Chapter 30: Comparing Invertebrates Section 1: Evolution of the Invertebrates

Evolution of the Invertebrates The evolutionary relationships between different groups of organisms can be shown in the form of a diagram called a phylogenetic tree Shows our best understanding of which phyla originate from a common ancestor and approximately when evolutionary lines diverged

Evolution of the Invertebrates The base of the tree represents the common ancestor of all the groups shown on the tree Branches that originate close to the bottom of the tree represent groups that evolved long ago Branches that originate near the top of the tree represent groups that evolved relatively recently The tips of the branches represent living groups Some phylogenetic trees show “dead” branches that do not reach the outside of the tree Dead branches represent extinct evolutionary lines There are no living groups from these lines

Evolution of the Invertebrates There are several major branches on a phylogenetic tree Figure 30 – 3 Protostomes, deuterostomes, acoelomates, pseudocoelomates, and coelomates These branches represent basic evolutionary lines in animals with bilateral symmetry

Evolution of the Invertebrates The division of animals into deuterostomes and protostomes is based on events in early development The division of animals into acoelomates, pseudocoelomates, and coelomates is based on the structure of the body cavity

Early Development Protostomes include flatworms, roundworms, annelids, mollusks, arthropods, and the members of most of the minor invertebrate phyla Deuterostomes include echinoderms, several small phyla of strange-looking marine animals we have not discussed, and all members of our own phylum, Chordata To understand the reasons for dividing animals into protostomes and deuterostomes, we must examine the earliest stages in the development of animals

Early Development Soon after an egg has been fertilized, it begins a series of divisions These divisions lead first to a two-cell stage and then to a four-cell stage When the embryo grows from four cells to eight cells, the new cells can be arranged in different ways In spiral cleavage, which occurs in almost all protostomes, the four new cells sit in between the four older cells In radial cleavage, which occurs in almost all deuterostomes, the four new cells sit directly on top of the four older cells

Early Development In both protostomes and deuterostomes, the cells of the embryo continue to divide until they form a hollow ball Then the ball becomes flattened on one side and folds in on itself The layer of cells on the outside of the ball is called the ectoderm The layer of cells that has folded inside the ball is called the endoderm Both the endoderm and ectoderm eventually develop into several different kinds of tissue

Early Development The round central cavity enclosed by the endoderm will become the digestive tract of the developing embryo The opening of this cavity to the outside is called the blastopore It is the blastopore that determines whether an animal is a protostome or a deuterostome If the blastopore becomes the mouth, the animal is a protostome If the blastopore becomes the anus and an opening that appears later becomes the mouth, the animal is a deuterostome

Early Development There is a third cell layer in embryos, called the mesoderm, which is located between the endoderm and the ectoderm Many important tissues, including muscles, develop from the mesoderm

Body Cavities Body cavities are important for several reasons Provide a space in which internal organs can be suspended so that they are not pressed on by muscles and twisted out of shape by body movements Allow room for internal organs to develop and expand Contain fluids that may be involved with internal transport, or the carrying of food, wastes, and other materials from one part of the body to another

Body Cavities Some phyla, such as flatworms, have no body cavity at all Acoelomates Other phyla, such as roundworms, have a body cavity that is partially lined with mesoderm Pseudocoelomates Still other phyla have a true coelom, or body cavity that is completely lined with mesoderm Coelomates More advanced than the other two The complete mesoderm lining makes it possible for the digestive tract to develop specialized regions and organs, allows for the formation of blood vessels, and makes it easier for complex organ systems to develop

Chapter 30: Comparing Invertebrates Section 2: Form and Function in Invertebrates

Form and Function in Invertebrates Each animal phylum represents an experiment in the design of body structures to perform the tasks necessary for survival The appearance of each phylum in the fossil record represents the random evolutionary development of a basic body plan that is different in some way from other body plans Evolution is random and undirected

Form and Function in Invertebrates Organisms are not better or worse than one another – they are simply different The body systems that perform the vital functions of life have taken many different forms in different phyla Some are complex, others are simple Some are efficient, others are not More complicated and efficient systems are not necessarily “better” than simpler systems

Movement Almost all animals use specialized tissues called muscles to move Without muscles, animals could not swim, fly, burrow, or run Muscles work only by contracting When muscles are stimulated, they generate force by getting shorter When they are not stimulated, they relax In most animals, muscles work together with some sort of skeletal system that provides firm support There are three main kinds of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons

Hydrostatic Skeletons Hydrostatic skeletons do not contain hard structures, such as bones or chitin plates, for muscles to pull against Instead, the muscles surround and are supported by a water-filled body cavity When the muscles contract, they push against the water in the body cavity Cnidarians, some flatworms, roundworms, some mollusks, and annelids have hydrostatic skeletons

Exoskeletons Exoskeletons usually refer to the hard nonliving coating that encloses an arthropod’s internal organs and muscles However, the shells of mollusks can also be considered exoskeletons Muscles attached to the inside of an arthropod’s exoskeleton are used to bend and straighten the joints Muscles attached to the shell in mollusks make it possible for snails to withdraw into their shell and for bivalves to close their two-part shell

