100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
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? 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 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
S
TRUCTURAL
O
RGANISATION IN
A
NIMALS
C
HAPTER
7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium. Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
Figure 7.1
Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
(a)
Flattened cell
Cube-like cell
Tall cell
(b)
(d)
(c)
2020-21
102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
are mainly of two types: unicellular,
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
(salivary gland). On the basis of the mode of
pouring of their secretions, glands are
divided into two categories namely
exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
than one layer (multi-layered) of cells and thus
has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular
unicellular
gland
Multicelluar
gland
(a)
(b
)
Figure 7.3 Compound epithelium
Multi-
layered
cells
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
103
Fat storage
area
Nucleus
Macro-
phage
Mast
cell
Fibroblast
Collagen
fibers
Plasma
Membrane
include cartilage, bone, adipose, and blood. In all
connective tissues except blood, the cells secrete fibres of
structural proteins called collagen or elastin. The fibres
provide strength, elasticity and flexibility to the tissue.
These cells also secrete modified polysaccharides, which
accumulate between cells and fibres and act as
matrix (ground substance). Connective tissues are
classified into three types: (i) Loose connective tissue,
(ii) Dense connective tissue and (iii) Specialised
connective tissue.
Loose connective tissue has cells and fibres loosely
arranged in a semi-fluid ground substance, for example,
areolar tissue present beneath the skin (Figure 7.4). Often
it serves as a support framework for epithelium. It
contains fibroblasts (cells that produce and secrete fibres),
macrophages and mast cells. Adipose tissue is another
type of loose connective tissue located mainly beneath the
skin. The cells of this tissue are specialised to store fats.
The excess of nutrients which are not used immediately
are converted into fats and are stored in this tissue.
Fibres and fibroblasts are compactly packed in the
dense connective tissues
. Orientation of fibres show a
regular or irregular pattern and are called dense regular
and dense irregular tissues. In the dense regular
connective tissues, the collagen fibres are present in rows
between many parallel bundles of fibres. Tendons, which
attach skeletal muscles to bones and ligaments which
attach one bone to another are examples of this tissue.
Dense irregular connective tissue has fibroblasts and
many fibres (mostly collagen) that are oriented differently
(Figure 7.5). This tissue is present in the skin. Cartilage,
Figure 7.4 Loose connective tissue : (a) Areolar tissue (b) Adipose tissue
(a)
(b)
fibres
Collagen fibre
(a)
(b)
Figure 7.5 Dense connective tissue:
(a) Dense regular
(b) Dense irregular
2020-21
104 BIOLOGY
bones and blood are various types of specialised
connective tissues.
The intercellular material of cartilage is solid and pliable
and resists compression. Cells of this tissue (chondrocytes)
are enclosed in small cavities within the matrix secreted by
them (Figure 7.6a). Most of the cartilages in vertebrate
embryos are replaced by bones in adults. Cartilage is
present in the tip of nose, outer ear joints, between adjacent
bones of the vertebral column, limbs and hands in adults.
Bones have a hard and non-pliable ground substance
rich in calcium salts and collagen fibres which give bone
its strength (Figure 7.6b). It is the main tissue that provides
structural frame to the body. Bones support and protect
softer tissues and organs. The bone cells (osteocytes) are
present in the spaces called lacunae. Limb bones, such as
the long bones of the legs, serve weight-bearing functions.
They also interact with skeletal muscles attached to them
to bring about movements. The bone marrow in some bones
is the site of production of blood cells.
Blood is a fluid connective tissue containing plasma,
red blood cells (RBC), white blood cells (WBC) and platelets
(Figure 7.6c). It is the main circulating fluid that helps in
the transport of various substances. You will learn more
about blood in Chapters 17 and 18.
