Classification of Living Things
The arrangement of living things into groups is known as classification or taxonomy.
Methods of Classifying Organisms
Taxonomy has three aspects, namely: classification, nomenclature and identification. Classification entails the orderly arrangement of organisms into groups. Such arrangement shows evolutionary relationship and genetic relatedness. Nomenclature on the other hand was to do with assigning names to the various taxonomic ranking of living organisms.
Levels of Hierarchy of Classification of Living Organisms
The main taxonomic groups in classification scheme are:
• Phylum or Division
NB: Phylum applies to animals while division is for plants.
Members of the kingdom share only one or very few characteristics and as you go down the ranking from kingdom to species the similarities among members increases.
The Five Kingdom of Living Things
Originally, all living things were classified into two kingdoms namely plants and animals. This was the scheme flavoured by Linnaeus. This scheme did not accommodate single celled organisms. Later further classification was made introducing the kingdom protista but even at that there were still problems with bacteria and fungi.
Today, there are five kingdoms generally accepted by biologists. All living things find their places in these groups except the viruses which have been considered by many scientists as mere life forms and not proper living things.
Viruses are extremely small organisms that can only be seen by the means of an electron MICROSCOPE. They consist mostly of protein and a complicated substance call Nucleic Acid.
Viruses may be classified as living things because they reproduce by replication. Also, they are classified as non-living things because they cannot live without host i.e. they cannot live alone.
Characteristics of Virus
1. Viruses are extremely small.
2. Viruses reproduce by replication.
3. Viruses are classified as living and non-living things.
4. Produces many diseases both to plants and animals.
5. They cause diseases like yellow fever, small pox and poliomyelitis to man.
6. They do not respire, excrete, grow or reproduce outside the tissue of a living plant or animal.
The method of classifying living things into five kingdoms is called the Robert Whittaker Scheme. The five kingdoms are:
• Fungi (mycetae)
The organisms in this group are usually single celled without membrane bound nucleus. They include the bacteria (bacterium, singular) and blue green algae, or cyanobacteria. They are very small, ranging in size from 0.1 to 10 mm. They are found in all environment, (ubiquitous) air, water and soil — and some can survive under very harsh condition in hot vents and volcanic lakes where the temperature is close to boiling point.
The protisis are unicellular organisms with true cells (eucarytes). Eucatyotes have cells with membrane bound nucleus and membrane bound organelles. They are also much larger than the prokaryotes e.g euglena in 100mm, paramecium 200mm and amoeba 1000mm. They are mostly aquatic living in Water said motile.
There are two types of protists. The plant-like – forms like chiamydomonas and chlorella are called protophytes while the animal like forms such as Amoeba paramecium and vorticella are known as protozoans.
Kingdom Mycetae (FUNGI)
There are about 100,000 known member of this group in this kingdom. They are grouped as non-mothiled organism, lacking chlorophyll and compose of thread — like structure (called hyphae which may contain one or many nuclei. The hyphae are (singular: hypha) organised into structures called mycelia (singular: mycelium) some are however single celled.
The members of kingdom plantae are multicellular non-motile chlorophyllous organisms. They are generally called the green plants. -The major phyla in this kingdom are the Thallophyta Bryophyta and Tracheophyta.
The members of this group are the simplest green plants. They do not have vascular bundles or systems for conveyance of nutrients or water through the body of the plant. They have filamentous or thread like or flat thallus bodies which are differentiated into roots, stems or leaves. All algae e.g. spirogyra, volvox, etc., are found in this group.
Characteristics of algae (Phylum Thallophyta)
i. Algae are the earliest and simplest green plants.
ii. The body structure is simple; hence they lack true roots, stems/leaves,
iii. They are mostly aquatics.
iv. They have chlorophyll.
v. They reproduce both sexually and asexually. Examples are Chlorella (one-celled), Spirogyra (filamentous) and volvox (colonial)
vi. They have cellulose cell wall.
vii They could exist as unicellular colonial or filamentous
Characteristics of fungi (Phylum Thallophyta)
i. They also have simple bodies; may be one-celled (yeast) or many cells with filaments called hyphae.
ii. They do not possess chlorophyll, hence are hetetrophic. They are either saprophytes (e.g. Rhizopus) or parasites (e.g. puccinia).
ii. The cell wall is made up of cellulose and chitin.
v. They reproduce both sexually (by fusion of gametes
produced in sexual organs of various types) and asexually (by spore formation). Examples are Rhzopus, Mucor, Mushroom and Penicillium.
