CLASSIFICATION OF ANIMALS AND ORGANIZATION OF LIFE

THE VERTEBRATES (PHYLUM CHORDATA)

All vertebrates have the following features:

1. A bilaterally symmetrical body which is divided into a head, trunk and a tail with a neck joining the head to the trunk in most vertebrates.
2. An internal skeleton (endoskeleton)
3. Well developed central nervous system and sense organs.
4. A closed blood system
5. Skin covered with scales, feather or hair.

The phylum chordata (animals with notochord) can be subdivided into five classes. Thee include

Class Pisces (tilapia, shark, dogfish)

1. These are fishes which are aquatic, cold blooded (poikilothermic) i.e. the body temperature varies with that of the surrounding).
2. Their bodies are covered with scales.
3. They breathe through their gills
4. Fertilization is external
5. They posses fins for locomotion
6. Their bodies are streamlined in shape

Class Amphibians (toad, frog, newt)

1. They are cold blooded animals.
2. They spend most of their time on land but reproduce (lay eggs) in water.
3. They carry out gaseous exchange using gills, lungs, skin or mouth living.
4. They have naked moist skin
5. Fertilization is external
6. They have two pairs of limbs with webbed toes

Class Reptilia (lizards, snakes, crocodiles etc)

1. Their skin is covered with dry scales which prevent water loss from the body.
2. Gaseous exchange is carried out through their lungs
3. Fertilization is internal and eggs are laid on land.
4. They are cold-blooded

Class Aves (birds)

1. They are warm- blooded (homoiothermic) i.e. they are able to keep their body temperature more or less constant.
2. Their body is covered with feathers
3. They have toothless beak for feeding.
4. Fore limbs modified into wings for flight
5. Fertilization is internal.

Class Mammalia (rats, bats, dogs, whales, monkey, kangaroos, man etc)

1. They are warm-blooded
2. They have skin covered with hair.
3. They possess heterodont teeth i.e. different sets of teeth.
4. They have a well-developed brain
5. They give birth to their young ones alive (viviparous)
6. They use lungs for gaseous exchange.

LEVELS OF ORGANIZATION OF LIFE

Every living thing is made up of a cell or a number of cells. A single-celled organism is called unicellular while a many-celled organism is called multicellular.

Organization of life is therefore the existence of life from a single-celled organism to a multicellular organism with complex forms that performs different functions. Four levels of organization of life exist, namely: cell, tissue, organ and system.

1. Cell: A cell is a functional and structural unit of protoplasm bounded by a membrane and containing nuclear and cytoplasmic materials. Some living things exist as cells which have cytoplasmic structures such as pseudopodia, cilia, flagella and vacuoles e.g. Amoeba, Chlamydomonas, Paramecium and Euglena. Common cells that perform distinct functions in plant and animals include sperm cells, nerve cells, bone cells, red blood cells, root hair cells etc.
2. Tissues: A tissue is a collection of cells which are similar in structure and functions. Hydra is an example of an animal that is made up of tissues only. Examples of tissues in animals and plants are epithelial tissue, blood, nerve tissue, muscle tissue, xylem and phloem tissues etc.
3. Organ: An organ is a collection of different tissues that perform a common function or functions e.g. Kidney, heart, brain, stem, roots etc. Example of organ in plant is onion bulb.
4. System: A system is a set of organs which co-operate to carry out one of the vital functions of life e. g. Digestive system, respiratory system, reproductive system, transport system etc.

COMPLEXITY OF ORGANIZATION IN HIGHER ORGANISMS

Higher organisms have bodies that are differentiated into specialised parts performing different functions. The different specialized parts made up of tissues, organs and systems make higher organisms more complex and efficient to carry out various activities of life.

Advantages of complexity

1. It leads to division of labour and specialization
2. It has led to the efficiency of the various organs of the body.
3. The complexity of higher organism has led to their survival (adaptation) in their different habitats.
4. Various systems operate side by side without adversely affecting each other
5. Reproduction in complex organisms does not lead to the disintegration of the parents

Disadvantages of complexity

1. Individual cells are not capable of existing independently and therefore depend on one another’s activities to exist
2. With complexity, ability to regenerate lost parts decreases.
3. Difficulties in acquisition of food materials and oxygen and loss of waste product
4. Difficulties in reproduction
5. It leads to slower rate of diffusion because the surface area to volume ratio is small and the distance from exterior to the center of the body is great