Basis of Classification | Biology for Grade 11 PDF Download

Earth is home to over a million described species of animals, each with unique structures and forms. Classification is essential to organize this vast diversity.
Classification helps in assigning a systematic position to newly described species. It provides a framework for placing new discoveries within the context of existing knowledge.

Diversity of Organisms

Basis of Classification

Classification enables a systematic study of the diverse animal kingdom. It allows scientists to group similar species together, making it easier to study and understand them.

The classification system assists in organizing animals by grouping them according to these essential characteristics given  below:

1. Levels of Organisation

(a) Cellular level Organisation

In this level of organisation , organisms consist of cells that are loosely gathered together. A prime example is seen in sponges. 

• In these simple animals, individual cells carry out various functions with limited specialization. There is minimal division of labor among cells, and they work collectively to perform basic life processes.

Sponges

(b) Tissue level Organisation

The tissue level of organization represents an intermediate stage between the cellular level and the organ level. 

• It allows for more specialization and division of labor among cells, leading to the formation of tissues with specific functions within these simple aquatic organisms.
• Example - Coelenterates, which include organisms like jellyfish and corals, exhibit a tissue level of organization. 

Coelenterates 

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(c) Organ level Organisation

At the organ level, the basic tissues are further organized into distinct organs. Organs are composed of multiple tissues that work together to perform specific, more specialized functions.

• This organization allows for a higher degree of complexity and efficiency in carrying out life processes.
• Organisms at the organ level possess specialized organs for various physiological functions. 
• For example, in Platyhelminthes, you can find organs such as the digestive system (with a mouth and gut), sensory organs like eyespots for light detection, and excretory structures for waste removal.

Platyhelminthes

(d) Organ system level Organisation

Organ system-level organisation refers to the hierarchical arrangement of organs and tissues within the human body to perform specific functions and maintain overall homeostasis. 

• The human body is composed of several organ systems, each with a unique set of organs and tissues that work together to carry out specific functions. 
• Example: Annelids, Arthropods, Molluscs, Echinoderms, and Chordates have organs organized into systems for specific functions.

Organisms with organ level organisation

2. Symmetry

Symmetry refers to a characteristic feature of an organism's body structure, specifically its arrangement and organisation of body parts. 

• Symmetry is used as a criterion to categorize and classify organisms into different taxonomic groups. There are three main types of symmetry:
  (a)  Asymmetry: Organisms with asymmetry lack any visible symmetry in their body structure.
  (i) They do not have a distinct plane or axis through which their body can be divided into similar or mirror-image halves.
  (ii) Asymmetrical organisms are often simple in structure and lack a clear orientation.
  (iii) Example: sponges ( Porifera)
  
  (b) Radial Symmetry: Radially symmetrical organisms have body parts arranged around a central axis, much like spokes on a wheel.
  (i) They can be divided into similar halves by multiple planes passing through the central point.
  (ii) Radial symmetry is commonly found in aquatic or sessile organisms like jellyfish, sea anemones, and starfish.
  (iii) Example: Coelenterates, Ctenophores, and Echinoderms
  
  (c)  Bilateral Symmetry: Bilaterally symmetrical organisms have a body plan in which their body can be divided into two nearly identical mirror-image halves by a single plane along their longitudinal axis.
  (i) This type of symmetry is typical of more complex, mobile animals. Humans, for example, exhibit bilateral symmetry.
  (ii) Example: Annelids, arthropods

Different types of Symmetry

3. Diploblastic and Triploblastic Organisation

Diploblastic and triploblastic organisation are terms used to describe different levels of germ layer development in the early embryonic development of animals. Germ layers are distinct layers of cells that form during gastrulation, the process by which the embryo transforms from a single-layered structure into a more complex, multi-layered structure.

(i) Diploblastic Organisation: In diploblastic organisms, there are two primary germ layers that develop during gastrulation: the ectoderm and the endoderm.

• Ectoderm: The outermost layer, the ectoderm, gives rise to the epidermis (outer skin layer) and the nervous system in more complex organisms.
• Endoderm: The innermost layer, the endoderm, forms the lining of the digestive tract and associated organs, such as the liver and pancreas, in more complex animals.

Diploblastic organisation is typically found in simpler, radially symmetrical animals like cnidarians (e.g., jellyfish and corals). These animals lack a true mesoderm, which is a third germ layer that gives rise to structures like muscles, bones, and most internal organs in more complex organisms.

