is up and posterior is down. The white region in the center of the body is a pharynx. (B) A model of the planarian brain. In the first study on the organization of the planarian brain, a homolog of the homeobox-containing gene Orthopedia (Otp) from planaria, Djotp, was isolated. Djotp is specifically expressed in the branch structures of the brain (Umesono et al., 1997). 1 Planaria possess a centralized nervous system which consists of a primitive brain and sensory organs located at the organism's anterior (cephalization), as well as of two ventral nerve cords running along the body from head to tail. 2  Decapitated planarians can completely regenerate a new brain. 1 The digestive system consists of a mouth, pharynx, and a branched intestine. 2  The food, sucked by the muscular mouth, passes the pharynx and gets distributed into the intestinal branches where it is digested and absorbed. Like the hydra, the jellyfish has a nervous system characterized by a series of interconnected nerve cells (a nerve net). The nerve net surrounds the whole body of the jellyfish. Shown to the left is a statocyst. When the animal moves and body is tilted, the statocyst makes contact with the cilium. The nervous system of the starfish is very simple...there is no brain and there are not even any ganglia to coordinate movement. The nervous system is characterized by a nerve ring that surrounds the mouth. A radial nerve branches off of the nerve ring and extends to each arm. A planarian is one of many flatworms of the Turbellaria class. It is also the common name for a member of the genus Planaria within the family Planariidae. Sometimes it also refers to the genus Dugesia. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas. Some planarians exhibit an extraordinary ability to regenerate lost body parts. DjTH protein is co-expressed with DjAADCA in the planarian CNS (Fig.4). DjTH-expressing cells are detected in the brain, the VNCs in the region anterior to the pharynx, and the head peripheral region. Basically, highly dense DjTH-expressing cells are restricted to the anterior portion of the body. In the first study on the organization of the planarian brain, a homolog of the homeobox-containing gene Orthopedia (Otp) from planaria, Djotp, was isolated. Djotp is specifically expressed in the branch structures of the brain (Umesono et al., 1997). Brain regeneration. Planarians are known as not only one of the most primitive animals which have a brain, but also as animals that have high regenerative ability. They can regenerate an entire body, including a brain, within 5 days after amputation from a small piece of the body in which no brain tissues remain. In the first study on the organization of the planarian brain, a homolog of the homeobox-containing gene Orthopedia (Otp) from planaria, Djotp, was isolated. Djotp is specifically expressed in the branch structures of the brain (Umesono et al., 1997). Cephalization is basically having a head. Planaria have concentrated sensory structures on one end of their body, so they have cephalization. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found on plants in humid a … reas. Sensory Structures of a Flatworm. Flatworms live in many environments. Jupiterimages/Photos.com/Getty Images. Flatworms, also called planarians, belong to the phylum Platyhelminthes. Characterized by flat bodies, they have no circulatory or respiratory organs and a very simple digestive tract. This body plan limits them to very small, flat shapes to allow them to absorb oxygen and moisture. However, a flatworm’s body contains the beginnings of more complex structures found in other animals. For example, eye spots resemble the ancestral forms found in more complex animals. Understanding flatworm sensory structures can help you understand the origins of these organs.
https://en.wikipedia.org/wiki/Planarian http://www.scholarpedia.org/article/Planaria_nervous_system http://www.scholarpedia.org/article/Planaria_nervous_system http://www.geochembio.com/biology/organisms/planarian/ https://faculty.washington.edu/chudler/invert.html https://en.wikipedia.org/wiki/Planarian http://www.scholarpedia.org/article/Planaria_nervous_system http://www.scholarpedia.org/article/Planaria_nervous_system http://www.answers.com/Q/Why_is_it_said_that_planaria_have_cephalization http://animals.mom.me/sensory-structures-flatworm-7189.html
what sensory structures do planarians have