OTHER IMPORTANT
FEATURES
Apart from the basic features of body, size and shape,
animals also possess some other important features as given
below
l. Cephalisation
It is the differentiation of head in anterior part of the body.
It involves the concentration of nervous tissue and sense
organs in the head.
2. Appendages
The projecting structures of the body that perform specific
functions like locomotion, capturing of food, sensation,
etc., are called appendages, e.g., wings, fins, limbs,
tentacles, parapodia, setae, etc.
3. Digestive System
Digestive tract is the passage through which food is taken
in for digestion, absorption and egestion. The digestive
tract that has a single opening for both ingestion and
egestion is called incomplete digestive tract, e.g., in
flatworms and coelenterates.
The digestive tract with two external openings, one for
ingestion and other for egestion is called complete
digestive tract. It is present in aschelminthes and higher
animals.
4. Respiratory System
Respiration occurs in different ways in different animals
(i) The tiny aquatic animals like Amoeba, Hydra, etc.,
respire through the body surface. This is called
body surface respiration.
(ii) Larger aquatic animals have special organs called
gills for respiration. This is called branchial
respiration, e.g., in prawns, fishes and mussels.
(iii) The land animals respire through lungs. This is
called pulmonary respiration. It occurs in frogs,
snails, lizards, birds and mammals.
(iv) Insects have tracheal respiration, which occurs
through trachea, i.e., an intercommunicating tube
through which gaseous exchange occurs.
(v) In animals like earthworm, leech, frogs, etc., moist
skin acts as respiratory surface. This is called
cutaneous respiration.
(vi) Scorpions have book lungs and king crabs have
book gills for respiration.
vii) In frog, gaseous exchange also occurs through the
lining of buccopharyngeal cavity. Hence, called
buccopharyngeal respiration. Frogs have three
modes of respiration, i.e., cutaneous,
buccopharyngeal and pulmonary.
5. Circulatory System
The circulatory system is responsible for the transport
materials in the body. It consists of a fluid, blood vessels and
a heart.
The circulatory system is closed when blood flows within
blood vessels.
If blood flows in spaces, and the sinuses are without proper
boundries, the circulatory system is called open.
In insects, such as prawn and Pila, blood is colourless that
contains haemocyanin (a copper containing pigment), while
in vertebrates, blood contains haemoglobin (an
iron-containing pigment).
6. Excretory System
Excretory system is involved in the removal of nitrogenous
waste products from the body of an organism with the help
of excretory organs.
The excretion occurs in different ways in different
organisms. Such as
(i) The excretory organs are absent in those organisms
where organisation level is below the tissue level.
Here, each individual cell takes part in excretion.
(ii) In animals like sponges, coelenterates, all the cells are
in contact with water. Excretion occurs by general
body surface.
(iii) In vertebrates, kidneys are the excretory organs.
Based on excretory products animals can be classified into
four categories as given below
(i) Aminotelic, excretory product is amino acids,
e.g., starfish, Unio, etc.
(ii) Ammonotelic, excretory product is ammonia,
e.g., most invertebrates and some molluscs.
(iii) Ureotelic, excretory product is urea,
e.g.. cartilaginous fishes, snail, prawn, mammals and
aquatic reptiles.
‘(iv) Uricotelic, excretory product is uric acid, e.g., Insects
terrestrial crustaceans, lizards, snakes, birds, etc.
7. Nervous System
The nervous system is the aggregation of nerve cells or
neurons that help in coordinating and controlling various
activities of the body.
8. Endocrine System
The endocrine glands are also called ductless glands. These
secrete hormones. Endocrine glands occur in all vertebrates
and in some invertebrates.
9. Sensory System
This system consists of specialised cells, tissues and organs
which can pick up a stimulus and transmit the same to the
nervous system. Sensory system consists of different
structures in different organisms, e.g., antennal (tactile and
skin (tactile), statocyst
smell),
tentacles
(tactile),
(balancing), ear (hearing), olfactory epithelium (smell),
taste buds (taste), eyes (vision), lateral line organs (current
receptors), etc.
10. Skeletal System
Skeletal system is a hard, internal or external framework
that provides support and shape to the body. Some animals
which are devoid of a skeleton have soft body, e.g.,
platyhelminthes, aschelminthes, annelids.
Skeleton system can be of following types
i. Exoskeleton
It is the hard supporting and protective framework present
on exterior of the body. It is made of non-living matter,
e.g., external shells of molluscs, cuticle of arthropods, scales
of fishes and reptiles, feathers of birds, hair, hoofs, nails,
horns and claws of mammals.
ii. Endoskeleton
It is a hard supporting framework present in the interior of
the body. In invertebrates such as sponges, it is made up of
calcareous or siliceous spicules. In vertebrates,
it is
composed of hard living tissues called cartilages and bones.
Endoskeleton supports whole body of an organism.
Knowledge Plus
• Nervous system was first evolved in cnidarians.
• Ascaris and earthworm are both ammonotelic and ureotelic,
• Development of living and growing endoskeleton in vertebrates
has enabled them to attain large Size.
11. Sex
Animals generally have sex organs to reproduce sexually.
When both male and female sex organs are found in same
individual, it is called hermaphrodite or bisexual or
monoecious, e.g., liver fluke, tapeworm, earthworm, leech,
etc.
The animals with either female or male sex organ is known
as unisexual or dioecious, e.g., frog, lizards, birds, dog, etc.
When male and female can be distinguished on the basis of
external features, the condition is called sexual dimorphism,
e.g., lion and lioness, man and woman, peacock and peahen,
etc.
12. Reproduction
Reproduction in organisms can be either asexual or sexual.
i. Asexual Reproduction
This kind of reproduction does not involve fusion of
It is found in lower animals like sponges, coelenterates,
annelids, platyhelminthes. The common methods are budding,
fission, fragmentation and regeneration.
ii. Sexual Reproduction
It involves formation and fusion of gametes. The male
gametes called sperms are motile while, the female gametes
called ova are generally non-motile.
13. Fertilisation
Fertilisation in animals is of two types
i. External Fertilisation
Animals such as many invertebrates, some marine fishes and
most amphibians, shed both eggs and sperms into water,
where fertilisation and development occur. This is called
ii. Internal Fertilisation
In land animals and some aquatic animals, the sperms are
introduced by the male into reproductive tract of female
during copulation. This is called internal fertilisation.