Biology IX | Diversity in the Living World | Part 2

August 24th, 2015 by admin No comments »

Diversity in the Living World | Part 2

Animal Kingdom (Animalia )

These are organisms which are eukaryotic, multicellular and heterotrophic. Their cells do not have cell-walls. Most animals are mobile.

They are further classified based on the extend and type of the body design differentiation found.

Sub-Kingdom (Porifera)

The word means organisms with holes. These are non motile animals attached to some solid support. There are holes or ‘pores’, all over the body. These lead to a canal system that helps in circulating water throughout the body to bring in food and oxygen. These animals are covered with a hard outside layer or skeleton. The body design involves very minimal differentiation and division into tissues. They are commonly called sponges, and are mainly found in marine habitats.

Sub-Kingdom Coelenterata

These are animals living in water. They show more body design differentiation. There is a cavity in the body. The body is made of two layers of cells : one makes up cells on the outside of the body, and the other makes the inner lining of the body. Some these species live in colonies (corals), while others have a solitary like-span (Hydra).

Sub-Kingdom Platyhelminthes

The body of animals in this group is far more complexly designed than in the two other groups we have considered so far. The body is bilaterally symmetrical, meaning that the left and the right halves of the body have the same design. There are three layers of cells from which differentiated tissues can be made, which is why such animals are called triplobastic. This allows outside and inside body linings as well as some organs to be made. There is thus some degree of tissue formation. However, there is no true internal body cavity or coelom, in which well-developed organs can be accommodated. The body is flattened dorsiventrally, meaning from top to bottom, which is why these animals are called flatworms. They are either free living or parasitic. Some examples are free-living animals like planarians, or parasitic animals like liverflukes.

Sub-Kingdom Nematoda

The nematode body is also bilaterally symmetrical and triploblastic. However, the body is cylindrical rather than flattened. There are tissues, but no real organs, although a sort of body cavity or a pseudo-coelom, is present. These are very familiar as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (roundworm or pinworms).

Sub-Kingdom Annelida

Annelid animals are also bilaterally symmetrical and triplobastic, but in addition they have a true body cavity. This allows true organs to be packaged in the body structure. There is, thus, extensive organ differentiation. This differentiation occurs in a segmental fashion, with the segments lined up one after the other from head to tail. These animals are found in a variety of habitats – fresh water, marine water as well as land. Earthworms and leeches are familiar.

Sub-Kingdom Arthropoda

This is probably the largest group of animals. These animals are bilaterally symmetrical and segmented. There is an open circulatory system, and so the blood does not flow in well-defined blood vessels. The coelomic cavity is blood-filled. They have jointed legs (the word ‘arthropod’ means ‘jointed legs’). Some familiar example are prawns, butterflies, houseflies spiders, scorpions and crabs.

Sub-Kingdom Mollusca

In the animals of this group, there is bilateral symmetry. The coelomic cavity is reduced. There is little segmentation. They have an open circulatory system and kidney like organs for excretion. There is a foot that is used for moving around. Examples are snails and mussels.

Sub-Kingdom Echinodermata

In Greek, echinos means hedgehog, and derma means skin. Thus, these are spiny skinned organisms. These are exclusively free-living marine animals. They are triploblastic and have a coelomic cavity. They also have a peculiar water driven tube system that they use for moving around. They have hard calcium carbonate structures that they use as a skeleton. Examples are starfish and sea urchins.

Sub-Kingdom Protochordata

These animals are bilaterally symmetrical, triplobastic and have a coelom. In addition they show a new feature of body design, namely a notochord, at least at some stages during their lives. The notochord is a long rod-like support structure (chord = string) that runs along the back of the animal separating the nervous tissue from the gut. It provides a place for muscles to attach for ease of proper notochord present at all stages in their lives or for the entire length of the animal. Protochordates are marine animals. Examples are Balanoglossus, Herdemania and Amphioxus.

Sub-Kingdom Vertebrata

These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.

Vertebrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs. All chordates possess the following features :

(i) have a notochord

(ii) have a dorsal nerve cord

(iii) are triploblastic

(iv) have paired gill pouches

(v) are coelomate.

Vertebrates are grouped into five classes.

(i) Division Pisces

These are fish. They are exclusively water living animals. Their skin is covered with scales/plates. They obtain oxygen dissolved in water by using gills. The body is streamlined, and a muscular tail is used for movement. They are cold-blooded and their hearts have only two chambers, unlike the four that humans have. They lay eggs. We can think of many kinds of fish, some with skeletons made entirely of cartilage, such as sharks, and some with a skeleton made of both bone and cartilage, such as tuna or rohu.

