STRUCTURAL ORGANIZATION AND CLASSIFICATION OF NON-CHORDATES Academic Script. 1 st Year undergraduate UGC Model-1 Syllabus. Animal Diversity-1 (Z-101B)

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1 STRUCTURAL ORGANIZATION AND CLASSIFICATION OF NON-CHORDATES Academic Script Course Name: Paper : Topic No. & Title: Zoology 1 st Year undergraduate UGC Model-1 Syllabus Animal Diversity-1 (Z-101B) Topic-1 Structural Organization & Classification of Non-Chordates

2 Academic Script:- Size of organism Different species of animals show an enormous range in size, from the tiny protozoan weighing a fraction of a milligram to the whale weighing more than 100 tons. Many animals, such as birds and mammals, have definite age limits for growth, but in reptiles and fishes growth may continue throughout life, although at a reduced pace. Mammals vary from forms as small as shrews to those as large as whales. One may surmise that the evolutionary trend has been from the small to the large, but this has not always been the case. Organs and Systems An organ is a group of tissues that performs a certain function. Organs are, in turn, associated in integrated groups to form systems. Each system is concerned with one of the basic functions common to all animals. There are eleven of these organ systems, but only the higher metazoans have all eleven systems. However, all living organisms, whether cellular or multicellular, have the same metabolic requirements and hence must perform the same basic functions. The need for procurement, processing, transportation, and utilization of food products and the need for movement, protection, perception and reproduction are as important to an ameba, or a whale as to man. Obviously, because of differences in size, structure, and environment, each must meet these problems in a different manner. We now discuss the eleven basic body functions or systems. Digestive tract and digestive processes

3 A digestive tract may occur in several forms, each form being a more or less gross indication of taxonomic similarity. The Porifera have no digestive cavity at all. The other group, the enterozoa, does possess them. Some organisms, such as the coelenterate, have only a mouth. Other multicellular organisms have a complete digestive tract. In most animals this takes place through the mouth, though in most Protozoans and some parasitic forms it occurs directly through the body surface. Most Protozoa, Porifera and Coelenterata have intracellular digestion whereas others have either extracellular or both intracellular and extracellular digestion. In most protozoans this process, like ingestion, occurs directly though the cell membrane. In most metazoans egestion occurs through the anus, if there is one, or through the mouth if there is no anus. Respiration Energy is bound up in food products and must be released by oxidation to be available for cellular use. Oxygen is required for this purpose. Respiration is the process of taking in oxygen and giving off carbon dioxide. In some animals this exchange of gases takes place by diffusion through the plasma membrane e.g protozoans or through the cells of the body wall e.g. lower metazoans. In most metazoans, however, there are specialized respiratory organs like gills, lungs, air passages etc. in which the diffusion of gases occurs between the environment and the animal s body fluids or blood. Circulation Nourishment and oxygen must reach all cells, and waste products must be carried away. Transportation of nutrients, oxygen and waste products is as important as digestion or

4 respiration. Transportation varies from the simple streaming of protoplasm within the walls of an ameba, or the movement of body fluids within a small multicellular animal, to the complicated circulatory systems of vertebrates, which include blood, a muscular heart, blood vessels, lymph nodes, and lymphatics. Excretion It is the duty of the excretory organs to get rid of excess water and nitrogenous wastes like urea, uric acid, ammonia, purine bodies, etc. In most protoxoans it occurs directly through the cell membrane by the process known as exocytosis. In most multicellular forms, however, special devices like flame cells, nephridia, green gland, malpighian tubules, coxal gland, etc. are used to remove liquid wastes from the blood or body fluids. In vertebrates there is a complex excretory system of kidneys, ureters, urinary bladder, and urethra. Nervous coordination The coordination of both the physical and the physiological activities of the body is under the control of both the nervous system and the endocrine system. In protozoans some parts of the protoplasm may be more sensitive to stimuli than others, for example, the light-sensitive eyespots of certain flagellates and the neuromotor apparatus that coordinates that beating or cilia. In even the lowest of metazoans, there are specialized nerve cells for transmission of stimuli. In other metazoans these cells are organized into nerves and nerve centers that regulate the transmission of stimuli from special receptor cells to effectors, such as muscles and glands. In still higher forms these nerve cells are organized into a complex central nervous system of brain, ganglia, nerve cord, and peripheral

