Living organisms -Classification and details discussion
Classification of Living organisms : For the students of USA UK Canada Australia India and other countries
The urge to classify things is a fundamental char- acteristic of man. Therefore, when the number of plants and animals known to mankind increased enormously, it was felt necessary to classify them. Also, these plants and animals were known by dif- ferent names at different places which made com- munication about them difficult. Thus there was also a need to devise a system of naming them (no- menclature) so that they could be recognised by a single name throughout the world. Although at- tempts had been made earlier, these needs were met by the work of the famous Swedish naturalist, Carolus Linnaeus, who described his system of classification in a book called Systema Naturae and also put forth the concept of binomial nomen- clature in 1758.
Binomial Nomenclature:
According to this system of nomenclature being universally followed since then, the name of every living organism consists of two words. The first word refers to the generic name (name of the ge- nus) which it shares with related forms and the sec- ond word is a specific epithet. The two words to- gether constitute the name of the species. The names were originally given in Latin. The branch of biology dealing with these aspects i.e. identifi- cation (of known organisms), nomenclature ( giving names to hitherto unknown) and classification is called taxonomy.
The system of classification proposed by Linnaeus was, however, artificial as it was based on only a few characters. Attempts were therefore made, and are still continuing, to devise more natu- ral and phylogenetic systems.
Classification:
The classical system of classification divides liv- ing organisms into two-the plant kingdom (Plantae) and the animal kingdom (Animalia). In this system, bacteria, actinomycetes, and fungi have always been included in the plant kingdom since they have a cell wall although they are other- wise quite different. Taxonomists in the last few decades have felt that this classification is artifi- cial and quite unsatisfactory. Therefore, there have been attempts during this period to devise systems on the basis of phylogenetic relationships. The sys- tem currently getting maximum support is that of R.H. Whittaker (1969) which divides organisms into five kingdoms.
(i) Monera: Organisms with prokaryotic cells and diverse nutritional habits including photosynthesis, chemosynthesis, and absorption. Reproduction is primarily asexual by simple cell division (fission) or budding, sexual recombination known in some species. Motile by simple flagella or gliding, or nonmotile.
The kingdom includes true-bacteria (Eubacteriales), fungus-like bacteria (Actinomycetales), and the blue-green algae (Cyanobacteriae). Whereas the cyanobacteria are photosynthetic because of the presence of chlorophyll and other pig- ments, the others are chemosynthesizers (use chemical energy of inorganic compounds to syn- thesize their food), parasites, or saprophytes (a few bacteria are also photosynthetic). It is the parasitic mode of nutrition of some of these organisms that is responsible for many of the dreaded plant and animal diseases (see tables). Some of our important antibiotic drugs like streptomycin,erythromycin, neomycin, terramycin and many others are obtained from actinomycetes.
(ii) Protista-Unicellular or colonial eukaryotes with diverse nutritional habits including photosyn- thesis, ingestion, and absorption. Reproduction is both asexual and sexual involving nuclear fusion and meiosis. Motility is by means of more ad- vanced type of flagella (9+2 strand flagella), or by other means.
The kingdom includes:
Euglenophytes (Euglenophyta)-flagellated unicells lacking cellulose cell walls but enclosed by a flexible or rigid pellicle;
Dinoflagellates (Pyrrophyta)-
unicells with- out or with a heavy cellulose wall (many species) sculptured into plates and two furrows, one trans- verse and one longitudinal, each with a long flagellum;
Golden algae and diatoms (Chrysophyta)-uni- cellular, colonial, or filamentous forms with elabo- rate siliceous skeletal structure or heavily calci- fied/siliceous and highly ornamental cell walls;
Yellow-green algae (Xanthophytes)-mostly unicells, a few filamentous or tubular (coenocytic- no transverse walls to separate cells). The kingdom Protista also includes five phyla of the protozoans (Sporozoa, Cnidosporidia, Zoomastigina, Sarco- dina, and Ciliophora) and two phyla of fungus-like organisms (Hyphochytridiomycota, Plasmodio- phoromycota).
(iii) Fungi-generally multi-nucleate organisms with eukaryotic nuclei dispersed in a walled, septate or sometimes aseptate (coenocytic) mycelium; non-photosynthetic due to lack of chloroplasts and photosynthetic pigments. Nutri- tion through absorption only. Tissue differentiation absent or limited to reproductive organs in higher forms. Primarily nonmotile (but protoplasmic flow in the mycelium); reproduction both asexual and sexual.
