Lecture 1: Animal Classification PDF

Lecture 1 INTRODUCTION Systematic Zoology or 'Animal Taxonomy' is the branch of Zoology concerned with the identification, description, nomenclature and classification of different kinds of animals. At present, more than one million species of animals are known. They present an interesting diversity...

Lecture 1 INTRODUCTION Systematic Zoology or 'Animal Taxonomy' is the branch of Zoology concerned with the identification, description, nomenclature and classification of different kinds of animals. At present, more than one million species of animals are known. They present an interesting diversity of structure, function, habits and mode of life. It is evident that some system is required for accurately naming and grouping such a large number of animals in order to facilitate their objective study and also to show the relationship between the various groups. The Greek philosopher Aristotle (384 - 322 B.C.) was among the earliest scientists who attempted classifying living organisms. He suggested that animals could be classified according to the presence or absence of 'red blood" into two groups: Enaima and Anaima, with and without red blood respectively. It was also once suggested that animals may be classified according to the type of environment they live in as terrestrial, aquatic or aerial, or according to the type of food they use into flesh-eaters (carnivorous) and plant-eaters (herbivorous). Gradually, however, emerged the idea of classifying animals according to the morphological similarities between them. This idea was formulated clearly in the 17th century by the English naturalist John Ray (1627-1705) whose system can be considered as the first scientific approach to animal classification. Moreover, he was the first to give a precise definition of the species, the basic unit for classification of living organisms. He defined the species as "an assemblage or grouping of animals which are morphologically similar and which interbreed freely with one another, but they commonly do not inter-breed with other species, and if they do so, they produce infertile (sterile) hybrids". Later came the Swedish naturalist Linnaeus (1707-1778) who laid down the basis of the system of classification we use nowadays. He classified living organisms according to the morphological and anatomical similarities between them. He also devised the system of "Binomial nomenclature" by which each type of organisms is given two names; the first is the name of the genus, and the second is the name of the species. Both are Latin and are written underlined or typed in italics. The name of the genus starts with a capital letter and that of the species with a small letter. The scientific name of man, for example, is Homo sapiens, where Homo denotes the generic name while sapiens represents the specific name. Nowadays, the classification of animals is not only based upon morphological and anatomical characteristics but also on biochemical, genetical, embryological and physiological features. Species having many features in common are placed in the same genus. Similarly, related genera are grouped in a family, and families with certain common characteristics constitute an order, and orders in turn are grouped into classes. From these classes, the higher taxonomic groupings known as phyla are formed. Although the relationships between the different phyla may not be obvious, it is always possible to arrange them according to the degree of complexity of structure and function into larger groups known as subkingdoms or branches. Thus, the Animal Kingdom is classified into three subkingdoms: I. Subkingdom Protozoa: This subkingdom includes a single phylum, Phylum Protozoa, the members of which are referred to as unicellular animals (or acellular) since the body of each is formed of a single protoplasmic mass that performs all life activities. II. Subkingdom Parazoa: This includes a single phylum, phylum Porifera, the members of which are simple multicellular animals commonly known as sponges. The body of the animal is formed of many cells which are not much differentiated and do not form proper tissues. III. Subkingdom Metazoa: This includes the rest of the animal phyla. The body in these animals is composed of many cells which are well differentiated into distinct types and form definite tissues. Division of labour and cell specialization are remarkable in these phyla. The different phyla of the Metazoa show gradations from simple structural organization to complicated organ systems in the body. 1- Coelentrata (Acoelomata) 2-Platyhelminthes (Pseudocoelomata) 3- Nematoda (Coelomata) 4- Annelida Fig. 1. Classification of the animal kingdom 5- Arthropoda 6- Mollusca Subkingdom Protozoa 7-Echinodermata Phylum Protozoa 8- Chordata The phylum includes more than 50,000 species which are found in almost all habitats, wherever moisture is present. General characters 1- The great majority are very small in size (unicellular animals) and can only be seen by the aid of the light microscope. Some of them are only 2 or 3 µm in length. 2- Many species live as solitary individuals, while a few live-in colonies.  3- Some Protozoa live as free living inhabiting fresh, salt water and damp places, others are parasitic as endoparasites. 4- In the majority of cases, there is only one nucleus in the cytoplasm with distinct nucleoli often referred to as the endosomes. The cytoplasm is usually differentiated into outer clear ectoplasm and inner granular endoplasm. The cell is covered by a delicate plasma membrane or by a firm living pellicle. Some forms have non-living external envelopes of gelatinous, cellulose or calcareous substances. 5- They move either by temporary protrusions of their cytoplasm known as pseudopodia. (sing, pseudopodium) by long whip-like filaments known as flagella (sing, flagellum) or by short hair-like filaments known as cilia (sing, cilium) and some have no locomotory organelles. 6- Holozoic nutrition (animal-like) is the most common type of nutrition with in this phylum. In this type, the animal feeds actively on other organisms (bacteria, algae, smaller protozoa, etc.) or on decaying organic matter. Some protozoa having chlorophyll-bearing organelles. Exhibit a plant-like mode of nutrition known as holophytic nutrition. In this type, the organism synthesizes its own complex organic food material in the presence of carbon dioxide, water and presence of sunlight, as plants generally do. Some protozoa were thought to adopt saprozoic nutrition feeding by diffusion through their body surfaces on the simple organic and inorganic substances found in the surrounding medium. 7- Osmoregulatory organelles, in the form of contractile vacuoles are characteristic of the great majority of the protozoa which live in fresh water. 8- Aerobic respiration is the rule in many protozoa. Exchange of respiratory gases occurs by simple diffusion through the body surface. 9- Excretion of catabolic nitrogenous watses (mostly in the form of ammonia) takes place also by diffusion through the body surface. There are no specialized excretory organelles for excretion.  10- All protozoa can reproduce asexually, either by binary fission or by multiple fission. But, in addition, some protozoa can reproduce sexually by forming male and female gametes. 11- Encystment is a common feature of the life cycle of many protozoa where the animal secretes a resistant protective cyst around itself to enable it to survive unfavourable environmental condition; and for dispersal.