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Statistical Methods. Flatworms have a cephalized nervous system that consists of head ganglion, usually attached to longitudinal nerve cords that are interconnected across the body by transverse branches. Excretion and osmoregulation by flatworms is controlled by "flame cells" located in protonephridia these are absent in some forms.
Flatworms lack a respiratory or circulatory system; these functions take place by absorption through the body wall. Nonparasitic forms have a simple, incomplete gut; even this is lacking in many parasitic species. Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle. Others move along slime trails by the beating of epidermal cilia. The development of directional movement is correlated with cephalization.
In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli. Other sense organs found in at least some members of this group not necessarily on the head include chemoreceptors, balance receptors statocysts , and receptors that sense water movement rheoreceptors. Most flatworms can reproduce sexually or asexually. Most are monoecious. When the cilium bends, it causes action potentials to fire in a nerve.
This provides information to move muscles. Flatworms Planaria. The nervous system of the flatworm has an organization different from the invertebrates describe above. It does have a nerve net, but these are connected by long nerve cords. These cords are connected to cerebral ganglia located in the head region. The central nervous system has been described as "ladder-like" because of the nerves connecting the nerve cords.
Flatworms have "auricles" that project from the side of the head. These auricles contain chemoreceptors that are used to find food. Flatworms also have eyespots called "ocelli". The ocelli are sensitive to light and are connected to the cerebral ganglia. Generally, the flatworm avoids light. The nervous system of the earthworm is "segmented" just like the rest of the body. The "brain" is located above the pharynx and is connected to the first ventral ganglion.
The brain is important for movement: if the brain of the earthworm is removed, the earthworm will move continuously. If the first ventral ganglion is removed, the earthworm will stop eating and will not dig. Each segmented ganglion gets sensory information from only a local region of its body and controls muscles only in this local region.
Earthworms have touch, light, vibration and chemical receptors all along the entire body surface. Sea Star "Starfish". The nervous system of the starfish is very simple 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. The picture on the left shows one of 3 nerve nets that extend throughout the body. Starfish have an interesting way of detecting light.
They have "eyespots" at the tip of each arm. The eyespot contains light sensitive pigments that allow the starfish to detect shadows and changes in the brightness of light. The nervous system is characterized by 6 ganglia. Some snails have chemosensors called "osphradia" in the mantle cavity. These osphradia are used to detect chemicals in the air or water. The aplysia has several ganglia that are connected by long nerves.
The cell bodies of some neurons are very large 1 mm in diameter. Neuroscientists like these cells because they are easy to: 1 see 2 record action potentials 3 inject chemicals. Bivalves clams, scallops.
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