The kingdom Fungi is classified into two divi- sions (phyla)-Myxomycophyta and the Eumyco- phyta.
A. Myxomycophyta (slime molds)-The slime molds are of two types. The true slime molds are multinucleate plasmoidal forms without cell walls. moving in an amoeboid fashion ingesting solid food particles (like amoebae) and dissolved nutri- ents. They produce spore-bearing structures on stalks. The other type-Cellular slime molds, on the other hand, have a one-celled body during feeding stage (indistinguishable from amoebae in appearance) and ingest bacteria; under certain conditions several one-celled individuals aggregate and produce a multicellular, slug-like pseudoplasmodium which moves about as a unit and later develops into a stalked spore-producing structure.
B. Eumycophyta (true fungi)-The true fungi are characterised by a definite cellular body which may be single-celled, coenocytic, or multicellular. The multicellular body is composed of much- branched filaments and is called a mycelium, the individual filament branches being called hyphae. The mycelium excretes digestive enzymes into the surrounding medium/environment and absorbs dissolved nutrients. Eumycophyta is further divided into the following classes:
(a) Chytridiomycetes (chytrids)-one-celled to typical mycelial forms (coenocytic) producing mo- tile reproductive cells, each with a single posterior, flagellum. They are primarily saprophytes or para- sites, mostly microscopic organisms, found in water.
(b) Oomycetes (water molds, white rusts, and downy mildews)-one-celled to a much-branched filamentous body (mycelium) producing motile re- productive cells, each with a pair of different types of flagella. They are saprophytes or parasites- some causing serious diseases in plants like white rust and downy mildews.
(c) Zygomycetes (bread molds, fly fungi, ani- mal trapping fungi)-much-branched mycelial (coenocytic) forms only, producing a sexual dor- mant spore called zygospore; flagellated cells lack- ing altogether. Saprophytes or parasites, some trapping and digesting small animals.
(d) Ascomycetes (sac fungi)-multicellular mycelial forms (one-celled in a few, e.g. yeasts) producing haploid (with a single set of chromo- somes) spores called ascospores inside a sac-like ascus, products of sexual reproduction. The yeasts, so important for our alcohol and bread-making in- dustries (breweries and bakeries), and Penicillium, the source of the wonder drug penicillin, are members of this class.
(e) Basidiomycetes (club fungi)-multicellular mycelial forms producing haploid spores, the basidiospores (usually 4) in club-shaped basidia (hence, the name), products of sexual reproduction. The rusts which sometimes devastate our wheat crops and the mushrooms are members of this class. The fruiting body (basidiocarp) of some basidiomycetes may be as several feet in diameter.
(f) Deuteromycetes (imperfect fungi)-mostly multicellular mycelial forms reproducing asexually. This constitutes an artificial group containing diverse kinds of fungi known only by their asexual reproductive structures (known as imperfect fungi due to apparent lack of sexual phase-the perfect stage).
(iv) Plantae-Multicellular organisms with cells being enclosed by a rigid cell wall and often vacuolate. Nutrition mostly photosynthetic, a few species being absorptive (saprophytes or para- sites); photosynthetic pigments in plastids (chloroplasts). Primarily nonmotile, often an- chored to the substratum. Structural differentiation leading towards organs of anchorage, support, and photosynthesis; in most species specialised photo- synthetic, vascular, and covering tissues present. Reproduction primarily sexual with cycles of al- ternating haploid and diploid generations, the haploid being greatly reduced in the more ad- vanced members of the kingdom. The kingdom Plantae is classified into a number of divisions:
A. Rhodophyta (red algae)-mostly multicellular organisms ranging from simple filaments to large plants over 10 feet long and with some tissue differentiation due to the aggregation of filaments. Reproduction mainly sexual through complex re- productive systems, but flagellated cells completely absent; mainly marine plants growing at- tached to rocks or other algae; reddish because of the pigment phycoerythrin'.
B. Pheophyta (brown algae)-multicellular plants ranging from only a few millimetres to mas- sive plants up to 200 feet long with some tissue differentiation. Reproduction both sexual and asexual. Mainly marine plants growing attached to rocks along ocean shores; brownish due to the xanthophyll pigment 'fucoxanthin'.