Note: Lichens are symbiotic association of fungi and algae.
Unlike the thallophyta, the bryophytes are essentially terrestrial organisms living on land or wherever there is enough moisture to sustain them. They have structures which are differentiated into leaves, root — like structure and shoot-like structures.
Two very good examples are the liver worts and mosses. Liver worts have creeping ribbon-like leafy body and erect slender spore-bearing structure while mosses on the other hand show a bit more complexity in that it does not lie flat on the ground but is rather erect.
The erect stem-like structure is surrounded by rosette leaves. The flexours naked portion of the ‘stem’ ends in a capsule which contains spores. The roots like rhizoids are prominent.
Characteristics of mosses and liverworts (Phylum Bryophyta)
i. They are green plants found in most habitats such as wet banks on soil, rock surfaces etc.
ii. They lack vascular tissue.
iii. They possess rhizoid which serves as roots; though liverworts posses stems and leaves.
iv They are multicellular organisms and reproduce both sexually and asexually.
i. The body of Mosses plant possesses an upright axis with leaves spirally arranged on it.
ii. The rhizoid are well developed and are multi-cellular in nature and branched.
iii. Sex organs are formed at the apex of stems.
iv. The capsules are bigger and are borne on long stalk above the leaves.
The body of liverworts possesses a flattened tongued-like shape called thallus.
The rhizoid are small, unicellular and un-branched.
The capsules are bigger and are borne on stalk above the leaves.
Tracheophytes include ferns, pines and flowering plants. They are plants that possess I vascular bundles. They are mainly terrestrial.
The Kingdom animalia contains those multi-cellular organisms that feed on other organisms and do not have chlorophyll. They are broadly classified into two groups namely the invertebrates those without vertebral column or backbone and the vertebrate i.e. those with vertebral column or backbone.
INVERTEBRATES are animals that lack the vertebral column/ backbone. They consist of simple unicellular and multi-cellular organisms. The major groups under here are:
c. Flat worms
d. Round worms
Characteristics of Protozoan
i. They are unicellular and microscopic organisms.
ii. Most protozoa are free-living in water, while others are parasitic. Examples are Euglena, Amoeba and Paramecium
This is the phylum in which hydra, sea jelly and sea anemone are found. They are radially symmetrical at the aquatic organisms. They are at the tissue level of organisation and have soft bodies without exo or endo skeletons.
The body has only one opening (the mouth) which is used for ingestion of food. It is surrounded by tentacles. They reproduce asexually by budding and sexually by producing gametes. They have stinging cell, for defence and capturing of preys
Characteristics of Coelenterates
i. They are simple multi-cellular aquatic organisms.
ii. They are cylindrical and radically symmetrical.
iii. They have a central gut cavity with a mouth surrounded by tentacles, but lack anus,
iv. They possess stinging cells for attack and defence.
v. Most are fee-living in water e.g. Hydra, Sea Anemone and jellyfish.
Platyhelminthes are the flat worms. They are bilaterally symmetrical and have definite anterior and posterior ends. They are acoelomate. They have alimentary canal with one opening which serves as mouth and anus. Most of them are hermaphrodites i.e. they posses male and female sex organs.
The nervous and excretory systems are equally well developed in some such as the planaria which are free living while others like the tremetodes (thicks) and cestodes (tape worms) are parasitic inhabiting the gut or other organ systems of their hosts.
Characteristics of Flat worms
i. Their body is thin and flat with a definite anterior end.
ii. They are bilaterally symmetrical i.e. they have dorsal/ventral surface.
iii. Their alimentary canal has one opening, performing as the mouth and anus.
iv. Most of them are hermaphrodites
v. They have nervous, excretory and reproductive systems.
vi. They may be free-living or parasitic e.g. liver fluke, tapeworm etc.
