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Chapter 8


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York University
BIOL 2030
German Reyes

CHAPTER 8 – acoelomorpha, platyzoa, mesozoa - Are worms and (most) have bilateral symmetry and are triploblastic animals - Have acoelomate bodies (lack coelom) and have mesoderm - Have only one internal space (digestive cavity) - Parenchyma is the region between the epidermis and the digestive cavity lining. - a pseudocoelomate has an internal cavity (pseudocoelom), surrounding the gut, but the cavity is not lined with mesoderm (unlike coelomate animal) Phylum Acoelomorpha - have a distinct anteroposterior axis - lack “true” brain Clade Platyzoa : are a part of lophotrochozoan protosome phyla. Contains Platyhelminthes, Gastrotricha, and phyla in Gnathifera Phylum Platyhelminthes - there are four classes: Turbellaria (nonparasitic), Trematoda, Monogenea, and Cestoda (all parasitic) - turbellarians have ciliated body ( cellular epidermis with rhabdites) while the parasitic classes have nonciliatd bodies (called syncytial tegument). The parasitic class have a body has a tegument - body acoelomate - have a mouth, pharynx, and intestine (intracellular digestion) - excretory system comprises two lateral canals with braches bearing flame cells - have auricles (earlike lobes), statocysts (balancing organ), rheoreceptors (sensing water current) and ocelli (light sensitive eyespot) Class Turbellaria: - some don’t have a larva stage and some have a ciliated larva stage - locomotion: quickly attach and detach from surfaces (epidermis has dual-gland adhesive organs) and have cilia - eg. Dugesia Class Trematoda - have a well-developed gut tube but the mouth is on the anterior end - Similar reproductive, excretory and nervous system as turbellaria. Sense organs are poorly developed - subclass Digenea: o life cycle: first host (intermediate host) is a mollusc and the final hose (definitive host) is a vertebrate. In the defentive host the parasite reproduces sexually o the larva is called miracidium and when it enters the tissue of the snail it turns into a sporocyst. Sporocyst reproduces asexually and creates more sporocysts and rediae. The radiae reproduce asexually and produce more radiae and carcariae which emerge from the snail and penetrate a second intermediate host (fish) and become metacercariae (juvenile flukes). When the host is eaten by the definitive host (humans) metacercariae become adults. o Eg. Clonochis o In the case of schostosomiasis there is no second intermediate host. So cercaria penetrates human skin and grows into an adult inside (no metacercaria) Class Monogenea - External
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