(ii) Division Amphibla

These animals differ from the fish in the lack of scales, in having mucus glands in the skin, and a three-chambered heart. Respiration is through either gills or lungs. They lay eggs. These animals are found both in water and on land. Frogs, toads and salamanders.

(iii) Division Reptilia

These animals are cold-blooded, have scales and breathe through lungs. While most of them have a three-chambered heart, crocodiles have four heart chambers. They lay eggs with tough coverings and do not need to lay their eggs in water, unlike amphibians. Snakes, turtles, lizards and crocodiles fall in this category.

(iv) Division Aves

These are warm-blooded animals and have a four-chambered heart. They lay eggs. There is an outside covering of feathers, and two forelimbs are modified for flight. They breathe through lungs. All birds fall in this category.

(v) Division Mammalia

Mammalia are warm-blooded animals with four-chambered hearts. They have mammary glands for the production of milk to nourish their young. Their skin has hairs as well as sweat and oil glands. Most mammals familiar to us produce live young ones. However, a few of them, like the platypus and the echidna lay eggs, and some, like kangaroos give birth to very poorly developed young ones.

Biology IX | Diversity in the Living World | Part 1

August 24th, 2015 by admin No comments »

Diversity in the Living World | Part 1

One of the cornerstones of science that is virtually unchallenged is that there is order in nature. The order found in living beings is a natural consequence of the shared evolutionary process that influences life. There are about 10 million species of organisms on earth, but only one-third of them have been identified so far.

The diversity is actually the product of the past 3.4 billion years of organic evolution. Also, during the same lengthy period, a lot many more species appeared and were lost (extinct). They are estimated to be at least fifty time more than the existing species.

The requirement is to assort the various varieties under species and then on the basis of similarities and dis-similarities (differences) arrange them into species and then into higher categories (such as genus, family, order, class and phylum).

The classification of organisms, therefore, is nothing but arranging the organisms into groups or sets on the basis of similarities and differences.

The Hierarchy of Classification-Group

Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms. The classification Whittaker proposed has five kingdoms. Monera, Protista, Fungi, Plantae and Animalia, and is widely used. These groups are formed on the basis of their cell structure, mode and source of nutrition and body organization. The modification Woese introduced by dividing the Monera into ARchaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use.

Further classification is done by naming the sub-groups at various levels as given in the following scheme :


Phylum (for animals) / Division (for plants)






Thus, by separating organisms on the basis of a hierarchy of characteristics into smaller and smaller groups, we arrive at the basic unit of classification, which is a ‘species’. So what organisms can be said to belong to the same species? Broadly, a species includes all organisms that are similar enough to breed and perpetuate.

Plant Kingdom (Kingdom Plantae)

Plant kingdom, as suggested by Eichler (1883) is subdivided into two sub-kingdoms : Cryptogamae and Phanerogamae.

Sub-kingdom Cryptogamae (Crypto-hidden, gamous-marriage) :

These are also known as lower plants, flowerless or seedless plants. These plants do not bear external flowers or seeds and hence are considered to have hidden reproductive organs. It is further divided into three divisions.

(i) Division Thallophyta (Thallus undifferentiated, phyta-plant) :

· The plant body is not differentiated into stem, root and leaves, but is in the form of an undivided thallus.

· There is no vascular system.

· The reproductive organs are single celled and there is no embryo formation after fertilization. This division includes three distinct sub divisions : algae, fungi and lichens.

(ii) Division Bryophyta :

They are the simplest land plants. Plant body is flat and lack true leaves and roots. A true vascular system is absent. Sex organs are multicellular. An embryo is formed upon fertilization. It includes liverworts, horn-worts and mosses. Some common Bryophyta are Riccia, Marchantic, Funeria.

(iii) Division Pteridophyta :

Plant body is made up of stem, leaves and roots. Vascular system is present. Reproductive organs are multicellular, Fertilized egg develops into embryo ; for example, all types of ferns.


Phanerogamae :

These are also called seed plants. Body is differentiated into true stem, leaves and root. Vascular system (xylem and phloem) is well developed. Sex organs are multicellular. An embryo develops from fertilized egg.

On the basis of absence or presence of fruits, there are two sub-divisions :

(a) Gymnospermae and

(b) Angiospermae

Both belong to division spermatophyta.

(a) Subdivision Gymnospermae :

Seeds are not enclosed in fruits. Examples : Pinus, Cycas, Cedrus.

(b) Subdivision Angiospermae :

Seeds are enclosed by a fruit. On the basis of the number of cotyledons, the angiosperms are distinguished into two groups : dicotyledons (pea, grams, etc). and monootyledons (wheat, rice, etc).