Chapter 33 An Introduction to Invertebrates 1 Chapter 33 An Introduction to Invertebrates Life Without a Backbone Invertebrates are animals that lack a backbone They account for 95 of known animal species Except for sponges and a few other groups all animals have tissues and are in the clade Eumetazoa Porifera 5 550 Species Animals in this phylum are informally called sponges Cnidaria 10 000 Species Sponges lack true tissue and are sessile animals They live as suspension feeders trapping particles that pass through the internal channel of their body Cnidarians include corals jellies and hydras These animals have a diploblastic radially symmetrical body plan that includes a gastrovascular cavity with a single opening that serves as both mouth and anus Chordata 52 000 Species More than 90 of all known chordate species have backbones and thus are vertebrates Annelida 16 500 Species Phylum Chrodata also includes three groups of invertebrates lancelets tunicates and hagfishes Annelids or segmented worms are distinguished from other worms by their body segmentation Earthworms are the most familiar annelids but the phylum consists primarily of marine and freshwater species Concept 33 1 Sponges are basal animals that lack true tissues Animals in the phylum Porifera are known informally as sponges Sponges are monophyletic Most species of sponges are marine though few live in fresh water Sponges are suspension feeders they capture food particles suspended in the water that passes through their body which in some species resembles a sac of perforated with pores Water is drawn through the pores into a central cavity the spongocoel and then flows out of the sponge through a larger opening called the osculum Sponges are basal animals they represent a lineage that originates near the root of the phylogenetic tree of animals Sponges lack true tissues groups of similar cells that act as a functional unit and are isolated from other tissues by membranous layers The sponge body does contain several different cell types Lining the interior of the spongocoel are flagellated choanocytes or collar cells These cells engulf bacteria and other food particles by phagocytosis By beating flagella the choanocytes create current that draws water in through the pores and out through the osculum The similarity between choanocytes and the cells of choanoflagellates support molecular evidence suggesting that animals evolved from a choanoflagellates like ancestor The body of a sponge consists of two layers of cells separated by a gelatinous region called the mesohyl The cell layers come in contact with water so gas exchange and waste removal can occur by diffusion across the membranes of these cells Amoebocytes are cells that move through the mesohyl They take up food from the surrounding water and from choanocytes digest it and carry nutrients to other cells Amoebocytes make tough skeletal fibers within the mesohyl In some sponges these fibers are sharp spicules made from calcium carbonate or silica Other sponges produce flexible fibers composed of a protein called spongin Amoebocytes are capable of becoming other types of sponge cells This gives the sponge body remarkable flexibility enabling it to adjust its shape in response to changes in its physical environment such as the direction of water currents Most sponges are hermaphrodites meaning that each individual functions as both male and female in sexual reproduction by producing both sperm and eggs Eggs reside in the mesohyl but sperm are carried out of the sponge by the water current Fertilization occurs in the mesohyl where the zygotes develop into flagellated swimming larvae that disperse from the parent sponge Cribostatin is a compound isolated from marine sponges that can kill penicillin resistant strains of the bacterium Streptococcus Concept 33 1 Summary Sponges are basal animals that lack true tissues Metazoa Phylum Porifera sponges Description lack true tissues have choanocytes collar cells flagellated cells that ingest bacteria and tiny food particles Concept 33 2 Cnidarians are an ancient phylum of eumetazoans All animals except sponges and a few other groups belong to the clade Eumetazoa animals with true tissues Most cnidarians still exhibit the relatively simple diploblastic radial body plan that existed in the early members of the group of some 560 million years ago The basic body plan of a cnidarian is a sac with a central digestive compartment the gastrovascular cavity Chapter 33 An Introduction to Invertebrates 2 A single opening to this cavity functions as both mouth and anus Polyps are cylindrical forms that adhere to the substrate by the aboral end of their body the end opposite the mouth and extend their tentacles waiting for prey contractions of its bell shaped body Medusa resembles a flattened mouth down version of the polyp It moves freely in the water by a combination of passive drifting and Some cnidarians exist only as polyps or only as medusa others have both a polyp stage and a medusa stage in their life cycle Cnidarians are carnivores that often use tentacles arranged in a ring around their mouth to capture prey and push the food into their gastrovascular cavity where digestion begins The tentacles are armed with batteries of cnidocytes cells unique to cnidarians that function in defense and prey capture Cnidae are capsule like organelles that are capable of exploding outward Nematocysts are specialized cnidae containing a stinging thread that can penetrate the body wall of the cnidarian s prey Cnidarians have the simplest form of contractile tissues and nerves The gastrovascular cavity acts as a hydrostatic skeleton Cells of the epidermis outer layer and gastrodermis inner layer have bundles of microfilaments arranged into contractile fibers Movements are coordinated by a nerve net Cnidarians have no brain and the non centralized nerve net is associated with sensory structures that are disrupted around the body Most hydrozoans alternate between the polyp and medusa forms Hydras among the few cnidarians found in fresh water are unusual hydrozoans in that they exist only in polyp form The polyp stage a colony of interconnected polyps in the case of Obelia is more conspicuous than the medusa When environmental conditions are favorable a hydra reproduces asexually by budding forming outgrowths that pinch off from the parent and live independently The medusa is the predominant stage in the life cycle of most scyphozoans Most