Unique characteristics of animals 1

Study Summary: Unique Characteristics of Animals (Kingdom Animalia)

This document provides a comprehensive overview of the Kingdom Animalia, detailing general characteristics, key anatomical features across phyla, and a phylum-by-phylum breakdown, along with comparative summaries and clarifications.

General Characteristics of Animals

• Multicellular organisms: Bodies composed of many cells with specialized functions.
• Germ Layers: Diploblastic (two germ layers: ectoderm, endoderm) or Triploblastic (three germ layers: ectoderm, mesoderm, endoderm).
• Nutrition: Heterotrophic – ingest food, cannot synthesize their own.
• Body Symmetry: Asymmetrical (sponges), Radial (cnidarians, echinoderms), or Bilateral (most animals).
• Cephalization: Concentration of sensory organs and nervous tissue at the anterior end (head).
• Body Cavity Types:
  • Acoelomate: No body cavity.
  • Pseudocoelomate: False cavity, not fully lined by mesoderm.
  • Coelomate: True cavity lined by mesoderm.
• Segmentation (Metamerism): Body divided into repeated units (e.g., annelids, arthropods, chordates).
• Circulatory System:
  • Open: Hemolymph bathes organs directly.
  • Closed: Blood confined to vessels.
• Reproduction: Mostly sexual; some reproduce asexually.
• Locomotion: Most animals are motile (muscles, limbs, cilia, flagella); few are sessile.

Key Anatomical Features Across Animal Phyla (Summary)

• Symmetry:
  • Radial: Body parts arranged around a central axis (e.g., Cnidaria, Echinodermata adults).
  • Bilateral: Mirror-image left/right halves (e.g., Platyhelminthes, Annelida, Arthropoda, Chordata).
• Body Cavity:
  • Acoelomate: No cavity.
  • Pseudocoelomate: Cavity not fully lined (e.g., Nematoda).
  • Coelomate: Cavity lined by mesoderm (e.g., Annelida, Arthropoda, Mollusca, Chordata).
• Segmentation: Repeated body segments (metameres) often with redundant organs (e.g., Annelids, Arthropods, Chordates).
• Cephalization: Concentration of nervous tissue and sense organs in head region. Well-developed in flatworms, arthropods, vertebrates; absent in sponges and cnidarians.

Phylum-by-Phylum Classification: Key Features

Phylum Porifera (Sponges)

• Common Names: Sponges ("pore-bearing").
• Features: Non-motile (sessile), multicellular, no true tissues/organs, outer skeleton (spicules/spongin).
• Canal System: Pores (ostia) and canals for water circulation, filter-feeding, gas exchange.
• Symmetry: No head or tail, asymmetrical.
• Habitat: Mostly marine; few freshwater.
• Reproduction: Sexual (sperm/eggs) and asexual (budding, gemmules).
• Examples: Spongia, Spongilla.

Phylum Cnidaria (Coelenterata)

• Body Form: Hollow body cavity (coelenteron/gastrovascular cavity), single opening for mouth/anus.
• Tissue Level: Diploblastic (two cell layers: ectoderm, endoderm) with mesoglea in between.
• Symmetry: Radial symmetry.
• Specialized Cells: Cnidocytes (stinging cells) for defense and prey capture.
• Forms: Polyp (sessile, e.g., Hydra) and Medusa (free-swimming, e.g., jellyfish).
• Examples: Hydra, jellyfish, corals, sea anemones.

Phylum Platyhelminthes (Flatworms)

• Characteristics: Dorsoventrally flattened body, bilaterally symmetrical, triploblastic.
• Body Cavity: Acoelomate (no true body cavity).
• Digestive System: Incomplete gut (single opening for mouth/anus) – some parasites lack a digestive system.
• Lifestyle: Free-living (Planaria) or parasitic (tapeworms, flukes).
• Nervous System: Primitive "ladder-type" with cerebral ganglia (primitive brain).

Phylum Nematoda (Roundworms)

• Shape: Cylindrical, unsegmented, tapered at both ends.
• Symmetry: Bilateral; triploblastic.
• Body Cavity: Pseudocoelomate (false body cavity, not fully lined by mesoderm).
• Habitat: Free-living in soil/water or parasitic (e.g., ascariasis, hookworm).
• Digestive System: Complete alimentary canal (mouth, intestine, anus).
• Examples: Ascaris, hookworm, pinworm.

