Zoology: Animal Classification, Invertebrates, Vertebrates and Animal Tissues
Zoology is the branch of biology that studies animals, their structure, classification, habits, distribution and relationship with the environment. The animal kingdom is enormously varied, ranging from the simple sponge that stays fixed to a rock to the complex human being. To study this variety in an orderly way, animals are classified into groups on the basis of their body plan, symmetry, presence of a backbone and other features. For candidates preparing for APSC, ADRE, DHS, SSC, Railway and police examinations in Assam, zoology questions are asked on the classification of animals, the characteristics of each phylum, important examples, and the record facts such as the largest and the fastest animal.
This article provides a complete deep-study revision of zoology. It first explains the basic vocabulary and the bases of classification, then takes up the invertebrate phyla and the vertebrate classes one by one, and after that discusses animal tissues, important examples and their economic importance. Finally, the examination trends, common mistakes and a practical preparation plan are discussed. Each group is described with its distinguishing feature and one or two clear examples, so that the classification becomes easy to remember.
Basic Terms
- Zoology: the branch of biology that deals with animals.
- Classification: the arrangement of organisms into groups on the basis of their similarities and differences.
- Taxonomy: the science of naming and classifying organisms.
- Invertebrate: an animal without a backbone.
- Vertebrate: an animal with a backbone.
- Symmetry: the arrangement of body parts around a central axis.
- Diploblastic: an animal with two germ layers, the ectoderm and the endoderm.
- Triploblastic: an animal with three germ layers, the ectoderm, mesoderm and endoderm.
- Coelom: a true body cavity lined by mesoderm.
- Cold-blooded: an animal whose body temperature changes with the surroundings.
- Warm-blooded: an animal that keeps its body temperature constant.
- Notochord: a flexible rod that is the characteristic of chordates.
- Oviparous: an animal that lays eggs.
- Viviparous: an animal that gives birth to young ones.
Core Concepts and Figures
- The animal kingdom is divided into invertebrates and vertebrates.
- The major invertebrate phyla are Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca and Echinodermata.
- The vertebrate classes are Pisces, Amphibia, Reptilia, Aves and Mammalia.
- Arthropoda is the largest phylum of the animal kingdom.
- Animals with a notochord are grouped under Chordata, and vertebrates form its largest sub-group.
- Fishes, amphibians and reptiles are cold-blooded, while birds and mammals are warm-blooded.
- The largest animal is the blue whale, the fastest land animal is the cheetah, and the largest land animal is the African elephant.
- The heart is two-chambered in fish, three-chambered in amphibians and most reptiles, and four-chambered in birds and mammals.
Bases of Animal Classification
Animals are classified using several criteria. The first is the presence or absence of a backbone, which divides them into invertebrates and vertebrates. The second is symmetry: sponges are asymmetrical, coelenterates show radial symmetry, and higher animals show bilateral symmetry. The third is the number of germ layers: coelenterates are diploblastic and the rest are triploblastic. The fourth is the presence and nature of the body cavity, or coelom: some animals are acoelomate, some are pseudocoelomate and some are true coelomate. The fifth is the mode of origin of the mouth and anus, which divides animals into protostomes and deuterostomes. Other features include segmentation, presence of appendages, and the nature of the skeleton. This step-by-step approach makes the huge animal kingdom easy to organise and remember.
Invertebrate Phyla
Porifera, or sponges, are the simplest animals. They are fixed to a surface, have pores all over the body, and have a canal system for circulating water; an example is Sycon. Coelenterata, or Cnidaria, are aquatic animals with radial symmetry and stinging cells called nematocysts; examples are the hydra and the jellyfish. Platyhelminthes are flatworms, which are triploblastic and mostly parasitic; the tapeworm and the liver fluke are examples. Nematoda, or roundworms, have a cylindrical body and a pseudocoelom; the Ascaris and the hookworm are examples. Annelida are segmented worms with a true coelom; the earthworm and the leech are examples. Arthropoda is the largest phylum and includes insects, crustaceans and arachnids; they have jointed legs and an exoskeleton, and examples are the prawn, the spider and the butterfly. Mollusca have a soft body usually covered by a shell; the snail and the octopus are examples. Echinodermata are marine animals with a spiny skin and water-vascular system; the starfish and the sea urchin are examples.
Vertebrate Classes
Vertebrates are chordates with a backbone, and they are divided into five classes. Pisces, the fishes, live in water, breathe through gills, have a two-chambered heart, and are cold-blooded; the rohu and the shark are examples. Amphibia live both in water and on land, breathe through gills when young and through lungs and skin when adult, have a three-chambered heart, and are cold-blooded; the frog and the salamander are examples. Reptilia are fully terrestrial, breathe through lungs, have a three-chambered heart except the crocodile, which has four, and have a dry scaly skin; the snake, lizard and tortoise are examples. Aves, the birds, are warm-blooded, have feathers, a four-chambered heart, hollow bones and lungs with air sacs; they lay eggs, and the crow and the peacock are examples. Mammalia are warm-blooded, have hair on the body, a four-chambered heart, and mammary glands to feed the young; most are viviparous, and the cow, the whale and the human being are examples. The duck-billed platypus is an egg-laying mammal, and the kangaroo is a marsupial that carries its young in a pouch.
