CHAPTER 9. ANIMAL CLASSIFICATION
In the Proterozoic Aeon (1500-580 mya) all the major classifications of the animal kingdom emerged. This chapter discusses the major types of animals.
There are 33 phyla of animals. Animals arose from protoctists, but no one is sure which protoctists (Margulis and Schwartz 1988:173). This divergence began around 1500 million years ago with the development of proto-animals. Actual animals arose about 1100 million years ago, the oldest fossils dating back to 700 million years ago. It may surprise the reader that animals are much older than plants. But it must be understood that botanists do not date the beginning of the plant kingdom until the emergence of proto-plants from the ocean onto the land.
Great progress was made in understanding life itself when the eighteenth century Swedish scientist Linnaeus came up with a system to classify all living things. The basic unit of biology is the species which consists of a genus and a specific epithet. Together they form the scientific name of a species. For instance, humans are Homo sapiens, in which Homo is the genus and sapiens the specific epithet. A number of species comprise a genera. A genera can in turn be placed into families and families into orders. Other higher categories of classifications are classes, phyla, and kingdoms. If subcategories are needed the prefix "sub" is merely added to the classification category, such as subclass and suborder.
The following classification of animals follows the course of animal evolution. There are at least three main variables by which zoologists classify animals (see Table 9.1). The first variable concerns embryo symmetry (asymmetrical, radiating, and/or bilaterial). The most primitive animals, the sponges and parazoa, have asymmetrical organization. Radially symmetrical means that the embryo (and sometimes the developed organism) can be divided any number of ways into equal halves by cuts that pass through an axis running through the oral (mouth containing) end and the aboral end (the end farthest away from the mouth). This includes animals known as Radiata, the hydras, jellyfish and comb jellies. The rest of the animals, which means most of them, are bilaterally symmetrical because their embryos can only be divided into equal halves by a single cut in a specific plane.
The second classification variable is the number and kings of layers making up the body cavity. A body cavity contains the various internal organs of the animal. The simplest organisms have only two layers of tissues, known as diploblastic and consisting of endodern and ectoderm. The ectoderm develops into the integumentary layer and the endoderm develops into the alimentary layer. These give way to more complex forms with three tissue layers, known as triploblastic and consisting of endoderm, mesoderm, and ectoderm. These add on a mesoderm layer and this develops all other body parts.
For a vertebrate animal the ectoderm develops into the skin and nervous system. The mesoderm develops into the backbone, skeleton, and excretory system, among other things. And the endoderm develops into the middle ear, liver, pancreas, and the lining of the bladder.
The third classification variable involves variations in the body cavity. Lower phyla have no true body cavity, or coelom. The round worms have an undeveloped body cavity. They have a pseudocoelom, a primitive body cavity not lined by mesoderm. Instead, the coelom is lined with a sheet of tissue called peritoneum, which forms mesenteries to support and anchor the internal organs. The remaining animals have a true coelom, one lined with mesoderm.
A way to distinguish among the large group of coelomates is to study their embryos. Animal life starts with the zygote (i.e., the fertilized egg). The zygote divides by mitotic cell division, becoming partitioned into an increasing number of smaller cells, known as blastomeres. Despite the increase in the number of blastomeres there is no increase in size in the overall embryo because there is no accompanying period of cellular growth. As the number of blastomeres increases the cells become arranged in the form of a hollow ball called a blastula.
The next stage brings the development of the gastrula. The process in which the gastrula forms is called gastrulation. There are many different methods of gastrulation. In one form of gastrulation (called involution) the outer surface of the blastula begins to inroll directly below the center of a lightly pigmented area known as the gray crescent. From the gray crescent develops a mouth-like opening in the blastula known as the blastopore. This blastopore is important because it forms the basis for distinguishing between two basic divisions within the animal kingdom: the protostomes and deuterostomes (see below). The involuting cells move away from the blastopore, developing around the inner margin of the outer surface of the blastula, thereby forming the walls of an enlarging chamber (the embryonic alimentary cavity).
The body cavity arises in two different ways. In the schizocoelous method the body cavity arises as a split in the mesoderm which is forming in bands near the blastopore. The protostome division of animals includes those phyla with schizocoelous body cavity formation. In these animals the mouth arises at or near the blastopore. In the deuterostome division of the Animal Kingdom are those phyla with enterocoelous body cavity formation. In this method the body cavity arises as pouches which bud off the embryonic alimentary cavity of the gastrula and subsequently fuse. In the deuterostomes the mouth arises away from the blastopore. To summarize, the first opening into the embryonic gastral cavity becomes the adult mouth in the Protostomia, hence the name, which means first mouth. In the Deuterostomia the first opening becomes the anus and the second opening becomes the mouth.