5 connections. In vertebrates the nervous system comprises two major parts, the cerebrospinal and the autonomic systems. Sensory perception To gain information about the environment, various sensory cells are receptive to certain kinds of outside stimuli light, sound touch, taste, chemical, heat, and cold, to name a few. Protozoans and sponges, which have no specialized sense organs, are nevertheless sensitive to light, touch and certain chemicals. Many invertebrates have developed simple sensory organs, such as light-sensitive eyes, statocysts for balance, tactile hairs, and chemoreceptors. In a few invertebrates and most vertebrates we find not only the simple receptors for touch, taste, and temperature but also highly specialized and complicated organs of sight, smell and hearing. Endocrine regulation Endocrine glands are ductless glands that secrete their products directly into the blood-stream. In arthropods there are hormones that control the darkening or lightening of skin pigmentation and others that control molting and metamorphosis. Much more is known about the endocrine glands in vertebrates. These include the pituitary, adrenal, thyroid, parathyroid and others and they control or integrate such body functions as growth, metabolism, sexual development, reproduction and many others. Integument The integument is the skin and its modification. Most invertebrates are covered with a single layer of epidermis. In some forms the epidermis secretes an outer noncellular cuticle, and in others an exoskeleton. The vertebrate integument

6 consists of the skin and its derivatives, such as hair, scales, feathers, nails, and hoofs. Besides being an outer protective covering, the integument is provided with sensory cells and glands. In some forms it has secretory and respiratory functions, and in worm-blooded animals like us, it helps regulate body heat. Skeletal support An organism s position in a taxonomic sequence may be indicated by its skeleton, or lack of one, and presence or absence of a notochord i.e. primitive vertebral column. Of those organisms possessing skeletons, three major types may be distinguished. Animals with a skeleton on the outside of the body, an exoskeleton, usually represent lower forms. These may be calcareous or siliceous tissues e.g protozoans, hard calcareous shell e.g. mollusks or tough but flexible chitinous covering e.g. insects and other arthropods. Some animals have components of both an internal and an external skeleton, as do some reptiles. Can you think of one? Those organisms with a completely internal skeleton, or endoskeleton, most frequently represent the few invertebrates and all chordate animals. These are composed of bone or cartilage which are live tissues and can grow as the animal grows. Muscular movement Most movement in animals is produced by the shortening of muscles or muscle fibers. Special contractile fibers called myonemes are found in some of the lower forms. In higher forms, muscles are arranged in antagonistic pairs or groups. They contract or shorten when stimulated by motor nerves and thus move the part to which they are attached. Reproduction

7 Reproduction may be either asexual or sexual. Asexual reproduction, found among the lower forms, includes fission and fragmentation or sporulation. Although some invertebrates possess only simple gonads that shed their gametes i.e. egg or ovum and sperm into the surrounding water, many invertebrates have reproductive systems as complex as those of vertebrates. Body cavity and coelom The coelom is the true body cavity. It is the space between the digestive tube and the outer wall; it contains the other visceral organs. Not all animals have coelom, for example, the jellyfish and flatworms. Those with a coelom, the eucoelomata, may be thought of as a tube-within-a-tube arrangement. A true coelom develops between two layers of mesoderm an outer somatic layer and an inner visceral layer and is lined with mesodermal epithelium called the peritoneum. The coelom is of great significance in animal evolution, for it provides spaces for visceral organs, permits greater size and complexity by exposing more cells to surface exchange. It also serves as hydrostatic fluid skeleton. A coelom may appear in various forms. In Rotatoria and Nematoda the body cavity is not lined with mesoderm and so is given the name pseudocoel. The Mollusca, Arthropoda, Onychophora and others have a hemocoel: a great cavity within which the blood sloshes around which is a rather primitive circulatory system.the coelom of the earthworm and of other annelids is divided into chambers by septa. Sometimes the vertebrate coelom also is divided into thoracic and abdominal cavities by the diaphragm. Grades of organization

