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Characteristics, distribution and ecology
Taxonomische classification
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(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classifications. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers).

In the two-kingdom (animal and plant) classification—older and not used on this DVD-ROM—animals composed of many cells (multicellular) were referred to as Metazoa to distinguish them from Protozoa (one-celled animals). In our system, there are no one-celled animals; traditional protozoans are placed in the Protoctista kingdom. We define animals as heterotrophic, diploid, multicellular organisms that usually (except sponges) develop from a blastula. The blastula, a multicellular embryo that develops from the diploid zygote produced by fertilization of a large haploid egg by a smaller haploid sperm, is unique to animals.
Because animal gametes—the egg and sperm—differ in size, they are called anisogametes. The diploid zygote produced by fertilization divides by mitotic cell divisions, resulting in a solid ball of cells that usually hollows out to become a blastula. In many animals, the blastula develops an opening called the blastopore, which is the opening to the developing digestive tract and will be the site of the mouth in animals belonging to some phyla or the anus in animals belonging to some of the other phyla. Animals in some phyla show neither of these two patterns; rather, some animals with spiral cleavage produce a blastula (stereoblastula) that is a solid ball of cells—their affinities remain unclear until more is known of their biology. Cephalopod molluscs (Phylum Mollusca), which have much yolk, lack blastocoels (embryonic cavities). Cell differentiation and cell migrations transform the blastula into a gastrula, an embryo with a dead-end indentation that is the embryonic digestive tract in most animals.
The details of further embryonic development differ widely from phylum to phylum. Nevertheless, common developmental patterns provide clues to relationships between the phyla. In many phyla, developmental details are known for very few species so far; in some phyla, for no species. Because development is intricate and complex, we cannot summarize it in a few words. For similar reasons, concise, accurate definitions of the phyla cannot always be given. Our descriptions are more informal.
Multicellularity is not unique to animals; multicellular organisms abound in all the kingdoms. Examples include most Cyanobacteria (Phylum Cyanobacteria) and Actinobacteria (Phylum Actinobacteria) in Kingdom Bacteria; Phaeophyta (Phylum Phaeophyta), Oomycota (Phylum Oomycota) and Rhodophyta (Phylum Rhodophyta) in Kingdom Protoctista; most members of Kingdom Fungi; and all members of Kingdom Plantae. However, multicellularity is most diverse in the animals; that is, many cells having highly specialized functions are grouped into tissues and tissues into organs. Complex junctions link cells into tissues in most phyla; two types of junctions unique to animals are desmosomes and gap junctions, which regulate communication and flow of materials between cells. Cell-to-cell connections can be seen with an electron microscope.
Most animals ingest nutrients. Many animals take food into their bodies through an oral opening and then either engulf solid particles into digestive cells by phagocytosis (“cell eating”) or, for liquid droplets, pinocytosis (“cell drinking”) or absorb food molecules through cell membranes. Parasites, such as the orthonectids (Phylum Orthonectida) and gordian worms (Phylum Nematomorpha), often lack digestive systems. Solar-powered animals, such as Convoluta paradoxa (a platyhelminth, Phylum Platyhelminthes) and Elysia (a mollusc, Phylum Mollusca) acquire photosynthesizing symbionts, just as did the protoctists that became plants (Phylum Chlorophyta).
Animals that inhabit deep-sea black smokers (hydrothermal vents) and cold seeps (cold water rising through the sea floor) do not directly depend on sunlight for energy. Rather, the energy that powers their symbioses comes from inorganic compounds such as sulfides and methane that seep up through vents in the sea floor. Tube worms, clams, and other vent and cold-seep animals are nourished by symbioses with internal chemolithoautotrophic bacteria. A chemolithoautotroph is a self-feeding bacterium that uses energy released by inorganic chemical oxidations as the source of energy for its life processes, including synthesis of organic molecules from CO2. Vent and seep animals either digest the bacteria directly or absorb organic molecules synthesized by their symbiotic partners. These vent and seep communities are rare today but were likely typical of Earth’s environment 3 billion years ago.
Animals exhibit behavior of various kinds, such as attraction to light, avoidance of noxious chemicals, and sensing of dissolved gases and temperature. Such behavior is found in members of all five kingdoms, but animals have most elaborated this theme. Early in the history of the animal kingdom, more than a half billion years ago, nervous systems including brains evolved in several lineages. Organisms in no other kingdom have nervous systems or brains.