This subkingdom contains two phyla. However, phylum Placozoa only has one species, which looks like a large amoeba. Phylum Porifera contains the sponges. The sponges differ so much from other multicellular animal groups that they are isolated in this special subkingdom, whereas the remainder of multicellular animals are placed in the subkingdom Eumetazoa, or true Metazoa. Sponges are found predominantly in salt water, although there are a few fresh water forms. Sponges lack the tissue and organ formation present in higher organisms, and they depend on the water currents to bring them food and oxygen and to carry away their body wastes.
|Assymetrical, Radial, or Bilateral Symmetry||Layers in Body Cavity||
|2.1||Grade Radiata||radial||two||none||hydras: comb jellies|
|2.2.1||Sub-grade Acoelmata||bilateral||three||acoelomate; lack a body cavity||flat worms|
|2.2.1||Sub-grade Pseudocoeleomata||bilateral||three||a false body cavity||round worms; worms|
|18.104.22.168||Deuterostomia||bilateral||three||true coelom||star-fish;reptiles; mammals|
Subkingdom Metazoa: Grade Radiata
The phylum Cnidaria contains animals such as the hydra and true jellyfish. These animals do not show any sign of the development of a head end. Nor do most have left and right sides for they are basically just a double wall containing the gastrovascular system that both digest and distributes nutritive materials. These animals have a nerve net in and under the body's epidermis of interlacing nerve cells. It is an example of a diffuse nervous system, which is a very primitive system. A nerve impulse starting in one area becomes directed in all directions.
Phylum Ctehnophora (comb jelly) is closely allied to the cnidarians. These marine dwelling animals have an unusual system of locomotion. They have eight rows of combs that are actually short plates of cilia (i.e., short hairs) attached together.
Subkingdom Metazoa; Grade Bilateria; Subgrade Acoelomata
Phylum Platyhelminthes contains the flat worms. In Class Turbellaria (free-living planarians) there is a well-developed front end that constitutes the beginnings of a brain. Extending posteriorly from the brain are nerve cords. Off these come lateral nerve branches that go to all parts of the body. The nervous system is involved in sensory reception and muscle control. Other flatworm classes include Trematoda (parasitic flukes) and Ecstoda (parasitic tapeworms). Other phyla are Nemertina (ribbon worms); Gnathostomulida (marine worms); and Mesozoa (small wormlike organisms).
The central nervous system consists of the brain and the spinal cord. The parts outside the central nervous system constitute the peripheral nervous system. In this system are the cranial and spinal nerves.
Phylum Nematoda contains the round worms. This phylum has the most primitive form of the coelom (i.e, body cavity). A cross section of the familiar intestinal parasite Ascaris shows the internal organs simply lying in an open cavity. Other phyla are: Nematomorpha (horsehair worms); Kinorhyncha (small wormlike animals); Loricifera (tiny animals with spiny heads); Gastrotricha (wormlike animals); Rotifera (small aquatic animals); Entoprocta (small marine animals); and Acanthocephala (spiny-headed worms).
Subgrade Coelomata; Protostomia
Animals in the subgrade Coelomata have true body cavities. At this point in the evolutionary tree there is a great division in the animal world. The Protostomes are those animals in which the first opening in the developing embryo becomes the mouth and a subsequent opening becomes the anus. The Deuterostomes are those animals in which the first opening in the embryo becomes the anus and the second opening the mouth.
Among the Protostomes the phylum Annelida contains the classes of leeches, segmented marine worms, and earthworms. The annelids are segmented, that is, they are divided into a series of nearly identical chambers. Each chamber is like a module containing representatives of each of the organ systems. The advantage of segmentation was that it brought a big improvement in movement. Each chamber moved individually without having to move the entire body. The annelids also have a well-developed nervous system. There is a bilobed brain, double nerve cord, and segmental ganglia that serve as relay stations for coordinating regional activity. There is also a distinct differentiation between afferent (sensory) nerves and efferent (motor) nerves.
Phylum Arthropoda contains the classes Chelicerata (horseshoe crabs, spiders, scorpions); Trilobita (extinct marine forms); Crustacea (crabs, shrimps, lobsters); Insecta (Insects); Diplopoda (millipedes); and Chilopoda (centipedes). During the late Silurian Period (440-395 million years ago) the first land animals were primitive scorpions which fed on centipedes and millipedes. Arthropods have an external, jointed exoskeleton, which is very heavy in proportion to the animal's body size. In addition, there is the problem of exchanging gases with the outer atmosphere. Arthropods developed a tracheal system consisting of a series of air-filled tubes. This system placed an upper limit on the size of insects, because of the difficulty in exchanging gases.
Compared to the annelids, the ganglia of the arthropods are larger and the sense organs more developed. However, their small size limits their brain capacity and they are very reflex-bound.
Phylum Mollusca contains the classes of primitive marine molluscs, chitons, snails, clams, tusk shells, and octopods and squids. Among the molluscs, only the chitons and the fossil-like Neopilina retain any significant sign of segmentation. The molluscs have a series of three pairs of well-defined ganglia. The cephalopods (squid and octopus) have textured nervous centers of great complexity. There are well developed organ systems. There are many other phyla, but they are mostly worms, such as velvet, tongue, spoon, and peanut worms.
