5th Standard Science Term-III
ANIMALS
Introduction
Our planet earth has countless number of organisms including plants and animals. Among them, animals are the most advanced organisms. Animals are a gift of nature to human beings. They are very closely associated with us in our daily life and contribute a lot to us. Man is exploiting nature nowadays more than ever before. Hence animals are affected and many of the plant and animal species are disappearing from the earth. We are going to study about this in this lesson. This lesson will also deal with reproduction in animals, extinction of animals and the ways to preserve them.
I. Reproduction in Animals
Reproduction is the biological process by which an organism gives rise to a new organism. This process is seen in all living organisms - both plants and animals. Reproduction is essential for the continuation of similar kinds of species, generation after generation. In animals, two types of reproduction are seen. They are : sexual reproduction and asexual reproduction.
1. Sexual Reproduction
Sexual reproduction is a natural way of reproduction in humans, animals and also in most of the plants. This type of reproduction is more complex and lengthy as compare to asexual reproduction. Different and unique offspring are produced by sexual reproduction. Sexual reproduction consists of the following stages.
a. Pre-fertilization b. Fertilization c. Post-fertilization
a. Pre-fertilization
This is the first stage of sexual reproduction. In this stage, gamete (sex cells formation and transfer of gametes take place. In animals, males and females have different reproductive organs. The male reproductive organ is called testes and the female reproductive organ is called ovary. The testes produce the male gametes known as sperms and the ovaries produce the female gametes known as ova or eggs. The male gamete reaches the female gamete during this stage.
b. Fertilization
When the male gamete reaches the female gamete they begin to fuse together. The fusion of gametes is known as fertilization. During fertilization, the nuclei of the sperm and the egg for a single nucleus together, resulting in the formation of a fertilized egg, known as zygote.
Fertilization in animals takes place in two ways. They are : External fertilization and Internal fertilization.
External fertilization takes place outside the animal's body. It usually takes place in aquatic environments where both eggs and sperm are released into the water. Fertilization in frogs and fish takes place by this method. When the fertilization takes place inside the animal's body, it is called internal fertilization. Internal fertilization takes place in animals like cat, dog, cow etc.
c. Post - fertilization
The zygote (fertilized egg) further divides repeatedly into group of cells. These cells develop into different tissues and organs constituting a full body. This structure is known as embryo.
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The embryo continues to develop in the uterus and it is developed into body parts such as head, face, hands, legs, etc. Based on whether the embryo develops outside or inside the body, animals are classified into oviparous animals and viviparous animals respectively.
✧ Oviparous animals
Animals in which embryo develops outside the body are called oviparous animals. They produce their offspring by laying eggs. In the case of birds new ones are produced from the eggs. The egg shell protects the embryo from outer environment and the embryo receives its nutrients from the egg yolk. In some of these animals the new born one will have different developmental stages.
For example, in butterfly, there are different developmental stages like egg, larva, pupa and adult. Each stage is different. The process in which a butterfly becomes an adult is called metamorphosis. The life cycle process can take a month to year.
State 1 - Eggs
In the first stage a butterfly lays eggs on the leaf. These eggs are very small and round. About five days after the eggs are laid, a tiny worm-like creature will hatch from the egg.
Stage 2 - Caterpillar (Larva)
The second stage is the caterpillar. It is also called larva. The caterpillar starts to eat leaves and flowers once it has hatched. It grows very fast because it eats a lot. As it grows fast, it sheds its old skin and gets new skin. A caterpillar shedding its outgrown skin is called moulting.
Stage 3 - Chrysalis (Pupa)
The third stage is the pupa. It is mostly brown or green. This is the resting stage as well as the changing stage. The caterpillar turns into a butterfly.
Stage 4 - Butterfly (Adult)
In the fourth stage, the pupa opens and a butterfly comes out. A butterfly is sometimes called an imago. It is also called as adult. Butterflies are very colourful. When the butterfly first comes out it is very tired and so it rests. Then the butterfly will lay eggs and the life cycle will start all over again.
✧ Viviparous animals
Animals in which the embryo develops inside the body are called viviparous animals. These animals give birth to the young ones. The developing embryo gets its nutrients from the mother. Human, cow, deer and dog are examples for viviparous animals.
|
Oviparous animals |
Viviparous animals |
|
The development of the
embryo takes place outside the animal. |
The development of the
embryo takes place inside the animal. |
|
They produce their
young ones by laying eggs |
They directly give
birth to the young ones. |
|
The embryo receives
the nutrients from the egg yolk. |
The embryo receives the
nutrients from the mother. |
|
Examples for oviparous
animals are insects, fish, reptiles, birds etc. |
Examples for
viviparous animals are cat, dog, lion, human etc. |
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2. Asexual Reproduction
The type of reproduction in which only a single parent gets divided into two or more new offspring is known as asexual reproduction. This type of reproduction takes place in micro organisms like hydra and amoeba. Asexual reproduction produces offspring that are identical to the parent. There are several ways by which animals reproduce asexually. Some of them are explained below.
✧ Fission
Fission occurs in some invertebrate (organisms without back bone), multi-celled organisms. In this method, an organism splits itself into two parts. For example, flatworms, sea anemones and sea cucumbers divide into two halves and regenerate the other half in each of the resulting individuals.
✧ Budding
Budding is a form of asexual reproduction that results from the outgrowth of a part of the body. Then, the bud is separated from the original organism forming two individuals. Budding occurs commonly in some invertebrate animals such as hydras and corals.
✧ Fragmentation
Fragmentation is the breaking of an individual into parts followed by regeneration. Reproduction through fragmentation is observed in sponges and sea stars. Fragmentation may occur through accidental damage, damage from predators, or as a natural form of reproduction.
✧ Spores
Some protozoan, bacteria, plants and fungi reproduce via spores. Spores are the structures that naturally grow as part of an organism's life cycle. They are separated from the organism and dispersed through a medium such as air or water. In a suitable environment, the spores will develop into a fully grown organism.
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