Notes on Animal Kingdom

4.1 Basis of Classification

The classification of animals is vital due to the vast number of different species—over a million described. A consistent classification helps in organizing and studying the biological diversity. Classification is based on several criteria:

4.1.1 Levels of Organisation

• Cellular Level:
  • Found in Porifera (sponges). Cells are not organized into tissues; they function as loose aggregates.
• Tissue Level:
  • Observed in Cnidaria and Ctenophora, where cells come together to form tissues that perform specific functions.
• Organ Level:
  • Examples include Platyhelminthes (flatworms) where tissues form organs.
• Organ System Level:
  • Seen in more complex animals such as Annelida, Arthropoda, and Chordata, where groups of organs work together in systems (e.g., digestive system).

4.1.2 Symmetry

• Asymmetry: Found in sponges. No symmetrical plane can divide the organism into equal halves.
• Radial Symmetry: Observed in Cnidaria and Ctenophora, allowing division into mirror images around a central axis.
• Bilateral Symmetry: Found in Annelida and Arthropoda; can be divided into two mirror-image halves along one plane.

4.1.3 Diploblastic and Triploblastic Organisation

• Diploblastic Animals:
  • Have two embryonic layers (ectoderm and endoderm), e.g., Cnidaria.
• Triploblastic Animals:
  • Possess an additional third layer (mesoderm), e.g., all animals from Platyhelminthes to Chordata.

4.1.4 Coelom

• Coelomates: Animals like Annelids and Molluscs possess a body cavity lined by mesoderm.
• Pseudocoelomates: E.g., Aschelminthes, have a body cavity that is not entirely lined by mesoderm.
• Acoelomates: Animals like Platyhelminthes, lack a body cavity.

4.1.5 Segmentation

• Refers to the division of the body into segments, seen in Annelids and Arthropods. This division facilitates specialization of body regions.

4.1.6 Notochord

• A dorsal rod-like structure present in some embryos. Chordates possess a notochord at some stage, which is used to classify them as chordates.

4.2 Classification of Animals

Overview of Major Phyla

1. Porifera:

   • Multicellular, exhibit cellular organization, asymmetrical.
   • Water enters through pores and flows into a central cavity (spongocoel).

2. Cnidaria (Coelenterata):

   • Radially symmetrical, with tissue-level organization.
   • Have stinging cells (cnidocytes), exist as polyps or medusae.

3. Ctenophora:

   • Known as comb jellies; have ciliated rows for movement and exhibit bioluminescence.

4. Platyhelminthes:

   • Flatworms, bilaterally symmetrical and acoelomate.

5. Aschelminthes:

   • Roundworms, pseudocoelomate, with separate sexes.

6. Annelida:

   • Segmented worms, coelomate, exhibiting organ-system level of organization.

7. Arthropoda:

   • Largest phylum with jointed appendages, segmented body, and chitinous exoskeleton.

8. Mollusca:

   • Soft-bodied, typically with a calcareous shell.

9. Echinodermata:

   • Marine organisms having endoskeleton and water vascular system; exhibit radial symmetry.

10. Hemichordata:

    • Simple worm-like marine animals featuring a stomochord.

11. Chordata:

    • Characterized by a notochord, hollow nerve cord, and pharyngeal gill slits. Includes vertebrates which have a complex structure and organization.

Chordata Subphyla

• Urochordata (Tunicata):
  Notochord present only in larval stages.
  • Examples: Ascidia.
• Cephalochordata:
  Notochord persists throughout life.
  • Examples: Amphioxus.
• Vertebrata:
  Notochord is replaced by vertebral column.

Vertebrate Classes

1. Cyclostomata: Jawless fish (e.g., lamprey).
2. Chondrichthyes: Cartilaginous fishes (e.g., sharks).
3. Osteichthyes: Bony fishes (e.g., salmon).
4. Amphibia: Frogs and salamanders, living in terrestrial and aquatic habitats.
5. Reptilia: Cold-blooded terrestrial animals with dry scaly skin.
6. Aves: Birds characterized by feathers and lightweight skeleton.
7. Mammalia: Warm-blooded, hair-covered animals with mammary glands for feeding offspring.

1. The classification of animals is essential for understanding biological diversity.
2. Animals exhibit varying levels of organization: cellular, tissue, organ, and organ system.
3. Symmetry types include asymmetry, radial, and bilateral.
4. Animals can be diploblastic or triploblastic based on the number of germ layers.
5. The presence of a coelom (body cavity) is critical for classification.
6. Segmentation is a defining feature in certain phyla, including Annelida and Arthropoda.
7. The notochord distinguishes chordates from non-chordates.
8. Classification is divided into several phyla, including Porifera, Cnidaria, and Chordata.
9. Chordates are characterized by a notochord, hollow nerve cord, and gill slits.
10. The vertebrate subphyla are further divided into distinct groups, including fish, amphibians, reptiles, birds, and mammals.