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Chapter 33 - Chapter 33 Invertebrates Chapter 33...

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Chapter 33 - Invertebrates Chapter 33 Invertebrates Lecture Outline Overview: Life Without a Backbone Invertebrates—animals without a backbone—account for 95% of known animal species and all but one of the roughly 35 animal phyla that have been described. o More than a million extant species of animals are known, and at least as many more will probably be identified by future biologists. Invertebrates inhabit nearly all environments on Earth, from the scalding water of deep- sea hydrothermal vents to the rocky, frozen ground of Antarctica. Concept 33.1 Sponges are sessile and have a porous body and choanocytes Sponges (phylum Porifera) are so sedentary that they were mistaken for plants by the early Greeks. Living in freshwater and marine environments, sponges are suspension feeders. The body of a simple sponge resembles a sac perforated with holes. o Water is drawn through the pores into a central cavity, the spongocoel, and flows out through a larger opening, the osculum. o More complex sponges have folded body walls, and many contain branched water canals and several oscula. Sponges range in height from about a few mm to 2 m and most are marine. o About 100 species live in fresh water. Unlike eumetazoa, sponges lack true issues, groups of similar cells that form a functional unit. The germ layers of sponges are loose federations of cells, which are not really tissues because the cells are relatively unspecialized. o The sponge body does contain different cell types. Sponges collect food particles from water passing through food-trapping equipment. o Flagellated choanocytes, or collar cells, lining the spongocoel (internal water chambers) create a flow of water through the sponge with their flagella and trap food with their collars. o Based on both molecular evidence and the morphology of their choanocytes, sponges evolved from a colonial choanoflagellate ancestor. The body of a sponge consists of two cell layers separated by a gelatinous region, the mesohyl. Wandering though the mesohyl are amoebocytes. o They take up food from water and from choanocytes, digest it, and carry nutrients to other cells.
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o They also secrete tough skeletal fibers within the mesohyl. In some groups of sponges, these fibers are sharp spicules of calcium carbonate or silica. Other sponges produce more flexible fibers from a collagen protein called spongin. ? We use these pliant, honeycombed skeletons as bath sponges. Most sponges are sequential hermaphrodites, with each individual producing both sperm and eggs in sequence. o Gametes arise from choanocytes or amoebocytes. o The eggs are retained, but sperm are carried out the osculum by the water current. o Sperm are drawn into neighboring individuals and fertilize eggs in the mesohyl.
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