NCERT Solutions For Class 11 Biology Chapter 4 Animal Kingdom

(c) Annelida

Water vascular system is the characteristic of Echinodermata.

An oviparous mother lays more eggs at one time as compared to the number of offspring a viviparous mother gives birth to at any given time. This is because of the following reasons:

• In oviparous mother, the eggs are laid soon after fertilisation, or eggs are externally fertilised. Therefore many eggs can be laid at one time. This also provides an evolutionary advantage as many eggs are liable to be eaten, destroyed during the period of development.
• In the case of the viviparous mother, she has to house the embryo in the uterus till the full term of development. The embryo also draws nutrition from its mother. The mother thus is constrained by space in the uterus.

Some of the modifications that help birds to fly are listed below:

1. The forelimbs are modified into wings.
2. Birds have feathers that help them fly, steer and balance.
3. Long bones are hollow with air cavities making their body weight light.
4. Birds have air sacs that connect to the lungs and provide them with additional oxygen which is essential for flying.
5. Birds have well-developed eyesight and brains that help them gauge any approaching danger as well as to forage for food.

Air bladder or swim bladder is a sac filled with air that regulates buoyancy in fishes. It plays a very important role in Pisces. The fishes (like Osteichthyes) where air bladder is present, it plays the following roles:

• The air bladder controls the buoyancy by enabling the fish to stay at a particular water depth without having to waste energy in swimming continuously.
• It acts as a resonating chamber to produce or receive sound.

In another kind of fishes (like Chondrichthyes), the air bladder is absent. Therefore these fishes have to swim constantly to avoid sinking resulting in the consumption of a lot of energy.

To understand the above statement, one needs to define the two words – notochord and vertebra – from which the terms chordates and vertebrates are derived.

Notochord: The notochord is a flexible rod-shaped structure found in embryos of all chordates.

Vertebral column: The vertebral column, also known as backbone or spine, is a bony structure found in vertebrates. It is formed from individual bones called vertebrae.

Chordata is a phylum which is divided into three subphyla - Urochordata, Cephalochordata and Vertebrata. All the chordates at some point of their lives possess a notochord. However, in vertebrates, this notochord is replaced by the vertebral column in the adult stage. Hence, it is right to state that “All vertebrates are chordates but all chordates are not vertebrates”.

Arthropoda is the largest phylum of animal kingdom. Over two-thirds of all known species on earth are arthropods. The following are the reasons for arthropods to constitute the largest group of the animal kingdom:

1. Arthropods are very diverse in structure and form.
2. Their diversity in form is responsible for their dispersion into varied habitats. Arthropods are known to inhabit the ocean depths to the tallest mountain peaks.
3. Their hard exoskeleton made of chitin is tough and protects the animal from extreme dryness. The exoskeleton is also insoluble in water, weak alkaline and acidic solutions.
4. They have joint appendages that allow for efficient and precise movements. Many appendages are modified to meet the need of the arthropod for a specific foraging behaviour or a habitat.
5. Their reproductive cycle is short and the female lays numerous eggs at one time.

The peculiar features of parasitic platyhelminthes are as follows:

1. Hooks and suckers are present to attach to the wall of the gut.
2. They have an extra coating (of glycocalyx) to protect themselves from being digested in the gut.
3. They absorb nutrients from the host directly through their body surface. Their digestive systems are less evolved as compared to the free-living platyhelminthes.
4. Locomotory organs like cilia and sensory organs like the primitive eye are absent in the parasitic forms of flatworms.
5. Parasitic flatworms are mostly hermaphrodite ensuring reproduction in the host. The reproductive capacity is enhanced with most of their body lumen packed with testes and eggs.

Coelom is a fluid-filled cavity formed between the gut wall and the body wall lined by the mesodermal cells. Presence or absence of coelom is an important criterion for animal classification. The animals with bilateral symmetry are further classified into three types of groups based on the presence or absence of coelom. They are as follows:

1. Coelomates: In these animals, true coelom lined by mesoderm is present. E.g. Annelids, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata  
2. Pseudocoelomates: In these animals, the body cavity is not lined by mesoderm instead the mesoderm is present in scattered pouches between the ectoderm and the endoderm. E.g. Aschelminthes
3. Acoelomates: In these animals, body cavity is absent. E.g. Platyhelminthes

Taxonomy is the study of principles and procedures of classification. Classification is the arrangement of living things according to their similarities and differences which bring order in the vast assemblage of living things in the world. A specimen can be classified in a stepwise process using the following classification categories:

For classification of animals, various common features such as the arrangement of cells, body symmetry, nature of coelom, circulatory system, the pattern of digestive system or reproductive system are taken into consideration. If these common features are not considered, the classification of each organism would become very difficult and chaotic. Classification of animals also helps in comparing different organisms. So, if only a single characteristic is considered while classifying the living organisms then the main objective would not be fulfilled.  Thus, to study the classification of animals then it has to be based on various common basic features. This process also helps in the addition of new discovery into the respective group.