In form, the animals are the most diverse of all organisms. The tiniest animals are termed microbes. Smaller than many protoctists, these animals require a microscope to be seen. Many of these minute animal species make up the heterotrophic fraction of the plankton (Greek planktos, wandering); planktonic animals—together with photosynthesizing planktonic species—constitute the base of freshwater and marine food webs.
The largest animals today are whales, sea mammals in our own class (Mammalia) and phylum (Craniata, Phylum Craniata). The members of most animal phyla inhabit shallow waters. Truly land dwelling forms are found in only four phyla: chelicerates such as spiders (Phylum Chelicerata), mandibulates (uniramians) such as insects (Phylum Mandibulata (Uniramia)), crustaceans such as sowbugs (Phylum Crustacea), and craniates such as reptiles, birds, and mammals (Phylum Craniata). Species that live on land in the soil (for example, earthworms) belong to several phyla, but, requiring constant moisture, they have not freed themselves from an aqueous environment. In fact, animals of most phyla are aquatic worms of one kind or another, except insects and others of Phylum Mandibulata. Probably more than 99.9 percent of all the species of animals that have ever lived are extinct and are studied in paleontology rather than zoology.
Of all organisms, only the animals have succeeded in actively invading the atmosphere. Representatives of all five kingdoms (for example, spores of bacteria, fungi, and plants) spend significant fractions of their life cycles airborne in the atmosphere, but none in any kingdom spends its entire life history in the air. Active flight evolved only in animals. Locomotion of animals through the air independently evolved several times but in only two phyla: Mandibulata, class Insecta, and Craniata, classes Aves (birds), Mammalia (bats), and Reptilia (several extinct flying dinosaurs).
For many years and even now, some biologists assign animals to one of two large groups: the invertebrates—animals without backbones, and the vertebrates—backboned animals. All animals, except members of our own phylum, Craniata, are invertebrates. Today, about 98 percent of all living animals are invertebrates. This invertebrate-vertebrate dichotomy amply accounts for our skewed perspective. Our pets, beasts of burden, and sources of food, leather, and bone—that is, terrestrial animals closest to our size and most familiar—are members of our own phylum. From a less human-centered point of view, traits other than lack of a backbone are better indicators of early evolutionary divergence. We prefer to describe these mostly marine animals by their unique traits rather than to collectively dismiss them as invertebrates.
The animal phyla are described here in approximate order of increasing morphological complexity. Two phyla of animals, Placozoa (Phylum Placozoa) with its single genus and Porifera, the better-known sponges (Phylum Porifera), constitute Subkingdom Parazoa (“alongside animals”); members of these phyla lack tissues organized into organs and most have an indeterminate form. The Rhombozoa and Orthonectida (Phyla Rhombozoa and Orthonectida) are animals whose evolution seems to have been independent of the true metazoa; rhombozoans and orthonectids do not fit the criteria of either Parazoa or Eumetazoa, the other animal subkingdom. The other 33 phyla constitute Subkingdom Eumetazoa (true metazoans); most have tissues organized into organs and organ systems.
Broad overviews of organ systems that carry out circulation, respiration, digestion, support, and reproduction will be discussed in each phylum essay. Certain organisms have open circulation with blood circulating at least partially in body spaces rather than in veins, arteries, and capillaries. In other organisms, blood is confined to arteries, capillaries, and veins in what are referred to as closed circulatory systems. A circulatory system transports dissolved gases—oxygen and carbon dioxide—whereas an excretory system functions to rid an organism of toxic wastes, such as nitrogenous wastes and salts. In regard to reproductive systems, some species are monoecious (one house, hermaphroditic), having both sexes within one individual organism; other species are dioecious (two houses), with separate male and female organisms. Monoecious organisms may be either simultaneous hermaphrodites or sequential hermaphrodites—first male and then female or first female, then male.
The Eumetazoa comprises two branches: radially symmetrical and bilaterally symmetrical animals. The Radiata—radially symmetrical organisms—are Cnidaria (Phylum Cnidaria) and Ctenophora (comb jellies), biradially symmetrical organisms (Phylum Ctenophora). Many species in these phyla are planktonic. Comb jellies and cnidarians encounter a uniform aquatic environment on all sides; their bodies are radially symmetrical both internally and externally. All other eumetazoan phyla have bilateral symmetry, at least at some time in their life cycles. For example, echinoderms often are radially symmetrical as adults, but all echinoderms are bilaterally symmetrical as larvae.
