Class 11-science NCERT Solutions Biology Chapter 4 - Animal Kingdom

1. There are numerous species which show great variety of life. The various common fundamental characters are taken into account for the classification of these animals. If this has not been done, it is not possible to deal with every living form separately at the individual level. Therefore, a classification has to be based on the common fundamental characters to study the diversity of animals.
2. The interrelationship among different animals will not be traceable.
3. The picture of all animals at a glance will not be projected.
4. It will be difficult to develop new animal species. 

Steps for the classification of a specimen:

i. Identify levels of organisation: Identify if it is a cellular level of organisation or tissue level of organisation.

ii. Identify body symmetry: Find if the animal has radial body symmetry or bilateral body symmetry.

iii. Presence or absence of body cavity: This will give a clue if the animal belongs to Acoelomata or Pseudocoelomata or Coelomata.

iv. Type of germ layers: This tells if an organism is diploblastic or triploblastic.

The body cavity which is derived and lined by a mesoderm is called the coelom and the animals are called coelomates. Examples: Annelids, molluscs, arthropods etc. When the body cavity develops from the blastocoel of the blastula, it is called pseudocoelom and the animals are called pseudocoelomates. Example: Aschelminthes. When the cavity is not derived from the mesoderm, it is called acoelom and such animals are called acoelomates. Examples: Porifers, cnidarians etc.

The presence or absence of the body cavity gives a clue about a particular animal's position in the evolutionary ladder. The absence of a coelom indicates that the animal is yet to develop a functional division of labour to carry out various activities. This indicates towards simplicity of activities going on inside the body. The presence of a coelom paves the way for evolution of a more complex and more efficient organ system for various activities.

Peculiar features in parasitic platyhelminthes:

1. A thick tegument resistant to the host's digestive enzymes and anti-toxins is present.
2. Adhesive organs such as suckers and hooks are found. Examples: Flukes have suckers and hooks and tapeworms have suckers for a firm grip in the host's body.
3. There is a loss of locomotory organs.
4. Digestive organs are absent in tapeworms because digested and semi-digested food of the host is directly absorbed through the body surface.
5. Sense organs are absent in parasitic flatworms.
6. Parasitic flatworms, such as liver fluke and tapeworms, perform anaerobic respiration.
7. They are mostly hermaphrodite. 

Arthropods are the first phylum to have well-developed systems to carry out different activities.

They constitute the largest group in the animal kingdom due to the following reasons:

1. They have jointed appendages for a variety of functions.
2. There is a distinct system for respiration, locomotion and reproduction. Example: They have trachea for respiration and Malpighian tubules for excretion.
3. They can survive in diverse conditions and can live in a variety of habitats like water, land and air.
4. They develop early as compared to animals of higher phylum.
5. They have a tough cuticle for protection of their body.
6. Nervous system and sense organs are better developed.
7. In some insects, pheromones help in communication.

(c) Echinodermata: The presence of a water vascular system or ambulacral system is the characteristic feature of Echinodermata. This system constitutes a perforated plate called madreporite. The pores of madreporite allow water into the system.

All chordates have a notochord present in some stage of life. Chordates include urochordates, cephalochordates and vertebrates. In vertebrates, the notochord is present in the embryonic stage and is replaced by a vertebral column during the adult stages. However, in protochordates, the vertebral column is not present. Therefore, all vertebrates are chordates, but all chordates are not vertebrates.

An air bladder in Pisces regulates buoyancy. It prevents them from sinking in water. This means that members of Pisces do not have to keep on swimming to remain afloat. It is present in Osteichthyes but is absent in Chondrichthyes, so they have to swim constantly to avoid sinking.

Following modifications in birds help them fly:

1. They have pneumatic or hollow bones which form a lightweight skeleton.
2. Forelimbs are modified into wings to assist in flight.
3. The urinary bladder is absent. Excretion of urine and faeces is through a single opening facilitating weight reduction.
4. Flight muscles are well developed.
5. Body is streamlined and offers least resistance while flying. 

Usually the number of eggs produced by oviparous mothers is greater than the number of young ones produced by viviparous mothers. The main reason for this is that in oviparous animals, the major part of development of the embryo takes place outside the uterus. This lessens the burden on the mother. On the other hand, in viviparous animals, the development takes place inside the uterus so lesser number of young ones can be successfully incubated. Moreover, when the eggs are outside, they are at a risk of getting eaten by some predator because of their immobility, so the need of more eggs occurs to ensure the continuity of progeny.

(c) Annelida: The annelids are characterised by metameric segmentation i.e. the body is divided externally by ring-like grooves called annuli and internally by transverse septa. The segments are called metameres.

(a)-(viii); (b)-(v); (c)-(iv); (d)-(i); (e)-(ii); (f)-(vii); (g)-(iii); (h)-(vi)

Parasites on human body:

1. Taenia (Tapeworm)
2. Ascaris (Round worm)
3. Ancylostoma (Hook worm)
4. Enterobius (Pin worm)
5. Wuchereria (Filarial worm)