7.1.3 Muscle Tissue
Each muscle is made of many long, cylindrical fibres
arranged in parallel arrays. These fibres are composed of
numerous fine fibrils, called myofibrils. Muscle fibres
contract (shorten) in response to stimulation, then relax
(lengthen) and return to their uncontracted state in a
coordinated fashion. Their action moves the body to adjust
to the changes in the environment and to maintain the
positions of the various parts of the body. In general,
muscles play an active role in all the movements of the body.
Muscles are of three types, skeletal, smooth, and cardiac.
Skeletal muscle tissue is closely attached to skeletal
bones. In a typical muscle such as the biceps, striated
(striped) skeletal muscle fibres are bundled together in a
parallel fashion (Figure 7.7a). A sheath of tough connective
tissue encloses several bundles of muscle fibres (You will
learn more about this in Chapter 20).
Platelets
WBC
RBC
(a)
(b)
(c)
Figure 7.6 Specialised connective
tissues : (a) Cartilage
(b) Bone (c) Blood
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
105
The smooth muscle fibres taper at both ends (fusiform) and do not
show striations (Figure 7.7b). Cell junctions hold them together and they
are bundled together in a connective tissue sheath. The wall of internal
organs such as the blood vessels, stomach and intestine contains this type
of muscle tissue. Smooth muscles are ‘involuntary’ as their functioning
cannot be directly controlled. We usually are not able to make it contract
merely by thinking about it as we can do with skeletal muscles.
Cardiac muscle tissue is a contractile tissue present only in the heart.
Cell junctions fuse the plasma membranes of cardiac muscle cells and
make them stick together (Figure 7.7c). Communication junctions
(intercalated discs) at some fusion points allow the cells to contract as a
unit, i.e., when one cell receives a signal to contract, its neighbours are
also stimulated to contract.
7.1.4 Neural Tissue
Neural tissue exerts the greatest control over
the body’s responsiveness to changing
conditions. Neurons, the unit of neural
system are excitable cells (Figure 7.8). The
neuroglial cell which constitute the rest of
the neural system protect and support
neurons. Neuroglia make up more than one-
half the volume of neural tissue in our body.
When a neuron is suitably stimulated,
an electrical disturbance is generated
which swiftly travels along its plasma
Nucleus
Striations
Junction
between
adjacent
cells
Nucleus
Striations
Smooth
muscle
fibers
Figure 7.7 Muscle tissue : (a) Skeletal (striated) muscle tissue (b) Smooth muscle tissue
(c) Cardiac muscle tissue
(a)
(b)
(c)
Figure 7.8 Neural tissue (Neuron with
neuroglea)
Dendrite
Cell
body
with
nucleus
Axon
Neuroglea
2020-21
106 BIOLOGY
membrane. Arrival of the disturbance at the neuron’s endings, or output
zone, triggers events that may cause stimulation or inhibition of adjacent
neurons and other cells (You will study the details in Chapter 21).
7.2 ORGAN AND ORGAN SYSTEM
The basic tissues mentioned above 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 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). You
are being introduced to morphology and anatomy of three organisms at
different evolutionary levels to show their organisation and functioning.
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 earthworm, cockroach and frog representing
invertebrates and vertebrates.
7.3 EARTHWORM
Earthworm is a reddish brown terrestrial invertebrate that inhabits the
upper layer of the moist soil. During day time, they live in burrows made
by boring and swallowing the soil. In the gardens, they can be traced by
their faecal deposits known as worm castings. The common Indian
earthworms are Pheretima and Lumbricus.
7.3.1 Morphology
Earthworms have long cylindrical body. The body is divided into more
than hundred short segments which are similar (metameres about
100-120 in number). The dorsal surface of the body is marked by a dark
median mid dorsal line (dorsal blood vessel) along the longitudinal axis of
the body. The ventral surface is distinguished by the presence of genital
openings (pores). Anterior end consists of the mouth and the prostomium,
a lobe which serves as a covering for the mouth and as a wedge to force
open cracks in the soil into which the earthworm may crawl. The prostomium
is sensory in function. The first body segment is called the peristomium
(buccal segment) which contains the mouth. In a mature worm, segments
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
107
14-16 are covered by a prominent dark band of glandular tissue called
clitellum. Thus the body is divisible into three prominent regions –
preclitellar, clitellar and postclitellar segments (Figure 7.9).