The nematodes are commonly called round worms. They have cylindrical thread like bodies, flexible, non-segmented and covered with non-living cuticle. The head is not well—formed although there is an anterior region with mouth and sense organs. They have separate sexes. They are mostly parasitic on plants and animals but some are free-living in soil or water. A good example is Ascaris.
Characteristics of Round worms
i. They have long cylindrical bodies with anterior pointed end.
ii. Anterior end has a mouth and sense organs but no well-formed head.
iii. Alimentary canal has two openings and a mouth and anus.
iv. They have nervous, excretory ad reproductive systems.
v. Sex organs are usually separate.
vi. They may be free-living or parasitic. Examples of round worms are hook worms, guinea worms, Ascaris etc.
These are the segmented worms. They exhibit metameric segmentation which means the body is divided by septa. The organs in the cylindrical segments are identical. The alimentary canal has two openings; the mouth and the anus. They have blood vascular systems. They include leaches (Hirudo) which is an ecto parasite and free-living polychaetes forms like Neuris and the common earthworm.
Characteristics of Annelids:
i. The body is long and cylindrical with a distinct head region.
ii The body is divided into many identical segments. The alimentary canal possesses both mouth and anus.
iv. They have blood vascular system in addition to nervous, digestive, excretory and reproductive systems. Examples of Annelids are Earthworms, Leeches e.t.c.
The molluscs are animals with shells. The short body is typically composed of a head, a ventral muscular foot and a visceral hump. The body is so and non-segmented. The organs a well developed. Sense organs are also well developed. The snails are main terrestrial and aquatics are found in moist places.
The shell which contains the body mass is coiled and opens at the coilar. The shell functions to protect the snail from drying predators and physical damage. Respiration is by lung located inside the mantle, The snail produces many eggs with shells. The genital opening is located very close to the head.
Characteristics of Molluscs:
i. The body of molluscs is short and composed of head, ventral muscular foot.
ii. They are soft bodied and un-segmented.
iii. Their bodies are enclosed by a calcareous shell, e.g. Slugs, Snails, Oysters etc.
Diagram of a snail showing external features
The arthropods are characterised by segmented bodies with jointed paired appendages. They are bilaterally symmetrical and have distinct head, throat and abdomen. The body is encased in chitinous exoskeleton.
They shed the exoskeleton at interval to aLlow growth by a process called moulting. There are five major classes of arthropods namely: the arachnida, crustacea, chilopoda, diplopoda and Insecta.
Characteristics of Arthropods:
i. This group of organisms has segmented body with a distinct head.
ii. The body is enclosed by a hard external exoskeleton.
iii. They posses jointed legs. Examples of Arthropods are insect, millipedes and centipedes, crabs/crayfish, arachnids, spider, scorpions, e.t.c.
The housefly belongs to the order of insect known as diptera because of their single pair of wings. The head of the housefly bears of pair of large compound eyes on either side, between these are three small simple eyes called Ocellus. The antennae are short and hairy. The mouth called proboscis is for sucking. The thorax is quite large, oval and made up of three segment, with the middle and largest segment hearing the only pair of large membranous wings used for flight. The third thoracic segment bears vestiges of wings or scales which are used for balancing in flight. There are three pairs of hairy legs on the thoracic segments, on the abdomen these are four segments.
Houseflies carry out complete metamorphosis with life cycle divided into distinct stages; larva, pupa and adult stage. The larva is called maggots. It is a very active stage which can be seen in rotten moist organic matter especially dead animal and excreta as greyish white spindle cylindrical organisms with tapered anterior. The pupa stage is more or less are sting state formed from the larva that has moved to a dry spot with its barrel shaped hardened skin.
External features of a housefly.
VERTEBRATES are animals with definite head, and a backbone of vertebral columns inside which is a nerve cord. They belong to the phylum chordata and sub-phylum vertebra. They have a well-developed brain and sense organs. They also posses two pairs of lumps and appendages.