C. Chlorophyta (green algae)-bright green plants due to chlorophyll in plastids, ranging from one-celled motile or nonmotile species to motile or nonmotile colonies, and from simple filaments to massive plants over 20 feet long with some tissue differentiation. Reproduction both asexual and sexual with wide diversity of life cycles; abundant in both fresh waters and marine waters, on tree trunks, and on moist rocks, leaf surfaces, and soil.
D. Bryophyta (bryophytes)-multicellular ter- restrial plants without vascular tissues or true roots. Reproduction mainly through vegetative means or sexual, the main plant body being the gametophyte (gamete-eggs and sperms-produc- ing plant body).
The bryophytes are divided into three classes- the Hepaticae (liverworts), Anthocerotae (horn- worts), and the Musci (mosses).
E. Psilophyta (psilophytes)-multicellular terrestrial plants with vascular tissues (xylem and phloem). The plant body consists of a sparingly branched, leafless, erect, aerial portion and a hori- zontal, rootless, underground portion; mostly fos- sil plants, only a few living species.
F. Lycopodophyta (club mosses)-multicellu- lar terrestrial plants with vascular tissues. The plant body is differentiated into root, stem, and leaves (primitive type). Stem not jointed; spores produced in sporangia borne in the axial of fertile leaves (sporophylls); sporophylls often aggregated into club-like terminal cones (strobili). Sperms flagellated.
G. Arthrophyta (horsetails)-multicellular plants with vascular tissues. Plant body differentiated into root, stem, and small whorled leaves (primitive type). Stem conspicuously jointed; spores produced in sporangia borne on highly modified sporophylls that are aggregated into a ter- minal cone-like structure (strobilus). Sperms flagellated.
H. Pterophyta (ferns)-multicellular plants with vascular tissues (without vessels). Plant body differentiated into root, stem, and leaves (advanced type). Stem not jointed but much reduced or creep- ing, or erect tree (in some tree ferns, e.g. Cyathea); spores produced in sporangia located usually on lower surface of the leaves (sporophylls) or on their margins. Sperms flagellated. Seeds not produced.
I. Cycadophyta (cycads)-multicellular terres- trial plants with vascular tissues (without vessels). Plant body distinctly differentiated into root, stem and leaves (advanced type), representing the sporophytic generation. Stem large and woody; leaves large, fern-like, pinnately compound. Gametophyte (male and female) small and depend- ent upon the sporophyte. Seeds produced in open carpels and, therefore, not enclosed. Sperms flagellated. Examples-Cycas (sago palm), Zamia (sago tree) etc.
J. Coniferophyta (conifers)-multicellular plants with well-developed vascular tissues (with- out vessels). Sporophyte (main plant body) large and usually woody; leaves simple, relatively small, usually not fernlike. Gametophyte (male and fe- male) small and dependent upon the sporophyte. Carpels open and, therefore, seeds not enclosed. Sperms nonflagellated. Examples-Pinus (pine), Cedrus (cedar, deodar), Taxus (yew), Sequoia (redwood), etc.
K. Anthophyta (flowering plants)-multicel lular flowering plants with well-developed vascular tissues; xylem usually with vessels. Sporophyte (main plant) ranging from small microscopic plants (Wolffia) to large trees (eucalyptus, banyan, mango etc.); herbaceous, shrubby, or woody trees. Leaves usually broad, borne alternately, opposite, or whorled. Gametophyte (male and female) highly reduced and dependent on the sporophyte; sperms nonflagellated, transferred to the ovule by a pollen tube. Seeds enclosed within an ovary which develops into a fruit. Anthophyta is divided into two classes:
(a) Dicotyledoneae (dicots)-multicellular plants with vascular tissue in distinct strands bundles arranged in a cylinder or circle; leaves net- veined. Floral parts (sepals, petals, stamens etc.) in fours or fives; embryo (young plant in seed) with two cotyledons (embryonal leaves). Examples: sunflower, dahlia, candytuft, mustard, mango, banyan, eucalyptus etc.
(b) Monocotyledoneae (monocots)-multi-cellular plants with vascular tissues usually in scattered bundles; leaves with parallel veins. Floral parts mostly in threes or multiples thereof; embryo in the seed with a single cotyledon. Examples- rice, wheat, millets, sugarcane, onion, lilies, etc.
(v) Animalia-Multicellular eukaryotic organ- isms with cells lacking a rigid cell wall and photosynthetic apparatus.
