Chapter 33 Invertebrates Overview Life Without a Backbone Invertebrates animals without a backbone account for 95 of known animal species and all but one of More than a million extant species of animals are known and at least as many more will probably be the roughly 35 animal phyla that have been described identified by future biologists Invertebrates inhabit nearly all environments on Earth 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 they capture food particles suspended 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 oscula More complex sponges have folded body walls and many contain branched water canals and several 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 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 Most sponges are hermaphrodites each individual produces both sperm and eggs in sequence 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 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 A single opening to this cavity functions as both mouth and anus 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 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 the mouth 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 through both a medusa stage and a polyp stage in their life cycle the food into the gastrovascular chamber for digestion Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push Some cnidarians exist only as polyps others exist only as medusas and still others pass sequentially The tentacles are armed with batteries of cnidocytes unique cells that function in defense and the The tentacles of a jelly dangle from the oral surface 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 Class and Examples Classes of Phylum Cnidaria Main Characters Hydrozoa Scyphozoa Cubozoa Anthozoa Most marine few freshwater both polyp and medusa stages in most species polyp stage often colonial All marine polyp stage reduced free swimming medusa up to 2 m in diameter All marine box shaped medusa complex eyes All marine medusa stage completely absent most sessile many colonial Concept 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 Class and Examples Turbellaria Monogenea Trematoda Cestoda Classes of Phylum Platyhelminthes Main Characters Most marine some freshwater a few terrestrial predators and scavengers body surface ciliated Marine and freshwater parasites most infect external surfaces of fishes life history simple ciliated larva starts infection on host Parasites almost always of vertebrates two suckers attach to host most life cycles include intermediate hosts 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 cnidarians Rotifers are tiny animals 5 m to 2 mm most of which live in freshwater Some live in the sea or in damp soil Rotifers 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 into the mouth Food particles drawn in by the cilia are captured by the jaws trophi in the pharynx and ground up parthenogenesis Other species produce two types of eggs that develop by parthenogenesis Some