Vertebrates are divided into five namely;
(i) Pisces (fishes)
(ii) Amphibians (toads, frogs)
(iii) Reptalia (snake, crocodiles)
(iv) Ayes (pigeon, hawk)
(v) Mammalian (man, rat, cows)
(i) Fish (Pisces)
The fishes are at the lowest level among vertebrates. They are adapted to life aquatic environment (fresh water, marine, or brackish water). Fishes typically have stream-lined bodies which reduce friction and obstruction when swimming in water. They are generally categorised into two based on the material the skeleton is made up of. The cartilaginous fish have skeleton made of cartilage these include dog, fish and rays. The bony fishes have skeletons made of bone for example, catfish, mackerel, tilapia etc.
External features of a Tilapia
Characteristic of Pisces (Fishes)
1. They are aquatic animals,
2. They breathe and respire with the gills.
3. They have fins, which helps them to move in water.
4. They are cold-blooded animals.
5. They have the same shape of teeth.
6. They are oviparous i.e. reproduce by laying eggs.
7. Their skins are enclosed with shiny scales.
The amphibians were the first group of vertebrates to venture on to land. They are characterised by their ability to live both in land and in water. Their body parts are specially modified and adapted for this purpose.
The adult unlike fish respire by means of lungs, body surface and surfaces of the mouth. They are cold-blooded animals with naked moist bodies. The adult have two pain of legs i.e. fore and hindlimbs.
They do not have external ears, but rather only middle and inner ears and on external ear drum. Amphibians have three- chambered hearts. Their legs are usually fertilised externally in water. Frogs, toads, newt and salamander are good example of amphibians.
The external features of a toad are shown in the figure below:
The toad has a stout body with the bead running directly into the trunk without neck. It also lacks a tail. The mouth is wide and has in it as ticky tongue which can be flipped out to pick insect.
The two nostrils are located a short distance behind the upper jaw. They have a pair of bulging eyes with movable eyelids behind which are located the ear drums. The limbs have four digits each. The fore-limbs are short and adapted for landing. Each one is divided into the upper arm, the fore-arm, the wrist and ends in the digits.
The hind limbs on the other hand are long and adapted for jumping. Each consists of the thigh, the shin and the webbed digits with ankle. The webbed digits are used for swimming when in water.
There are other features which the toads possess which specifically adapt it to terrestrial life. The skin has mucus gland which secret mucus that keeps the skin moist and ensure exchange of gas by diffusion. They are also able to change the intensity of the colour of their skin to blend with the environment. This serves as a form of camouflage. They equally have poisonous glands on the surface of the body which makes them unpalatable to predators.
In terms of reproduction, they still have in common with fish the fact that the egg has no amnion or shell and fertilization still takes place in water.
However, the fertilised eggs hatch into fish-like tadpoles with gills which go through a process of metamorphosis to become adults. Three methods of respiration are identifiable in the toad namely pulmonary involving the living; buccal which taken place across the highly vascularised mucous membrane, mouth cavity and the cutaneous which takes place on the highly vascularised skin by simple diffusion.
Characteristics of Amphibians (Toads, Frogs)
1. They live both on land and water.
2. Their skins are always moist and do not posses scales.
3. They respire with gills at the larval stage and lungs at the adult stage.
4. They are cold-blooded animals.
5. They possess the same shape of teeth.
(iii) Reptiles (Reptilia)
The reptiles are essentially terrestrial animals. They are well developed and adapted for life on land. Most of their features clearly show this. The skin of typical reptile is covered with dry horny scales which prevent loss of water from if body. They are also cold-blooded and they respire solely by means of lungs which they used to obtain atmosphere oxygen.
The reptiles have a very effective reproductive adaptation to life on land. First, they have copulatory organs which ensure that the sperms are deposited into the female genital and fertilisation is internal. It therefore means that there is no need for water which the sperms have to swim and also a danger of the loss of sperm through drying.