It is divided into two main sub-kingdoms-Pro- tozoa and Metazoa. While Protozoa are unicellular organisms, Metazoa are multicellular. Metazoa is further sub-divided into two branches-Parazoa and Eumetazoa. In Parazoa, the animal body is not organized into tissues and organs, instead it is formed by loosely aggregated cells (e.g. sponges). In contrast, in Eumetazoa, the cells are properly arranged into tissues and organs. The Eumetazoa are divided into two divisions depending on the number of germ layers present in the embryo. Ani- mals having two germ layers (ectoderm and endoderm and a non-cellular jelly-like layer, the mesogloea in between the two) are designated diploblastic and those having three (ectoderm, mesoderm and endoderm) triploblastic.
The animals in these two divisions are further grouped into grades-Acoelomata, Pseudocoelomata and Eucoelomata. The acoelomates lack a body cavity since the space between the body wall and digestive tract is filled with parenchymatous cells. In pseudocoelomates a body cavity, though present, is not lined by epithelial cells. In eucoelomates, the body cavity is lined by epithelial cells and is referred to as 'true coelom'. These grades are sub-divided into smaller groups known as phyla while considering similarities in the main features of animals and the presence or absence of the backbone or vertebral column.
The main animal phyla consist of
(a) Protozoa These microscopic animals are one-celled and occur either singly or in colonies. They exist all over the world in fresh and salt wa- ters, and many are parasitic. They are classified on the basis of the presence or absence of locomotory structures, such as pseudopodia, cilia and flagella. Examples-Trypanosoma, Euglena (with flage- lla); Amoeba, Entamoeba (with pseudo-podia); Paramecium, Vorticella (with cilia); Plasmodium (without locomotory structures).
(b) Porifera: These animals exhibit radial sym- metry and have cylindrical bodies. Cells are loosely aggregated and are not differentiated into tissues. Their skeleton is made up of spicules or fibres or both, Examples-Sycon, Euspongia (bath sponge), Spongilla (fresh water sponge).
(c) Coelenterata: These are the first multi-cel- lular radially symmetrical diploblastic animals possessing tissues and a digestive cavity. The body shows two main forms, the polyp and the medusa. The polyp (asexual) and the medusa (sexual) forms regularly alternate in the life cycle. Examples- Hydra, Obelia, Aurelia (jelly fish), Metridium, (sea anemone).
(d) Platyhelminthes: The body is bilaterally symmetrical, flattened, thin and soft, and appears leaf-like or ribbon-like. These are wormlike crea- tures. Some called 'planarians' live in fresh water under stones, others are parasites. Examples- Fasciola (liver fluke), Schistosoma (blood fluke), Taenia solium (pork tapeworm).
(e) Aschelminthes: These are pseudocoelomate worms with round bodies, pointed at both ends with no rings or segments. Examples-Ascaris (roundworm), Oxyuris (pinworm), Ancylostoma (hookworm).
(f) Annelida: This includes worms with round bodies marked externally into rings or segments. The body is soft, elongated and may have one or more pairs of bristles on each segment for locomotion. True coelom is present. These worms possess a closed blood vascular system. Examples- Pheretima (earthworm), Hirudinaria (leech), Nereis (clamworm).
(g) Arthropoda: The body is segmented and has a hard chitinous outer covering (exoskeleton). Jointed limbs are present. Coelom is reduced. Circulatory system is open and consists of a dorsal heart. This phylum is the largest of all and is divided into four large classes:
(1) Crustaceans-arthropods with limbs at- tached to most segments. Examples-Palaemon (prawn), Daphnia (water-flea), Branchipus (shri- mp).
(ii) Insects-arthropoda with three pairs of legs and body divided into three parts. Examples- Periplanata (cockroach), Papilio (butterfly), Musca (housefly), Culex (mosquito).
(iii) Arachnids-arthropods with four pairs of legs and body divided into two parts. Examples- Buthus (scorpion), Lycosa (wolf spider), Limulus (king crab).
(iv) Myriapods-arthropods with body divided into a number of similar segments with one or two pairs of legs to most segments. Examples- Scolopendra (centipede), Julus (millipede).
(h) Mollusca: These animals possess a soft unsegmented body without appendages. Body is covered by a fold of skin called mantle which usually secretes a shell. They possess a ventral muscular organ called foot for locomotion. Examples- Pila (apple or pond snail), Limax (slug), Patella (limpet), Unio ( mussel), Sepia ( cuttlefish), Loligo (squid).