Subgrade Coelomata: Deuterostomia
Some of the relatively less commonly known phyla are Hemichordata (small marine animals living in U-shaped burrows); Pogonophora (beard worms); and Chaetognatha (arrow worms). A much better known phylum is that of the Echinodermata. The classes of this phylum are Crinoidea (stalked sea lilies, feather stars); Asteroidea (starfish); Ophiuroidea (brittle stars); Echinoidea (sea urchins, sand dollars); and Holothuroidea (sea cucumbers). Although echinoderms start life with bilateral symmetry, they later become secondarily radially symmetrical. They are interesting as an example of an "experiment" in evolution that led into a blind alley. The reversion to radial symmetry prevented further development of the nervous system. There is no evidence of significant development of the central nervous system.
Also limiting was the development of a stiff, armored exoskeleton and the reliance for locomotion upon an entirely new mechanism, the water vascular system. General locomotion is achieved by hundreds of tube feet, which stick out through the armor and are moved by hydrostatic pressure generated in the water vascular system and by muscles.
Echinoderms and protochordates (because they have very similar larvae) probably had a common ancestral form that gave rise to both of them. The unusual construction of the echinoderms appears to have prevented them from giving rise to other forms.
During the Archaean aeon, some early proto-chordates developed. The chordates eventually developed into the well-known vertebrates. The primary chordate features are: gill slits in the pharyngeal region at the anterior end of the digestive tract; a dorsally situated, hollow nervous system; and a dorsal, internal axial skeletal structure, which in the primitive chordates is called a notochord. The notochord (a semirigid rod) made swimming possible via a wriggling motion. This notchord also exists in vertebrate larve and embryos, but, except for the most primitive chordates, for example the lampreys, the notochord is surrounded or replaced by the backbone during embryonic development.
Development of the Nervous System
Plants and animals have various chemicals that control and regulate their behaviors. However, in addition to chemical controls, most animals also have neural control systems. Through this nervous system, animals can stimulate a specific organ and do so exclusively, without the side effects that often accompany the more general actions of hormones. When discussing the evolution of animals this book will pay special attention to the nervous system because of its importance to understanding the nature of human beings with their larger brain capacity.
The nervous system is composed of nerve cells called neurons. There are more than two hundred varieties of neurons, but they all consist of the same basic parts. The cell body of the neuron has a centrally located nucleus containing DNA. From one end of the cell body extends an axon, which is a single fiber over which motor messages are transmitted, like those which move an arm or a leg. Usually extending from the other end of the cell body are the dendrites, which are shorter than axons. The dendrites are branching nerve fibers with thousands of sprouts that make contact with other dendrites and also with axons and neurons. Most dendrites receive nerve signals, but some do send signals.
Animals obviously need to respond to various stimuli in their environments. To do so, the higher animals developed special kinds of nerve cells (called receptors) that respond to various kinds of stimuli. Some obvious examples of receptors are the traditional five senses of sight, hearing, touch, smell, and taste. In sensory (or afferent) neurons, the part of the cell analogous to the dendrite (input end) is especially adapted to be short-circuited by some specific type of stimulus (for example, light, heat, sound, chemicals, and so on). A stimulus, such as the finger prick of a pin, affects a receptor, which then generates nerve impulses that travel to the central nervous system. The central processing area interprets the stimulus and responds by initiating other nerve impulses that pass through motor (or efferent) neurons in order to excite effectors (such as muscles and glands) to respond, such as pulling back the hand in the case of the pin prick.
Nerve messages themselves are both electrical and chemical. The nerve messages start out as high-speed spiky electrical waves sent by neurons. However, most dendrites and axons do not make direct contact with their targets. They end at a slitlike gap, called a synapse, that separates them from their intended destination. The nerve message has to cross this synapse. In order to proceed, the message has to be "ferried" across by special chemicals called neurotransmitters. Neurotransmitters are stored in extremely small bubbles, called vesicles, at the ends of dendrites and axons. When the electrical wave of a nerve signal arrives at a vesicle, a flow of ions makes the bubble burst, thereby discharging the neurotransmitters into the synapse. The neurotransmitters cross the synapse and lodge in chemical "docks" (also known as receptors) on the surface of the target cell. Neurotransmitters must fit into the receptors perfectly, like a key in a lock. Otherwise the signals will not go through. The sudden descent of neurotransmitters on the target cell then generates fresh electrical waves to send an electrical signal through the neuron to the next synapse.
It is important to understand the basics of the nervous system and the neurotransmitters, because without these we cannot fully understand animal evolution, nor can we understand the psychiatric difficulties caused for human beings through imbalances in neurotransmitter levels.
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