Structural Organisation in Animals

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. But 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 the 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 multicellular animals, a group of similar cells along with intercellular substances perform a specific function. Such an organisation is called a 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 proportions and patterns 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 an 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 Organ and Organ System

The basic tissues organise to form organs, which in turn associate to form organ systems in 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 types of tissues. For example, our heart consists of all four types of tissues — epithelial, connective, muscular, and neural. We also notice, after some careful study, that the complexity in organ and organ systems displays certain discernable trends. This discernable trend is called the evolutionary trend (you will study the details in Class XII).

In this chapter, you are being introduced to the morphology and anatomy of the frog. Morphology refers to the study of form or externally visible features. In the case of plants or microbes, the term morphology precisely means only this. In the 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.

Figure 7.0 — The four primary animal tissue types (epithelial, connective, muscular, neural) and their major subtypes. Every organ is built from a combination of these.

7.2 Frogs — An Overview

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 — the Indian bullfrog.

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 — this is called 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:

7.2.1 Morphology

Have you ever touched the skin of a 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 the dorsal side of the body is generally olive green with dark irregular spots. On the ventral side, the skin is uniformly pale yellow.

Body of a frog is divisible into head and trunk. A neck and tail are absent. Above the mouth, a pair of nostrils is present. Eyes are bulged and covered by a nictitating membrane that protects them while in water. On either side of eyes, a membranous tympanum (ear) receives sound signals.

Limbs and Locomotion

The forelimbs and hind limbs help in swimming, walking, leaping, and burrowing. The hind limbs end in five digits and they are larger and muscular than forelimbs that end in four digits. Feet have webbed digits that help in swimming.

Sexual Dimorphism

Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of:

• Vocal sacs — sound-producing structures used for mating calls
• Copulatory pad — present on the first digit of the forelimbs (absent in females)

Figure 7.1 — External morphology of Rana tigrina showing body regions, limbs (4 digits forelimb, 5 digits webbed hindlimb), and sexual dimorphism

7.2.2 Anatomy — An Overview

The body cavity of frogs accommodates different organ systems such as digestive, circulatory, respiratory, nervous, excretory, and reproductive systems with well-developed structures and functions.

Figure 7.2 — Organ systems of the frog (Rana tigrina) — all six major systems and their interconnection

7.2.3 Digestive System

The digestive system consists of the alimentary canal and digestive glands. The alimentary canal is short because frogs are carnivores and hence the length of intestine is reduced.

Digestive Glands

• Liver — secretes bile that is stored in the gall bladder
• Pancreas — produces pancreatic juice containing digestive enzymes

Digestion Process

Food is captured by the bilobed tongue. Digestion of food takes place by the action of HCl and gastric juices secreted from the walls of the stomach. Partially digested food called chyme is passed from stomach to the first part of the small intestine, the duodenum.

The duodenum receives bile from gall bladder and pancreatic juices from the pancreas through a common bile duct. Bile emulsifies fat and pancreatic juices digest carbohydrates and proteins. Final digestion takes place in the intestine.

7.2.4 Respiratory System

Frogs respire on land and in the water by two different methods. This dual mode of respiration is a remarkable adaptation for their amphibious lifestyle.

The lungs are a pair of elongated, pink-coloured sac-like structures present in the upper part of the trunk region (thorax). During aestivation and hibernation, gaseous exchange takes place through the skin.

7.2.5 Circulatory System

The vascular system of frog is well-developed and of the closed type. Frogs have a lymphatic system also. The blood vascular system involves heart, blood vessels, and blood. The lymphatic system consists of lymph, lymph channels, and lymph nodes.

Heart

Heart is a muscular structure situated in the upper part of the body cavity. It has three chambers — two atria and one ventricle — and is covered by a membrane called pericardium.

Special Portal Systems

Special venous connections between organs are present in frogs:

• Hepatic portal system — connection between liver and intestine
• Renal portal system — connection between kidney and lower parts of the body

Figure 7.3 — Frog circulatory system showing the 3-chambered heart, arterial and venous systems, and the hepatic & renal portal systems

Blood Composition

The blood is composed of plasma and cells. The blood cells are:

7.2.6 Excretory System

The excretory system of the frog consists of a pair of kidneys, a pair of ureters, a urinary bladder, and the cloaca.

• Kidneys — paired, dark-red, bean-shaped organs located on either side of the vertebral column
• Ureters — in males, the ureters also serve as urinogenital ducts carrying both urine and sperms; in females, the ureters carry only urine
• Urinary bladder — thin-walled, present ventral to the rectum, opens into the cloaca
• Cloaca — a small, median chamber that receives the digestive, excretory, and reproductive openings

7.2.7 Nervous System & Sense Organs

The system for control and coordination is highly evolved in the frog. It includes both the neural system and endocrine glands. The chemical coordination of various organs of the body is achieved by hormones which are secreted by the endocrine glands.

Endocrine Glands

The prominent endocrine glands found in the frog are:

Nervous System Organisation

Brain Structure

The brain is divided into three parts:

• Fore-brain — includes olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon
• Mid-brain — characterised by a pair of optic lobes
• Hind-brain — consists of cerebellum and medulla oblongata

The medulla oblongata passes out through the foramen magnum and continues into the spinal cord, which is enclosed in the vertebral column.

Sense Organs

Frogs have different types of sense organs:

7.2.8 Reproductive System

Frogs have well-organised male and female reproductive systems. The reproductive organs are distinct and exhibit clear sexual dimorphism.

Male Reproductive System — Details

• Vasa efferentia — 10-12 in number, arise from testes
• They enter the kidneys on their side and open into Bidder's canal
• Finally communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca

Female Reproductive System — Details

• Ovaries — situated near kidneys, no functional connection with kidneys
• Oviducts — a pair arising from the ovaries, open into the cloaca separately
• A mature female can lay 2500 to 3000 ova at a time

Fertilisation & Development

7.2.9 Ecological Importance of Frogs

Frogs are beneficial for mankind because they eat insects and protect the crop. Frogs maintain ecological balance because these serve as an important link of food chain and food web in the ecosystem.

✦ Summary

Cells, tissues, organs, and organ systems split up the work in a way that ensures the survival of the body as a whole and exhibit division of labour. A tissue is defined as a group of cells along with intercellular substances performing one or more functions in the body. Epithelia are sheet-like tissues lining the body's surface and its cavities, ducts, and tubes.