(i) Echinodermata: This includes spiny- skinned animals built on a five-radial plan. They move by numerous soft tube-like structures called tube feet. They possess a system of tubes known as the water vascular system which aids in slow loco- motion and respiration. They are exclusively ma- rine. Examples-Astropecten (starfish), Ophiura (brittle star), Echinus (sea urchin), Antedon (sea- lily), Cucumaria (sea cucumber).
(J) Chordata: The three unique characteristics of chordates are: (i) a dorsal, hollow, tubular nerve cord; (ii) a notochord which is ventral to nerve cord and is replaced by cartilage or bone to form a vertebral column in higher vertebrates; and (iii) gill slits in the pharynx. Animals with backbone (vertebral column) constitute the major sub-phylum. However, there are a few forms that lack backbones and are considered primitive protochordates.
The phylum chordata is further divided into the following five sub-phyla:
(a) Hemichordata: These soft, worm-like animals resemble chordates in that they have pharyngeal gill slits and a dorsal hollow nerve cord which is confined to the anterior half of the body. Example-Balanoglossus (tongue worm).
(b) Urochordata: In these bag-like sessile creatures, nerve cord and notochord are absent in the adult but present in the tadpole-like larva. Examples -Herdmania, Doliolum (tunicate).
(c) Cephalochordata: These animals possess all three unique chordate characters throughout life. Example-Branchiostoma (amphioxus or lancelet).
(d) Agnatha: They form a small group of vertebrates that have a suctorial mouth but no jaws. Such animals are called cyclostomes. Examples- Petromyzon (sea lamprey), Myxine (hagfish).
(e) Gnathostomata: These are familiar verte- brates which possess jaws and include diverse ani- mals such as fishes, amphibians, reptiles, birds and mammals. This sub-phylum is divided into five classes.
(1) Pisces: These are aquatic cold-blooded, jawed vertebrates adapted to life in water. Their bodies are covered with scales, they respire through gills, move with the help of fins (no legs) and have a two-chambered heart.
They are broadly classified into two groups: Elasmobranchii (Chondrichthyes or Cartilaginous fishes) These fishes have a bony endoskeleton exclusively made up of cartilage. Mouth is ventral in position and gill cover (operculm) is absent. Examples-Scoliodon (shark or dogfish), Torpedo (electric ray), Raja (skate), and Teleostomi (Osteichthyes or bony fishes)-These have a endoskeleton. Mouth is terminal in position with gills covered with gill cover. Examples-Labeo (rohu), Catla (catla), Mystus (singhara), Gambu- sia (mosquito fish), Hippocampus (sea horse), Exocoetus (flying-fish) Pleuronectes (sole-fish).
(ii) Amphibia: These are cold-blooded vertebrates with four legs and scaleless skins, which are usually moist. They usually have a larval stage (tadpole) which is aquatic, and an adult stage which has lungs and lives most of the time on land. Heart is three chambered with two auricles and one 1 ventricle. Examples-Rana (frog), Bufo (toad), Salamandra (salamander), Uraeotyphlus (caecilian, limbless amphibian).
(iii) Reptilia: These cold-blooded vertebrates possess hard scaly skins and are adapted for life in dry places. They breathe by means of lungs (even though some live in water). The heart is three- chambered. Only crocodiles have a four cham- bered heart. Examples-Testudo (tortoise), Trionyx (turtle), Hemidactylus (house lizard), Naja (cobra), Crocodylus (crocodile).
(iv) Aves: Birds are the only animals with a body covering of feathers. Their forelegs adapted as wings and are usually used in flight. are They are warm-blooded and have a body tempera ture higher than that of other animals. Living birds have no teeth. The heart is four-chambered. Cloaca is present. Examples-Passer (sparrow), Corvus (crow), Columba (pigeon), Pavo (peacock) Struthio (ostrich), Bubo (owl).
(v) Mammalia: These warm-blooded verte- brates constitute the highest class of animals. They have a body more or less covered with hair. They also have milk glands by which the young are nourished. Another unique character is the presence of a muscular partition (diaphragm) that separates the thoracic from the abdominal cavity. The heart is four-chambered. The embryo receives nourishment and oxygen through the placenta and disposes of wastes through maternal blood circulation
Examples-Macropus (Kangaroo, pouch-bearer),duck billed platypus and spiny anteater (egg-layers), Felis (cat), Canis (dog), Oryctolagus (rabbit), Panthera (lion, tiger, leopard), Macaca(monkey), Elephas (elephant), Ursus (bear), Balaena (whale), Homo (man).