Characteristics of the body cavity including its embryonic origin allow most of the bilaterally symmetrical phyla to be assigned to one of three groups, but about eight phyla cannot yet be assigned because too little is known about the nature of the origin of their body cavities. Those that lack a body cavity between gut and outer body wall musculature are Acoelomata (Phyla Platyhelminthes through Nemertina), although neither Phylum Rhombozoa nor Phylum Orthonectida has outer body wall musculature; those that have a body cavity called a pseudocoelom—not a coelom—are Pseudocoelomata (Phyla Nematoda through Kinorhyncha); and those that develop a true coelom are Coelomata (Phyla Chelicerata through Craniata). What is the difference between a pseudocoelom and a coelom? Gastrulation leads to the development of two, and eventually three, tissue layers in all animals more complex than the placozoans (Phylum Placozoa), sponges (Phylum Porifera), cnidarians (Phylum Cnidaria), ctenophores (Phylum Ctenophora), rhombozoans (Phylum Rhombozoa), and orthonectids (Phylum Orthonectida). The three tissue layers are called the endoderm, mesoderm, and ectoderm (in order from the inside out) and are the masses of cells from which the organ systems of animals develop. In general, the intestine and other digestive organs develop from endoderm; the muscle, skeletal, and all other internal organ systems except the nervous system develop from mesoderm; and the nervous tissue and outer integument develop from the ectoderm. In the coelomates, the embryonic mesoderm opens to eventually form an internal body cavity lying between the digestive tract and outer body wall musculature. This body cavity is called the coelom. A pseudocoelom is also an internal body cavity lying between the outer body wall musculature and gut. Unlike the coelom, though, it is lined by loose cell masses rather than with mesoderm, and the pseudocoelom generally forms by persistence of the blastocoel, the embryonic cavity of the blastula.
Problematic phyla that do not easily fit these categories include nemertines (Phylum Nemertina). If one accepts that the nemertine proboscis cavity and blood vessels are homologues of the coelom, then Phylum Nemertina may be assigned coelomate status and located beside sipunculans (Phylum Sipuncula) on the phylogenetic tree.
For animals of eight phyla whose members are bilaterally symmetrical—particularly for kinorhynchs (Phylum Kinorhyncha) and loriciferans (Phylum Loricifera) and a newly proposed Cycliophora phylum—the nature of the body cavity is uncertain. Embryological studies are needed to determine the origin of the body cavity for members of these phyla before they can be placed in any of these groups. For priapulids (Phylum Priapulida), gastrotrichs (Phylum Gastrotricha), entoprocts (Phylum Entoprocta), ectoprocts, or bryozoans (Phylum Bryozoa), brachiopods (Phylum Brachiopoda), and phoronids (Phylum Phoronida), relationships to other phyla in the animal phylogeny are uncertain; some of the uncertainties are discussed in relevant essays. In establishing classification schemes, when we cannot place a species with any previously established phylum—Loricifera, for example—we create a new place for it. In creating a new phylum, we set that phylum forth as a hypothesis to be tested by studying relationships. Investigation of the origin of the body cavity is one mode of studying relationships between organisms. Affinities of the new Cycliophora (Symbion) phylum to existing phyla Entoprocta and Ectoprocta warrant testing.
Two groups of coelomate animals are distinguished according to the fate of the blastopore—the site of invagination of the blastula. In protostome (“first mouth”) animals (Phyla Chelicerata through Onychophora), the blastopore is the site of the mouth of the adult. In deuterostomes (Phyla Chaetognatha through Craniata), the blastopore becomes the anus—the rear end of the intestine; the mouth forms as a secondary opening at the end opposite the anus. The deuterostome phyla are thought to have common ancestors more recent than their protostome ancestors. This protostome-deuterostome divergence occurred at least 520 million years ago, as judged from the presence of both protostomes and deuterostomes in the Lower (early) Cambrian fauna. The relationship of lophophorates, for example, to either deuterostome or protostome coelomates is not established.
Most biologists agree that animals evolved from ancestral protoctists. Which protoctists, when, and in what sort of environments are questions that are still debated. Earl Hanson (Wesleyan University) amassed much information on the protoctist–animal connection and suggested that the question remains open. Patricia Wainright of Rutgers University (and colleagues) did the same for the fungus–animal connection. The Porifera (Phylum Porifera) evolved from the choanomonads (Class Choanomonada in Phylum Zoomastigota, including the zooflagellates), deduced from both molecular systematics (ribosomal nucleotide sequences) and details of fine structure of the cells. It is likely that the animal phyla other than poriferans, especially the eumetazoans, had different ancestors among the protoctists.

Kingdom Animalia