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.

TRUCTURAL

RGANISATION IN

NIMALS

HAPTER

7.1 Animal Tissues

7.2 Organ and Organ

System

7.3 Earthworm

7.4 Cockroach

7.5 Frogs

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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)

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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)

)

Figure 7.3 Compound epithelium

Multi-

layered

cells

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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

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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

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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

(a)

(b)

(c)

Figure 7.8 Neural tissue (Neuron with

neuroglea)

Dendrite

Cell

body

with

nucleus

Axon

Neuroglea

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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

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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

-9

segments. A single female

genital pore is present in the mid-ventral line of 14

segment. A pair of

male genital pores are present on the ventro-lateral sides of the 18

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

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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

segment onwards and continues

till the last segment. A pair of short and conical

intestinal caecae project from the intestine on the

segment. The characteristic feature of the

intestine after 26

segment except the last

-25

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

, 5

and 6

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

Figure 7.10 Alimentary canal of earthworm

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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

, 5

and 6

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

and 4

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

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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

and 11

segments.

Their vasa deferentia run up to the 18

segment

where they join the prostatic duct. Two pairs of

accessory glands are present one pair each in

the 17

and 19

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

segment. Four

pairs of spermathecae are located in 6

-9

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

and 13

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

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

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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

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