Endoskeletons Endoskeletons are frameworks located inside the body of animals Sponges, echinoderms, and vertebrates have endoskeletons Animals with endoskeletons typically have muscles that attach to the outside surface of the endoskeleton

Feeding As you move through the invertebrate phyla from simpler animals such as sponges to more complex animals such as arthropods, you can observe three major evolutionary trends First, simpler animals such as sponges, cnidarians, and flatworms break down their food primarily through intracellular digestion More complex animals use extracellular digestion

Feeding In intracellular digestion food is digested, or broken down, inside the cells In extracellular digestion, food is broken down outside the cells – specifically, in a digestive tract Mollusks, annelids, arthropods, echinoderms, and chordates typically rely on extracellular digestion

Feeding Second, cnidarians and some flatworms have a simple digestive system that has a single opening through which food enters and through which solid wastes are expelled More advanced digestive systems, such as those found in roundworms, mollusks, annelids, arthropods, echinoderms, and chordates, have two openings – a mouth at one end and an anus at the other

Feeding Third, the digestive tract tends to acquire more and more specialized regions The digestive system is not the only system to become more specialized as you move from simpler animals to more complex animals This evolutionary trend is seen in most of the other systems responsible for performing essential life functions

Internal Transport All cells of multicellular animals must be supplied with oxygen and nutrients and must dispose of metabolic wastes The smallest and thinnest multicellular animals manage to fulfill their internal transport needs through diffusion between their body surface and the environment Most complex multicellular animals have a collection of pumps and tubes called a circulatory system There are two basic types of circulatory systems: open and closed

Respiration In order to supply oxygen to and remove carbon dioxide from their tissues, animals must exchange these gases with the environment Two features are common to all respiratory systems First, they almost always have structures that maximize the amount of surface area in contact with air or water Second, they have some way of keeping the gas exchange surfaces moist so that diffusion can occur

Respiration Some animals that live in water or in very moist soil, such as cnidarians and flatworms, respire through their skin Aquatic organisms – mollusks, crustaceans, some insects, and many annelids, for example – have gills that help them exchange gases with the water around them Terrestrial invertebrates have evolved several organs for breathing air These include the highly modified mantle cavities of land snails, the book lungs of spiders, and the tracheal tubes of insects

Excretion Multicellular animals, whether they live in water or on land, must control the amount of water in their tissues At the same time, all animals must get rid of toxic nitrogenous wastes produced as a result of cellular metabolism Excretory systems in invertebrates have evolved in ways that enable these animals to both regulate the amount of water in the body and get rid of nitrogenous wastes

Excretion In all animals, the breakdown of amino acids during cellular metabolism produces ammonia Many aquatic animals simply allow ammonia to diffuse through their body tissues and out into the surrounding water, which immediately dilutes it and carries it away

Excretion Terrestrial animals must do two things: conserve body water and get rid of nitrogenous wastes at the same time In order to do this, many invertebrates convert ammonia into urea Urea is soluble in water and is much less toxic than ammonia The waste product produced by the excretory system, which is called urine, is expelled from the body Terrestrial animals can get rid of more wastes in less water than their aquatic counterparts

Excretion Sponges, cnidarians, and roundworms Diffusion through body surfaces Freshwater flatworms Flame cells Insects and some arachnids Malpighian tubules Annelids, mollusks, and chordates Nephridia

Response Nervous systems gather information from the environment, process information, and allow animals to respond to it Invertebrates show three obvious trends in the evolution of the nervous system: centralization, cephalization, and specialization

Response Cnidarians and some flatworms Nerve nets Other flatworms Ganglia Mollusks and arthropods Ganglia are organized into a brain

Reproduction and Development Many simple invertebrates reproduce asexually through fragmentation or budding Asexual reproduction allows animals to produce offspring rapidly from a single individual Sexual reproduction maintains genetic diversity in a population Although sexual reproduction does not create new genes, it does result in new combinations of genes Most of the more complex animals reproduce sexually

Fertilization There are two basic ways in which sperm cells and egg cells are brought together in sexual reproduction: external fertilization and internal fertilization External fertilization is generally associated with less complex animals The eggs are fertilized outside the body Internal fertilization is associated with more complex animals The egg is fertilized inside the female’s body

Parental Care Many invertebrates do not take care of their fertilized eggs or young The eggs are ignored as soon as they are laid Most of the young are eaten or are exposed to adverse environmental conditions and die Some invertebrates take care of their offspring

Parental Care Some of the ways in which invertebrates care for their offspring may seem horrifying to humans For example, the eggs of some species of mites hatch within the female’s body The larvae immediately begin to devour their mother from the inside! Within two days – while still inside their mother’s nearly empty exoskeleton – the young mites mature, mate, and eat their way to the outside The males die within a few hours The females seek out prey in the form of insect eggs and begin to feed – even as their own offspring start chewing on their internal organs