Chapter 33: InvertebratesOverview: Life Without a Backbone- Invertebrates—animals without a backbone—account for 95% of known animal species and all but one ofthe roughly 35 animal phyla that have been described. 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 EarthConcept 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: they capture food particlessuspended in the water that passes through their body, which typically resembles a sac perforated with pores.- Water is drawn through the pores into a central cavity, the spongocoel, and flows out through a larger opening, the osculum. More complex sponges have folded body walls, and many contain branched water canals and several oscula.- Unlike eumetazoa, sponges lack true issues, groups of similar cells that act as a functional unit and are isolated form other tissues by membranous layers.- However, the sponge body does contain several different cell types. Flagellated choanocytes, or collar cells, lining the spongocoel (internal water chambers) create a flow ofwater 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.- Most sponges are hermaphrodites, each individual produces both sperm and eggs in sequence.- Sponges produce a variety of antibiotics and other defensive compounds.Concept 33.2 Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes- All animals except sponges belong to the Eumetazoa, the animals with true tissues.- One of the oldest animal groups in this clade is the phylum Cnidaria. ◦ Cnidarians (hydras, jellies, sea anemones, and coral animals) have diversified into a wide range of both sessile and floating forms They exhibit a relatively simple, diploblastic, radial body plan that arose 570 million years ago.- The basic cnidarian body plan is a sac with a central digestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus.- This basic body plan has two variations: the sessile polyp and the floating medusa.- Polyps are cylindrical forms that adhere to the substrate by the aboral end of the body (the end opposite themouth) and extend their tentacles, waiting for prey.- Medusas (also called jellies) are flattened, mouth-down versions of polyps that move by drifting passively and by contracting their bell-shaped bodies. The tentacles of a jelly dangle from the oral surface.- Some cnidarians exist only as polyps, others exist only as medusas, and still others pass sequentially through both a medusa stage and a polyp stage in their life cycle.- Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push the food into the gastrovascular chamber for digestion. The tentacles are armed with batteries of cnidocytes , unique cells that function in defense and the capture of prey. Cnudicytes contain cnidae capsule-like organielles  Cnidae called nematocysts are stinging capsules.- Muscles and nerves exist in their simplest forms in cnidarians.- Cells of the epidermis and gastrodermis have bundles of microfilaments arranged into contractile fibers. True muscle tissue appears first in triploblastic animals. When the animal closes its mouth, the gastrovascular cavity acts as a hydrostatic skeleton against which the contractile cells can work.1- Movements are controlled by a noncentralized nerve net associated with simple sensory receptors that are distributed radially around the body.- The phylum Cnidaria is divided into four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa.Classes of Phylum CnidariaClass and Examples Main CharactersHydrozoa Most marine, few freshwater; both polyp and medusa stages in most species; polyp stage often colonialScyphozoa All marine; polyp stage reduced; free-swimming; medusa up to 2 m in diameterCubozoa All marine; box-shaped medusa; complex eyesAnthozoa All marine; medusa stage completely absent; most sessile; many colonialConcept 33.3: Most animals have bilateral symmetry- The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.- Most bilaterians are also coelomates.- During the Cambrian explosion, most major groups of bilaterians emerged.Classes of Phylum PlatyhelminthesClass and Examples Main CharactersTurbellaria Most marine, some freshwater; a few terrestrial; predators and scavengers; body surface ciliatedMonogenea Marine and freshwater parasites; most infect external surfaces of fishes; life history simple; ciliated larva starts infection on hostTrematoda Parasites, almost always of vertebrates; two suckers attach to host; most life cycles include intermediate hosts. Cestoda Parasites of vertebrates; scolex attaches to host; proglottids produce eggs and break off after fertilization; no head or digestive system; life cycle with one or more intermediate hosts.- A planarian’s head is equipped with a pair of light-sensitive eyespots amd lateral flaps that function mainly to detect specific chemicals. The planarian nervous system is more complex and centralized than the nerve nets of cnidariansRotifers- Rotifers are tiny animals (5 µm to 2 mm), most of which live in freshwater. Some live in the sea or in damp soil.- Rotifers are smaller than many protists but are truly multicellular, with specialized organ systems.- Rotifers have an alimentary canal, a digestive tract with a separate mouth and anus.- Internal organs lie in the pseudocoelom, a body cavity that is not completely lined with mesoderm. The fluid in the pseudocoelom serves as a hydrostatic skeleton. Through the movements of nutrients and wastes dissolved in the coelomic fluid, the pseudocoelom also functions as a circulatory system.- The word rotifer, “wheel-bearer,” refers to the crown of cilia that draws a vortex of water