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Both sexes must infect the human if the cycle is to continue. Once within the alternate host, a snail, a single miracidium may produce as many as 200,000 infectious cercariae. The diagram gives the life cycle of the blood fluke, Schistosoma mansoni. All of these have at least two different stages in their life cycle, each parasitic in a different host - one of which is usually a snail.įigure 19.1.10.5 Lifecycle of Schistosoma.But there is no separate exit so undigested food has to leave by the mouth. It leads to an elaborate gastrovascular cavity. Planarians feed through a mouth on their ventral surface. The bilateral symmetry of planarians is associated with active locomotion by secreting a layer of mucus underneath them and propelling themselves forward with the many cilia on their ventral surface and by swimming and a concentration of sense organs in the head (called cephalization). Planaria share with the other members of the phylum (1) a flat, almost ribbonlike, shape and (2) bilateral symmetry. Turbellaria, free-living forms of which the planarian is a commonly-studied example. This phylum contains some 20,000 species distributed among three classes. Have only the polyp stage.įlatworms (Phylum Platyhelminthes) Figure 19.1.10.4 Flatworm The jelly of the medusa is a much-enlarged mesoglea.
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Scyphozoa Jellyfishes (the medusa stage is dominant).
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The body wall is perforated with pores (hence the name Porifera) through which water containing food particles is filtered. Diploblastic that is, the body wall is made of two layers of cells with a jellylike mesoglea between them. Most are marine although some live in fresh water. Sponges are sessile, spending their lives anchored to a solid surface underwater.