Worm Anatomy: There are 3 major phyla of worms: Annelida: segmented worms Platyhelminthes: flat worms Nematoda: round worms Interestingly, none of these worms are closely related to each other. Have bilateral symmetry Soft-bodied Invertebrates Common ex: Tapeworms, flukes No respiratory or circulatory organs and requires diffusion (resulting in the flat shape) Distinct brain Specialized digestive system, no anus Complex lifecycles that involve parasitism and infestation of hosts. (see diagram for example) Un-segmented Bilateral Have adapted to every ecological niche Represent 90% of all life on the sea floor. Mostly microscopic Can live in virtually any Moist environment Have Digestive systems Common examples: Hookworm, Pinworms Most are parasitic Contain a complete digestive system No respiratory or circulatory system Simple nervous system- main ventral nerve, smaller dorsal nerve cord Simple sensory organs at both ends Usually sexual. Males are smaller and have bent tails. Some are hermaphroditic. Annelids are some of the most well-recognized worms. ex: leeches and earthwormsearthworms Found in most wet environments Range in size from one millimeter to over 3 meters Hard chitin body Major identifying characteristic is a segmented body. Invertebrates Bilateral symmetry Closed circulatory system Digestive tract includes a gut Has a nerve cord and nerve cells Contain photoreceptors (but not eyes) Asexual reproduction still possible! Using fission, posterior part of worm breaks off to form a new clone worm. Considered the most highly organized animal to be able to perform complete regeneration Sexual reproduction can be hermaphroditic or have distinct sexes. Aquatic segmented worms perform external fertilization. Terrestrial worms are typically hermaphrodites and exchange gametes when laying next to each other.