Phylum Annelida (Segmented Worms)

• Key Features: Segmented body (metamerism), cylindrical, bilaterally symmetrical, triploblastic.
• Body Cavity: True coelom (schizocoelom).
• Circulatory System: Closed circulatory system.
• Habitat: Marine, freshwater, moist terrestrial (earthworms).
• Movement: Uses setae (bristles) and muscular hydrostatic skeleton.
• Examples: Earthworm (Lumbricus), leech (Hirudo), marine bristle worms.

Phylum Arthropoda (Jointed Appendages)

• Largest Phylum: Over 80% of all animal species.
• Key Features: Jointed appendages, segmented body (head, thorax, abdomen), exoskeleton of chitin.
• Symmetry: Bilateral; triploblastic; true coelom (reduced).
• Circulatory System: Open circulatory system (hemolymph, no differentiated blood vessels).
• Growth: Molting (ecdysis) to grow.
• Examples: Insects (mosquito, butterfly, bedbug, fly), crustaceans, arachnids, myriapods.

Phylum Mollusca (Mollusks)

• Characteristics: Soft-bodied, often with a hard calcareous shell (reduced in some).
• Body Plan: Muscular foot, visceral mass, mantle.
• Symmetry: Bilateral; triploblastic; coelomate.
• Circulatory System: Open circulatory system (except cephalopods like octopus – closed).
• Habitat: Marine, freshwater, terrestrial.
• Examples: Snail, octopus, squid, clam, oyster.

Phylum Echinodermata (Spiny Skins)

• Examples: Starfish (sea star), sea urchin, brittle star, sea cucumber.
• Symmetry: Adults radial (usually pentamerous), larvae bilateral – secondarily derived radial symmetry.
• Skeleton: Endoskeleton of calcareous plates (ossicles) with spines.
• Body Cavity: Coelomate; triploblastic.
• Unique System: Water vascular system for locomotion, feeding, respiration.
• Habitat: Exclusively marine.

Phylum Hemichordata & Chordata (Dorsal Nerve Cord)

• Hemichordata: Acorn worms; open circulatory system, separate sexes, external fertilization; possess pharyngeal slits but no notochord.
• Phylum Chordata (Most complex):
  • Possess notochord at some stage (flexible rod supporting the body).
  • Bilaterally symmetrical, triploblastic, coelomate.
  • Circulatory System: Closed circulatory system (most chordates).
  • Key Features: Dorsal hollow nerve cord, pharyngeal slits, post-anal tail.
  • Subphyla: Urochordata (tunicates), Cephalochordata (lancelets), Vertebrata (fish, amphibians, reptiles, birds, mammals).

Comparative Summary: Major Animal Phyla

Additional Important Notes

• Circulatory System Types:
  • Open: Arthropods, most mollusks – hemolymph bathes organs directly.
  • Closed: Annelids, chordates, some mollusks – blood remains in vessels, more efficient.
• Skeletal Support:
  • Hydrostatic skeleton: Annelids.
  • Exoskeleton: Arthropods (chitin).
  • Endoskeleton: Echinoderms (calcareous plates); Chordates (bone/cartilage).
• Reproduction: Asexual reproduction common in some phyla (sponges, cnidarians). Most animals reproduce sexually with complex life cycles.
• Locomotion: Most animals are motile (muscles, cilia, flagella). Sessile animals include sponges, corals, barnacles.
• Body Cavity Functional Significance: Coelom provides space for organ development, acts as a hydrostatic skeleton, and allows internal transport.
• Germ Layers:
  • Diploblastic: Ectoderm and endoderm (cnidarians).
  • Triploblastic: Ectoderm, mesoderm, endoderm (all higher animals).

Quick Revision: Key Defining Features & Unique Facts

Corrections & Clarifications

• Circulatory System in Chordates: Chordates have a closed circulatory system. Only a few primitive chordates have a less developed system, but vertebrates have a closed system with a heart.
• Pseudocoelomates: Nematodes are true pseudocoelomates – body cavity not completely lined by mesoderm.
• Segmentation: Annelids show internal and external segmentation; arthropods have segmented bodies with jointed appendages.
• Bilateral Symmetry and Cephalization: Flatworms show early cephalization; arthropods and chordates have highly developed heads with concentrated sense organs.