Phylum Platyhelminthes, Nematoda, Annelida - Zoology PDF

The flatworms are acoelomate organisms that are dorsoventrally compressed. Since flatworms are acoelomates, their bodies are solid between the outer surface and the cavity of the digestive system. They include many free-living and parasitic forms. The free-living forms have primitive sense organs an...

The flatworms are acoelomate organisms that are dorsoventrally compressed. Since flatworms are acoelomates, their bodies are solid between the outer surface and the cavity of the digestive system. They include many free-living and parasitic forms. The free-living forms have primitive sense organs and parasitic forms include important parasites of humans. Flatworms have three embryonic tissue layers (triploblastic) that give rise to surfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system (from endoderm). **Class Cestoda** -- **Tapeworms** -- internal parasites of mainly vertebrates **Class Monogenea** -- ectoparasites of mostly fish **Class Trematoda -- Flukes** - internal parasites of humans, mollusks, and many other groups **Class Turbellaria** This class includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. Turbellaria are hermaphroditic. ***Dugesia* (Planaria)** A common is a freshwater Turbellaria. The head has **eyespots** that are light sensitive and surrounded by **ganglia**. From the ganglia **nerve cords** extend the length of the body. **Auricles** located on the head and are tactile and olfactory sensory lobes. Planaria suck food items into their **mouth** and into the **pharynx** which are then passed into the **gastrovascular cavity**. The excretory system is made up of tubules connected to excretory pores on both sides of the body.  [Phylum Nematoda] Roundworms are bilaterally symmetrical, worm-like organisms that are surrounded by a strong, flexible noncellular layer called a **cuticle**. The cuticle is resistant to digestive enzymes and is only permeable to dissolved gasses, water, and some ions. Roundworms are pseudocoelomates. A simple nervous system consists of a ring of nervous tissue around the pharynx that gives rise to dorsal and ventral nerve cords running the length of the body. Nematodes have colonized nearly every conceivable habitat on earth. Nematodes are unbelievably abundant. Some species are generalists, occurring across wide areas and in many habitats; others are much more specialized. Many nematodes are free living and play critical ecological roles as decomposers and predators on microorganisms. But nematodes also include parasitic species, a number of which affect humans directly or indirectly through their domestic animals. Common roundworm **Example:** *Trichinella* -- causes trichinosis (can get by eating raw or undercooked meat that is infected with the larvae. Hookworms -- live in the small intestine **Example:** *Ascaris* Whipworms -- resides in the cecum Pinworms -- can live in the colon and rectum Filarial worms -- primarily tropical parasites that cause diseases such as elephantiasis and river blindness  [Phylum Annelida] The phylum includes earthworms, polychaetas, and leeches. Annelids are found in marine, freshwater, and terrestrial habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. Annelids are protostomes and are often called "segmented worms" due to their key characteristic of **metamerism**, or true segmentation. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization. All annelids except leeches also have chitinous hair-like structures, called **setae**, projecting from their cuticle. **Class Polychaeta** Polychaetas include such forms as sand worms, tube worms, and clam worms. Most have well developed, paired, paddle-like appendages (**parapodia**). Polychaetas are extremely abundant in some areas. They play essential ecological roles, serving on one hand as predators on small invertebrates, and on the other as food for fish and large invertebrates. **Class Oligochaeta -- Earthworm** Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the **clitellum** as well as few, reduced setae. **Class Hirudinea - Leeches** A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of setae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates.  [Terminology] **[Platyhelminthes]** Eyespots Nerve cords Ganglia Auricles Mouth Pharynx Gastrovascular cavity **[Nematoda]** Cuticle **[Annelida]** Metamerism Setae Parapodia Mouth Head end Tail end Clitellum Anterior sucker Posterior sucker