Four pairs of spermathecal apertures are situated on the ventro-lateral
sides of the intersegmental grooves, i.e., 5
th
-9
th
segments. A single female
genital pore is present in the mid-ventral line of 14
th
segment. A pair of
male genital pores are present on the ventro-lateral sides of the 18
th
segment. Numerous minute pores called nephridiopores open on the
surface of the body. In each body segment, except the first, last and
clitellum, there are rows of S-shaped setae
, embedded in the epidermal
pits in the middle of each segment. Setae can be extended or retracted.
Their principal role is in locomotion.
7.3.2 Anatomy
The body wall of the earthworm is covered externally by a thin non-cellular
cuticle below which is the epidermis, two muscle layers (circular and
longitudinal) and an innermost coelomic epithelium. The epidermis is made
Figure 7.9 Body of earthworm : (a) dorsal view (b) ventral view (c) lateral view
showing mouth opening
2020-21
108 BIOLOGY
up of a single layer of columnar epithelial cells
which contain secretory gland cells.
The alimentary canal is a straight tube and
runs between first to last segment of the body.
(Figure 7.10). A terminal mouth opens into the
buccal cavity (1-3 segments) which leads into
muscular pharynx. A small narrow tube,
oesophagus (5-7 segments), continues into a
muscular gizzard (8-9 segments). It helps in
grinding the soil particles and decaying leaves,
etc. The stomach extends from 9-14 segments.
The food of the earthworm is decaying leaves and
organic matter mixed with soil. Calciferous
glands, present in the stomach, neutralise the
humic acid present in humus. Intestine starts
from the 15
th
segment onwards and continues
till the last segment. A pair of short and conical
intestinal caecae project from the intestine on the
26
th
segment. The characteristic feature of the
intestine after 26
th
segment except the last
23
rd
-25
th
segments is the presence of internal
median fold of dorsal wall called typhlosole. This
increases the effective area of absorption in the
intestine. The alimentary canal opens to the
exterior by a small rounded aperture called anus.
The ingested organic rich soil passes through the
digestive tract where digestive enzymes
breakdown complex food into smaller absorbable
units. These simpler molecules are absorbed
through intestinal membranes and are utilised.
Pheretima exhibits a closed type of
blood vascular system, consisting of blood
vessels, capillaries and heart. (Figure 7.11). Due
to closed circulatory system, blood is confined
to the heart and blood vessels. Contractions
keep blood circulating in one direction. Smaller
blood vessels supply the gut, nerve cord, and
the body wall. Blood glands are present on the
4
th
, 5
th
and 6
th
segments. They produce blood
cells and haemoglobin which is dissolved in
blood plasma. Blood cells are phagocytic in
nature. Earthworms lack specialised breathing
devices. Respiratory exchange occurs through
moist body surface into their blood stream.
Mouth
Pharynx
Oesophagus
Gizzard
Stomach
Pre-typhlosolar
part of intestine
Intestinal
caecum
Lymph gland
Typhlosolar
part of intestine
Intestinal lumen
Typhlosole
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
Figure 7.10 Alimentary canal of earthworm
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
109
The excretory organs occur as segmentally
arranged coiled tubules called nephridia
(sing.: nephridium). They are of three types:
(i) septal nephridia, present on both the sides of
intersegmental septa of segment 15 to the last
that open into intestine, (ii) integumentary
nephridia, attached to lining of the body wall of
segment 3 to the last that open on the body
surface and (iii) pharyngeal nephridia, present
as three paired tufts in the 4
th
, 5
th
and 6
th
segments (Figure 7.12). These different types of
nephridia are basically similar in structure.
Nephridia regulate the volume and composition
of the body fluids. A nephridium starts out as a
funnel that collects excess fluid from coelomic
chamber. The funnel connects with a tubular part
of the nephridium which delivers the wastes
through a pore to the surface in the body wall
into the digestive tube.
