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STRUCTURAL ORGANISATION IN ANIMALS 79C HAPTER 7
S TRUCTURAL O RGANISATION IN
ANIMALS
7.1 Organ and Organ In the preceding chapters you came across a large variety of organisms,
System both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
7.2 Frogs
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? As you have already learnt in your earlier classes, in
multicellular animals, a group of similar cells alongwith intercellular
substances perform a specific function. Such an organisation is called
tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 O RGAN AND ORGAN SYSTEM
The basic tissues as you have learnt in earlier classes, organise to form
organs which in turn associate to form organ systems in the multicellular
organisms. Such an organisation is essential for more efficient and better
coordinated activities of millions of cells constituting an organism. Each
Rationalised 2023-24
, 80 BIOLOGY
organ in our body is made of one or more type of tissues. For example,
our heart consists of all the four types of tissues, i.e., epithelial, connective,
muscular and neural. We also notice, after some careful study that the
complexity in organ and organ systems displays certain discernable trend.
This discernable trend is called evolutionary trend (You will study the
details in class XII). In this chapter, you are being introduced to
morphology and anatomy of frog. Morphology refers to study of form or
externally visible features. In the case of plants or microbes, the term
morphology precisely means only this. In case of animals this refers to
the external appearance of the organs or parts of the body. The word
anatomy conventionally is used for the study of morphology of internal
organs in the animals. You will learn the morphology and anatomy of
frog representing vertebrates.
7.2 FROGS
Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry. You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.
7.2.1 Morphology
Have you ever touched the skin of frog? The skin is smooth and slippery
due to the presence of mucus. The skin is always maintained in a moist
condition. The colour of dorsal side of body is
Head generally olive green with dark irregular spots. On
Trunk
the ventral side the skin is uniformly pale yellow.
Eye The frog never drinks water but absorb it through
the skin.
Body of a frog is divisible into head and trunk
(Figure 7.1). A neck and tail are absent. Above the
Fore limb mouth, a pair of nostrils is present. Eyes are bulged
and covered by a nictitating membrane that
Hind limb protects them while in water. On either side of eyes
a membranous tympanum (ear) receives sound
Figure 7.1 External features of frog signals. The forelimbs and hind limbs help in
Rationalised 2023-24
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