(ii) Triploblastic Organisation: In triploblastic organisms, there are three primary germ layers that develop during gastrulation: the ectoderm, mesoderm, and endoderm.

• Ectoderm: Similar to diploblastic organisms, the ectoderm gives rise to the epidermis and the nervous system.
• Mesoderm: The mesoderm is the middle layer and is a significant development in triploblastic animals. It gives rise to various tissues, including muscles, connective tissues, the circulatory system, and many internal organs such as the kidneys and reproductive organs.
• Endoderm: As in diploblastic organisms, the endoderm forms the lining of the digestive tract and associated organs.

Triploblastic organisation is found in more complex animals, including most vertebrates (animals with a backbone) and many invertebrates. These animals have a greater level of developmental complexity compared to diploblastic organisms because they have three distinct germ layers, which allows for the formation of a wider range of tissues and organs.

4. Coelom

The term "coelom" refers to the body cavity found in many animals. This body cavity is a fluid-filled space that separates the digestive tract (endoderm) from the outer body wall (ectoderm). It is lined by a layer of mesodermal tissue

There are three such categories divided on the basis of the presence or absence of coelom.

(a) Coelomates:  Animals that possess a true coelom, which is a fluid-filled body cavity entirely lined by mesoderm, one of the three primary germ layers.

• They are typically more complex in terms of anatomy and organization because the presence of a coelom allows for the development of specialized organs and systems.
• Examples of coelomates include annelids (segmented worms), molluscs (like snails and clams), arthropods (including insects and crustaceans), echinoderms (such as starfish and sea urchins), hemichordates (a group of marine invertebrates), and chordates (including vertebrates like mammals, birds, reptiles, amphibians, and fish).

(b) Pseudocoelomates: Pseudocoelomates are animals with a body cavity known as a pseudocoelom, which is not fully lined by mesoderm but is located between the ectoderm (outer tissue layer) and the endoderm (inner tissue layer).

• This arrangement results in scattered pockets or pouches of coelom-like spaces within the body.
• Pseudocoelomates, such as aschelminthes (roundworms), benefit from having a body cavity, allowing for some internal organ development and mobility, but it is less advanced than a true coelom in terms of structural support and complexity.

(c) Acoelomates: Acoelomates are animals that lack a true body cavity. They do not have a coelom or pseudocoelom.

• Their bodies are typically solid and filled with tissues. As a result, they have limited space for organ development and mobility.
• Examples of acoelomates include platyhelminthes, which are flatworms. These animals are relatively simple in structure due to the absence of a body cavity.

5. Segmentation

Metamerism, also known as serial segmentation, is a fundamental biological phenomenon observed in certain animals where the body is divided into a series of repeating, similar segments. 

• These segments often exhibit a high degree of external and internal similarity and typically contain a set of organs or structures that repeat from one segment to the next. 
• One of the well-known examples of metamerism is found in earthworms.

Segmentation in Earthworm

6. Notochord

 The notochord is a rod-like structure that forms during embryonic development in some animals, particularly those belonging to the group called chordates. Animals with a notochord are called chordates, while those without it are called non-chordates.

• Chordates: These animals have a notochord at some point during their development, and it serves as a key structural feature. Chordates include a wide range of animals, including humans and other vertebrates, as well as some simpler organisms like tunicates and lancelets.
• Non-chordates: These animals do not develop a notochord. This group encompasses many different animal phyla, such as sponges, jellyfish, flatworms, insects, and starfish.

Phylum ChordatesThe presence or absence of the notochord is a fundamental characteristic used in the classification of animals into these two major categories, reflecting their evolutionary relationships and developmental traits.

Body Cavity or Coelom

On the basis of coelom, animals can be:

(i) Acoelomate: The animals in which the coelom is absent are called as Acoelomates, for example flatworms. In them, the space between ectoderm and endoderm is filled with parenchyma eg. Platyhelminthes.

(ii) Pseudocoelomate: The body cavity is not completely lined with mesoderm. Instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called as pseudo-coelom e.g. roundworm. 

(iii)  Eucoelomate: The true coelom is a body cavity that arises as a cavity in embryonic mesoderm. In this case, the mesoderm of the embryo provides a cellular lining, called as coelomic epithelium or peritoneum, to the cavity. The coelom is filled with coelomic fluid secreted by the peritoneum. The coelom is found in Arthropods, Molluscs, Annelids, Echinoderms, Hemichordates, and Chordates. 

The true coelom is of two types:

(a)  Schizocoelom: It develops by the splitting up of mesoderm. It is found in annelids, arthropods, and molluscs. The body cavity of arthropods is called hemocoel.      