Furthermore, the eggs have membranes and shells which protect them from mechanic damage, elemental damage, desiccation and infection by bacteria and other diseases carrying organisms. The lizards, crocodiles, snakes, and tortoise are examples – reptiles.
External Features of a lizard
Characteristics of Reptalia: (Snake and Crocodile)
1. They are poikilothermics i.e. cold-blooded animals.
2. Scales covers their skin.
3. Live mostly on land even though a few live in water.
4. Their teeth are of the same shape i.e. homodont.
5. They reproduce by laying eggs i.e. they are oviparous.
6. They posses jaws with teeth set in sockets.
(iv) Birds (Aves)
The birds are adapted to life that involves movement by flight. Generally, birds have their bodies covered with feathers which are considered by evolutionists to be modified scales.
The forelimbs are also modified into wings for flight, while the hind limbs are scaly and adapted for walking. They have four toes each ending in a claw. They have regulated body temperature that makes them homoeothermic or warm blooded. The fact that the body temperature is higher than that of the environment makes them active at all times.
The jaws of the birds arc modified into structures called beak used for feeding; they lack teeth. Birds have only inner and middle ears which are sparsely covered with feathers. They have good eye sights located on the side of the head and designed to have a wide field of vision. Their four chambered heart adapts them for the strenuous exercises that flight entails.
Furthermore, the skeleton of the birds are hollow and hard; this helps them during flight i.e. the animals are not weighed down in flight. The lungs are highly developed and there are air sacs in the body for proper oxygen supply.
They have the breast plate modified into a keel which altogether give the bird a boat like shape and provide surface area for attachment of the powerful muscles involved in flight.
Birds lay large eggs protected with shells; fertilisation is internal. The small opening and the genital openings are combined to form cloaca just as in reptiles, although there is a penile structure in the cloaca.
The birds have very short tails. The feathers attached to the tail from the tail wings are used for steering in flight. Another adaptation is the presence of oil gland in the tail which produces oil that the bird uses its tail to pick, preen and straighten out the barbs of the feather when loosened.
The feathers which are unique features of birds are of different types, each type serving a distinct purpose. There are basically four types of feathers namely; contour feather semiplume feather, down feather and filoplume feather.
External Features of a Pigeon
Characteristics of Ayes (Pigeon, Birds, Hawk)
1. They are warm-blooded animals.
2. They are both terrestrial and aboreal animals.
3. Birds have feathers with which they fly.
4. They reproduce by laying eggs.
5. They feed on concentrated foods like grains.
6. They have scaly legs and hollow bones.
The mammals are the most highly developed vertebrates on the evolutionary scale. They are warm-blooded like birds and have four chambered heart in which deoxygenated blood from the body and the oxygenated blood from the lungs do not meet just as in birds.
Mammals have different types of glands on their skin. These are the sebaceous gland which secretes oil, sweat gland which serves to regulate body temperature and mammary gland which produce milk, protein and growth nutrient rich formula for young one.
The mammals have eyes with eye and ears with external ears called pinnae. They respire by means of a pair of lungs which are contained in the thoracic cavity separated from the abdominal cavity muscular sheet called diaphragm.
The brain of the mammal is well developed. In most mammals the fertilised eggs are developed in the uterus of the female and the young ones are born alive (viviparity). The young is fed in the early stage with milk produced by the mammary gland of the mother. Man, goat, camel, guinea pig, rat, elephant, kangaroo, etc. are examples of mammals and generally share these features which may be further modified to fit the specific mode of life of the specie within its habitat.
Characteristics of Mammalian: (Man, Cat and Cow)
1. They posses heterodont dentition i.e. they have different kinds of teeth.
2. They are homoeothermic i.e. warm-blooded animals.
3. They are viviparous animals since they deliver their young ones alive; although a few mammals lay eggs like Duck-billed platypus and echidna.
4. Their chest cavity is separated from the abdominal cavity by the diaphragm.
5. They breast feed their young ones with milk.
6. They have mammary gland known as mammae.
.7. Their bodies are usually closed by their hairs.
8. They are mostly terrestrial animals even though some live in water.
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