Nervous system is basically represented by
ganglia arranged segmentwise on the ventral
paired nerve cord. The nerve cord in the anterior
region (3
rd
and 4
th
segments) bifurcates, laterally
encircling the pharynx and joins the cerebral
ganglia dorsally to form a nerve ring. The cerebral
ganglia alongwith other nerves in the ring
integrate sensory input as well as command
muscular responses of the body.
Figure 7.11 Closed circulatory system
Figure 7.12 Nephridial system in earthworm
2020-21
110 BIOLOGY
Sensory system does not have eyes but does
possess light and touch sensitive organs (receptor
cells) to distinguish the light intensities and to feel
the vibrations in the ground. Worms have
specialised chemoreceptors (taste receptors) which
react to chemical stimuli. These sense organs are
located on the anterior part of the worm.
Earthworm is hermaphrodite (bisexual), i.e.,
testes and ovaries are present in the same
individual (Figure 7.13). There are two pairs of
testes present in the 10
th
and 11
th
segments.
Their vasa deferentia run up to the 18
th
segment
where they join the prostatic duct. Two pairs of
accessory glands are present one pair each in
the 17
th
and 19
th
segments. The common prostate
and spermatic duct (vasa deferentia) opens to
the exterior by a pair of male genital pores on
the ventro-lateral side of the 18
th
segment. Four
pairs of spermathecae are located in 6
th
-9
th
segments (one pair in each segment). They receive
and store spermatozoa during copulation. One
pair of ovaries is attached at the inter-segmental
septum of the 12
th
and 13
th
segments. Ovarian
funnels are present beneath the ovaries which
continue into oviduct, join together and open on
the ventral side as a single median female genital
pore on the 14
th
segment.
A mutual exchange of sperm occurs between
two worms during mating. One worm has to find
another worm and they mate juxtaposing
opposite gonadal openings exchanging packets
of sperms called spermatophores. Mature sperm
and egg cells and nutritive fluid are deposited in
cocoons produced by the gland cells of clitellum.
Fertilisation and development occur within the
cocoons which are deposited in soil. The ova
(eggs) are fertilised by the sperm cells within the
cocoon which then slips off the worm and is
deposited in or on the soil. The cocoon holds the
worm embryos. After about 3 weeks, each cocoon
produces two to twenty baby worms with an
average of four. Development of earthworms is
direct, i.e., there is no larva formed.
Figure 7.13 Reproductive system of earthworm
2020-21
STRUCTURAL ORGANISATION IN ANIMALS
111
Earthworms are known as ‘friends of farmers’ because they make
burrows in the soil and make it porous which helps in respiration and
penetration of the developing plant roots. The process of increasing fertility
of soil by the earthworms is called vermicomposting. They are also used
as bait in game fishing.
7.4 COCKROACH
Cockroaches are brown or black bodied animals that are included in
class Insecta of Phylum Arthropoda. Bright yellow, red and green coloured
cockroaches have also been reported in tropical regions. Their size ranges
from ¼ inches to 3 inches (0.6-7.6 cm) and have long antenna, legs and
flat extension of the upper body wall that conceals head. They are
nocturnal omnivores that live in damp places throughout the world. They
have become residents of human homes and thus are serious pests and
vectors of several diseases.
7.4.1 Morphology
The adults of the common species of cockroach, Periplaneta americana
are about 34-53 mm long with wings that extend beyond the tip of the
abdomen in males. The body of the cockroach is segmented and divisible
into three distinct regions – head, thorax and abdomen (Figure 7.14).
The entire body is covered by a hard chitinous exoskeleton (brown in
colour). In each segment, exoskeleton has hardened plates called sclerites
(tergites dorsally and sternites ventrally) that are joined to each other by
a thin and flexible articular membrane (arthrodial membrane).
Figure 7.14 External features of cockroach
2020-21