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Ch33WordLectureOutline - CHAPTER 33 INVERTEBRATES...

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CHAPTER 33 INVERTEBRATES Introduction More than a million extant species of animals are known, and at least as many more will probably be identified by future biologists. Animals are grouped into about 35 phyla. Animals inhabit nearly all environments on Earth, but most phyla consist mainly of aquatic species. Most live in the seas, where the first animals probably arose. Terrestrial habitats pose special problems for animals. Only the vertebrates and arthropods have great diversity. Our sense of animal diversity is biased in favor of vertebrates, the animals with backbones, which are well represented in terrestrial environments. But vertebrates are just one subphylum within the phylum Chordata, less than 5% of all animal species. Most of the animals inhabiting a tide pool, a coral reef, or the rocks on a stream bottom are invertebrates , the animals without backbones. A. Parazoa 1. Phylum Porifera: Sponges are sessile with porous bodies and choanocytes Based on both molecular evidence and the morphology of their choanocytes, sponges represent the lineage closest to the colonial choanoflagellates. The germ layers of sponges are loose federations of cells, which are not really tissues because the cells are relatively unspecialized. Sponges are sessile animals that lack nerves or muscles.
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However, individual cells can sense and react to changes in the environment. The 9,000 or so species of sponges range in height from about 1 cm to 2 m and most are marine. About 100 species live in fresh water. The body of a simple sponge resembles a sac perforated with holes. Water is drawn through the pores into a central cavity, the spongocoel , and flows out through a larger opening, the osculum . More complex sponges contain branched canals and several oscula. Nearly all sponges are suspension feeders, collecting food particles from water passing through food-trapping equipment. 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. The body of a sponge consists of two cell layers separated by a gelatinous region, the mesohyl . Wandering though the mesohyl are amoebocytes. They take up food from water and from choanocytes, digest it, and carry nutrients to other cells. 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 hermaphrodites , with each individual producing both sperm and eggs.
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