Animal Kingdom

Learning Outcomes:

1. Understand fundamental classification principles of the animal kingdom.
2. Identify major characteristics used in categorizing animals.
3. Differentiate between diploblastic and triploblastic animals.
4. Distinguish between open and closed circulatory systems.

Animals present vast diversity in form and structure. With over a million species identified, classification is vital to understand their relationships and systematic positioning. Here, the basis of classification and major animal phyla are discussed in detail.

Basis of Classification

Though animals exhibit diverse structures, several fundamental features form the basis of their classification. These include cellular arrangement, body symmetry, coelom development, and various physiological systems. Below are the core principles of animal classification.

Levels of Organisation

Animals in the kingdom Animalia are all multicellular, but they do not have the same organisation of cells:

1. Cellular Level: Sponges have cells organized as loose aggregates, with minimal specialization. This represents the cellular level of organisation.
2. Tissue Level: In coelenterates, cells performing similar functions form tissues, showing the tissue level of organisation.
3. Organ Level: Flatworms (Platyhelminthes) and higher animals demonstrate an organ level, where tissues form distinct organs for specific functions.
4. Organ System Level: In animals such as annelids, arthropods, and chordates, organs group into functional systems. These animals exhibit organ system level organisation.

Organ systems vary in complexity. For example:

• The digestive system in Platyhelminthes is incomplete, having only one opening.
• The circulatory system can be open, where blood bathes tissues directly, or closed, where blood flows within vessels.

Symmetry

Symmetry describes the arrangement of body parts:

1. Asymmetry: Found in sponges, where no plane divides the body into equal halves.
2. Radial Symmetry: Seen in coelenterates and echinoderms, where any plane passing through the central axis creates identical halves.
3. Bilateral Symmetry: Animals like annelids and arthropods exhibit bilateral symmetry, where only one plane divides the body into two equal halves.

Diploblastic and Triploblastic Organisation

Based on the number of embryonic layers:

1. Diploblastic Animals: Coelenterates have two layers—ectoderm and endoderm—separated by a non-cellular layer called mesoglea.
2. Triploblastic Animals: Animals from Platyhelminthes to Chordata have a third layer called mesoderm between the ectoderm and endoderm.

Important Concept:
Diploblastic organisms, like cnidarians, have simpler tissue structures, while triploblastic organisms exhibit more complex body systems due to the presence of mesoderm.

Coelom

The coelom is the body cavity formed between the body wall and the gut, crucial for classification:

1. Coelomates: Animals with a coelom, such as annelids and chordates.
2. Pseudocoelomates: Animals like aschelminthes, which have a body cavity not fully lined by mesoderm.
3. Acoelomates: Animals such as Platyhelminthes that lack a coelom.

Segmentation

Certain animals show a division into segments, such as earthworms (metamerism), where both external and internal segmentation occurs.

Notochord

The notochord is a rod-like structure seen in chordates during embryonic development, distinguishing them from non-chordates.

Classification of Animals

Kingdom Animalia is broadly classified into various phyla based on fundamental features.

Phylum – Porifera

1. Sponges are aquatic, mostly marine, and asymmetrical. They exhibit a cellular level of organisation and possess a water transport system for feeding, respiration, and excretion.
2. Reproduction: Hermaphrodites; reproduction can be both sexual and asexual.

Phylum – Coelenterata (Cnidaria)

1. These animals have tissue-level organisation and are diploblastic. They show radial symmetry and possess cnidoblasts, which are specialized stinging cells used for defense and capturing prey.
2. Two Body Forms: The sessile polyp and the free-swimming medusa.
3. Reproduction: Sexual and asexual (metagenesis).

Phylum – Ctenophora

1. Known as comb jellies, these marine animals show radial symmetry and have eight rows of ciliated comb plates for locomotion.
2. Bioluminescence: They can emit light, a characteristic of ctenophores.

Phylum – Platyhelminthes

1. Commonly known as flatworms, they are acoelomates with bilateral symmetry.
2. Reproduction: Hermaphrodites with internal fertilization and complex life cycles.

Phylum – Aschelminthes

1. Also known as roundworms, they have bilateral symmetry and a pseudocoelom.
2. Reproduction: Dioecious with internal fertilization.

Phylum – Annelida

1. Segmented worms with a true coelom, they show metamerism.
2. Circulatory System: Closed, and they have specialized excretory structures called nephridia.

Phylum – Arthropoda

1. The largest phylum, includes insects and other species with jointed appendages and a chitinous exoskeleton.
2. Respiratory Systems: Gills, book lungs, or tracheal systems.

Phylum – Mollusca

1. Molluscs are soft-bodied animals, often with a calcareous shell.
2. Respiration: Through gills or lungs.

Phylum – Echinodermata

1. Spiny-skinned animals, exclusively marine, exhibit radial symmetry in adulthood and bilateral symmetry in the larval stage.
2. Unique Features: Water vascular system aids in movement and food gathering.

Phylum – Hemichordata

1. Worm-like marine animals with a rudimentary notochord (stomochord).
2. Circulation: Open type with external fertilization.

Phylum – Chordata

Chordates are defined by the presence of:

1. Notochord.
2. Dorsal, hollow nerve cord.
3. Pharyngeal gill slits.

Note: Not all chordates possess a vertebral column, but all vertebrates are chordates.

Comparison Table: Chordates vs Non-Chordates

Feature	Chordates	Non-Chordates
Notochord	Present	Absent
Nervous System	Dorsal, hollow, single	Ventral, solid, double
Gill Slits	Present	Absent
Heart	Ventral	Dorsal (if present)
Tail	Present (post-anal)	Absent

Chordates are further divided into:

1. Urochordata: Notochord present only in the larval tail.
2. Cephalochordata: Notochord extends from head to tail and persists throughout life.
3. Vertebrata: Notochord replaced by a vertebral column during development.

Subphylum Vertebrata

Vertebrates possess a muscular heart, kidneys for osmoregulation, and paired appendages. The subphylum is divided as follows:

Class Cyclostomata

1. Jawless vertebrates that are parasitic on fishes.
2. Respiration: Through gill slits.

Class Chondrichthyes

1. Cartilaginous fishes with streamlined bodies.
2. Notochord persists throughout life.
3. Reproduction: Mostly viviparous.

Class Osteichthyes

1. Bony fishes with a terminal mouth and air bladder for buoyancy.
2. Reproduction: Oviparous with external fertilization.

Class Amphibia

1. Dual existence (land and water); respiration occurs through lungs, gills, and skin.
2. Reproduction: Oviparous with external fertilization.

Class Reptilia

1. Cold-blooded animals with dry skin and scales.
2. Heart: Three-chambered (except in crocodiles, which have four).

Class Aves

1. Birds with feathers and wings.
2. Reproduction: Oviparous with internal fertilization.

Class Mammalia

1. Mammals possess mammary glands and body **hair

**.

2. Reproduction: Mostly viviparous, with internal fertilization and direct development.

Important Note: Mammals are distinguished by their capacity for viviparity, nourishing young with milk.

MCQ:
Which of the following classes is characterized by cartilaginous endoskeletons?
Answer: Chondrichthyes

This extensive discussion outlines the classification of the animal kingdom, considering critical aspects such as levels of organization, symmetry, coelom, and distinct phylum characteristics.