Animal Tissues
Animals are made of four basic types of tissue. Epithelial tissue covers and lines the body surfaces and internal cavities and also forms glands. Muscular tissue is responsible for movement and is of three kinds: striated, smooth and cardiac. Connective tissue joins and supports other tissues and includes bone, cartilage, blood, tendon, ligament and adipose tissue. Nervous tissue is made of neurons and carries messages in the form of impulses. These four tissues combine to form organs and organ systems. The study of tissues is called histology, and it helps in understanding how the different organs of the body are built and how they work together. In higher animals, organs are grouped into systems such as the digestive, circulatory, respiratory, nervous and excretory systems, each specialised for a particular function.
Important Examples and Records
Zoology questions often test record facts and typical examples. The largest phylum is Arthropoda, and the largest class is Insecta. The blue whale is the largest animal that has ever lived, the cheetah is the fastest land animal, and the peregrine falcon is the fastest bird. The ostrich is the largest living bird and cannot fly, while the hummingbird is the smallest bird. The shark is the largest fish, and the whale shark is the largest of all fishes. Among mammals, the elephant is the largest land animal, the giraffe is the tallest, and the bat is the only mammal that can truly fly. The blue whale is a mammal, not a fish, because it is warm-blooded, gives birth to live young and feeds its young with milk. Animals such as the earthworm, called the farmer's friend, enrich the soil, and many insects act as pollinators and as decomposers.
Economic Importance of Animals
Animals are important to human beings in many ways. Domestic animals such as the cow, buffalo, goat, sheep, pig and poultry provide milk, meat, eggs, leather, wool and manure. Fishes, prawns and crabs are important sources of food and income, especially in Assam, where fisheries are a major livelihood. Silkworms give silk, honeybees give honey and beeswax, and lac insects give lac. Earthworms improve soil fertility, and many insects help in pollination. Animals also help in transport and in agriculture as draught animals. Some animals, however, are harmful: mosquitoes spread malaria and dengue, rats damage crops and stores, and certain insects and worms are pests or parasites. The balance of nature also depends on animals, as they form part of food chains and food webs and help in the recycling of nutrients.
Examination Trends
In competitive examinations, zoology questions usually test the distinguishing feature of a phylum or class, the example that belongs to a particular group, the animal that is the largest, fastest or tallest, the number of heart chambers in each vertebrate class, the difference between cold-blooded and warm-blooded animals, and the difference between oviparous and viviparous animals. Objective questions frequently ask the largest phylum, the phylum of the starfish, the class of the frog, the animal that lays eggs but is a mammal, and the animal that is the farmer's friend. A smaller number of questions test animal tissues, symmetry and the meaning of coelom and notochord.
Common Mistakes to Avoid
- Do not call the whale or the dolphin a fish; they are mammals.
- Remember that the crocodile has a four-chambered heart, unlike other reptiles.
- Do not confuse the hydra and the jellyfish with worms or fish; they belong to Coelenterata.
- Do not interchange cold-blooded and warm-blooded animals.
- Do not forget that Arthropoda, not Mollusca or Annelida, is the largest phylum.
How to Prepare This Topic
Prepare a table of the invertebrate phyla with one distinguishing feature and one example for each, and a separate table of the five vertebrate classes with their habitat, heart chambers, body temperature and examples. Learn the record facts, largest, smallest, fastest and tallest, as a small list. Draw simple diagrams of the hydra, the earthworm and the frog, because diagram-based questions are common. Remember the exceptions, such as the four-chambered heart of the crocodile and the egg-laying platypus, since examiners like to ask about them. Finally, practise previous years' question papers so that the exact style of questioning becomes familiar.
Frequently Asked Questions
Which is the largest phylum of the animal kingdom?
Arthropoda is the largest phylum of the animal kingdom. It includes insects, crustaceans and arachnids, and its members have jointed legs and an exoskeleton made of chitin.
What is the difference between cold-blooded and warm-blooded animals?
Cold-blooded animals cannot keep their body temperature constant, so it changes with the surroundings; fishes, amphibians and reptiles are cold-blooded. Warm-blooded animals keep their body temperature constant; birds and mammals are warm-blooded.
Why is the whale classified as a mammal and not a fish?
The whale is warm-blooded, breathes through lungs and not gills, gives birth to live young, and feeds its young with milk from mammary glands. These are the features of mammals, so the whale is a mammal and not a fish.
How many chambers are there in the heart of different vertebrates?
Fish have a two-chambered heart, amphibians and most reptiles have a three-chambered heart, and birds and mammals have a four-chambered heart. The crocodile is an exception among reptiles, as it has a four-chambered heart.
Which animal is called the farmer's friend and why?
The earthworm is called the farmer's friend because it burrows through the soil, loosens it, improves aeration and drainage, and adds nutrients through its castings, all of which increase the fertility of the soil.