8 Animals though they show different shapes and size possess either cellular, tissue, organ or organ system of organization. The well developed nucleolus and different lamellar organells are the specific characteristics of kingdom eukaryota. The cellular grade of organization can be observed in all protozoans where all the vital activities of the body are performed by a single cell i.e. unicellular organism, e.g. amoeba. The next higher level of body organization is observed in multicellular animals, which are called metazoans. Based on complexity of organization, metazoans are further subdivided into tow subkingdoms, the Parazoa and Eumetazoa. In Parazoa the cells are loosely aggregated and do not form tissues or organs hence, they have cellular grade organization in multicellular organisms which lacks different organs & organ systems and they have no physiological division of labour e.g. sponges. In Eumetazoa, which includes the rest of the multicelluar animals the cells are organized into structural and functional units called tissues i.e. tissue grade organization; organs and organsystem develop and show division of labour. Eumetazoa is further divided on the basis of number of embryonic cell layer. They are Diploblastic animals with two embryonic cell layers - ectoderm and endoderm e.g. Hydra, where as Triploblastic animals with three embryonic cell layer named ectoderm, endoderm and mesoderm. On the basis of body cavity i.e. coelom the Triploblastic animals are further divided into Acoelomate, Pscudocoelmate and Coelomate. Acoelomate does not have coelom e.g. Tapeworm and Ascaris. Pscudocoelomate have coelom, but it does not result in a splashed mesoderm. On the basis of cleavage and the development of the mouth the Coelomate is further divided into Protostomia and Deuterostomia. In Protostomia animals there is a determinate radial cleavage, mouth develop from the blastopore and mesoderm develop from a single cell of the blastocyst; e.g. Earthworm, Spider, Octopus. All other animals which have specific characteristics like indeterminate radial

9 cleavage and in which the mouth develops on the opposite side of blastopore are Deuterostomia which are further divided on the basis of presence or absence of notochord; an elastic skeletal rod running along the back beneath the nerve cord in the embryo or adults. All the animals in which notochord is absent are known as Non-chordates. The animal in which notochord is present in embryo as well as in adults known as Hemichordates e.g. Balanoglossus, Ascidia. The animal in which notochord is present during the embryonic period and replaced by vertebral column in adult are known as Vertebrata. Now as we have understood all the major criteria and concepts of classification we will see the classification or major phyla upto the class with examples of animals. Note that the Nematoda has lowest no. of classes i.e. only two where as Arthropoda has different subphyla under that maximum no. of classes. Recent Development In Classification System Recent development in the field of classification systems has taken place. The recent classification system proposed by Carl Woese and Co-worker in 1990 has been shown here in which all animals are put under domain Eukaryota. This domain is further divided into two kingdom named Protista and Animalia. Protista has Protozoa as sub-kingdom where as kingdom Animalia has four different sub-kingdom. A total 44 different phylums are described by Dr. Carl Woese. In 1998 Thomas Cavalier-Smith divided Protista in 2 new kingdoms: Chromista the phylogenetic group of golden-brown algae that includes those algae whose chloroplasts contain chlorophylls a and c, as well as various colorless forms that are closely related to them, and Protozoa, the kingdom of protozoans. On the basis of above findings in 2004 T. Cavalier-

10 Smith published paper named Only six kingdoms of life in the proceedings of the Royal Society of London. This is the chart describe the six kingdoms of life and the 34 microbial phyla proposed by Thomas Cavalier-Smith.