(b) Enterocoelom: The mesoderm arises from the wall of the embryonic gut or enteron as hollow outgrowths or enterocoelomic pouches. It occurs in Echinoderms, Hemichordates, and Chordates.

Segmentation

–    In some animals, the body is externally and internally divides into segments or metameres with serial repetition of atleast some organs. For example, in earthworm, the body shows metameric segmentation and the phenomenon is known as metamerism.

Notochord 

– Notochord is a mesodermally derived rod-like structure formed on the mid-dorsal surface during embryonic development in some animals. Animals with notochord are called chordates and those animals that do not form this structure are called non-chordates, e.g., Porifera to Echinoderms or Hemichordates.

Blood Vascular System

Blood vascular system is basically of two types:
Open and Closed

(i)   Open type: In open type, the blood is pumped by the heart into the blood vessels that open into blood spaces (sinuses). There is no capillary system (i.e., most arthropods, some molluscs except cephalopods and tunicates). These sinuses are actually the body cavities and are called hemocoel. The pressure of the blood is low; it moves slowly between the tissues, and finally, returns to the heart via the opened veins. In fact, the distribution of blood in the tissues is very poorly controlled. The pigments, which carry oxygen, remain dissolved in blood plasma. Body tissues and visceral organs exchange respiratory gases, nutrients, and waste products, directly with blood.  

(ii) Closed type: Many invertebrates and all vertebrates, including humans, have a closed circulatory system.  In the closed type, the blood flows around the body through the specific blood vessels. In this system, the same blood regularly circulates in the body under high pressure and returns back to the heart without leaving the system of tubes. The heart pumps the blood into the aorta, which branches in the body into the arteries, and in the tissues into the arterioles, to form the capillary network. The venules of the capillary network carry the blood back to the heart via veins and the vena cava. This helps in supplying the nutrients and oxygen to the tissues, and removing waste materials and caron dioxide from it reveals a comparison between open and closed circulatory systems.  

Habitat

• Habitat is the place where an organism lives. On the basis of habitat, animals are divided into - aquatic and terrestrial.
• Aquatic animals live in water. Types of aquatic animals are zooplankton, nekton and benthon.

• Zooplanktons are passively floating or animals in water. E.g., protozoans, protists etc.
• Nekton are actively swimming aquatic organisms in water, able to move independently of water currents. E.g., Shark, Bony fishes.
• Benthon lives at bottom. It may be sedentary or motile. E.g., starfish, sponges, etc.
• Types of terrestrial animals are
  • Cursorial (Runfast) – E.g., kangaroo, dog
  • Fossorial (lives in burrows/underground) – E.g., earthworm, rabbit.
  • Arboreal (lives on trees) E.g., bat, monkey
  • Aerial/flying (can fly) – E.g., birds, bats winged insects.

Level of Organisation

• All animals are multicellular, but they have different levels of organisation.
• It is of five types – acellular, cellular, tissue, organ and organ system.
  

• Sponges exhibit cellular level of organisation where cells are loosely arranged.
• Coelenterates exhibit tissue level of organisation where cells of similar functions are grouped together.
• Platyhelminthes and higher phyla exhibit organ-level of organisation where tissues form organs specialised for specific functions.
• Annelids, Arthropods, Molluscs, Echinoderms and Chordates exhibit organ system level of organisation where organs function together to perform specific physiological functions.

Patterns of Organ Systems

Digestive System: A digestive system is a group of organs in animals that work together to break down food into nutrients that can be absorbed by the body. It includes organs such as the mouth, esophagus, stomach, small intestine, large intestine, liver, and pancreas, each with its own specific function in the process of digestion and absorption.

There are two forms of digestive systems Complete and Incomplete Digestive system.

• Incomplete Digestive System – This form of digestive system has one and only opening to the outside of the body, i.e., a solitary opening serving as both mouth and rear-end. Hence, the digestive system is incomplete.
• Complete Digestive System – In this form there are two different openings to the outside of the body, a mouth and a rear-end or anus.

Circulatory System: A circulatory system is a group of organs and tubes that moves blood, oxygen, nutrients, and other important things through an animal's body. It includes the heart, blood vessels, and blood. It helps the body stay healthy by controlling temperature, pH, and fluids.

The circulatory system are of two types:

• Open type, where blood is pumped out of the heart and directly bathes the cells and tissues.
• Closed type, where blood is circulated through vessels of varying diameters.