Introduction to Animal Diversity

Animal Diversity Overview

• Diversity refers to the variety and forms of animals, ranging from the smallest to the largest species.
• Terrestrial animals struggle with temperature variations due to weak chemical bonds regulating proteins and enzymes, which are affected by temperature changes.
• Adaptations to temperature vary:
  • Endotherms (birds and mammals) maintain a constant internal temperature.
  • Ectotherms adjust their body temperature according to the environment for energy efficiency.

General Features of Animals

• Animals are unicellular or multicellular heterotrophs, dependent on plants for food.
• About 98% of animals are invertebrates, existing in diverse forms and sizes, from microscopic to gigantic.
• Animal phyla distribution: predominantly found in the sea, followed by freshwater, then land.
• Major phyla dominating terrestrial life: Arthropoda, Mollusca, and Chordata.
• Animals lack rigid cell walls, allowing cells to form flexible tissues for specific functions (e.g., nerve tissues).

Movement and Reproduction

• Active movement is a distinctive feature, facilitated by the evolution of nervous and muscular tissues.
• Most animals reproduce sexually, with immobile eggs larger than flagellated sperm and typically no alternation of generations.

Embryonic Development

• Similar embryonic development patterns across most animals:
  • Zygote undergoes mitosis to form a morula, a solid ball of cells, leading to the blastula (hollow cell ball).
  • Gastrula stage may involve the formation of the blastophore, which develops into the digestive system.

Subkingdom Classification

• Two main subkingdoms:
  • Parazoa: Symmetry, tissues, and organs are absent (e.g., sponges).
  • Eumetazoa: Exhibits symmetry, definitive shapes, and organized tissues into organ systems.

Body Plan Features

• Animal classification hinges on morphological and developmental characteristics, culminating in bilateral symmetry.
• Key distinguishing traits include tissue layer count during development, presence/absence of body cavities, and embryological features.

Key Transitions in Body Plan Evolution

• Evolution of tissues marks the initial key transition; simplest animals (parazoans) lack defined tissues and organs.
• Bilateria display enhanced mobility, aiding in food acquisition and predator avoidance, with sense organs concentrated at the anterior region.
• Cephalization: Evolutionary trend towards the development of a head and brain area.

Body Cavity Evolution

• Emergence of body cavities marks a crucial evolution in complexity, allowing efficient organ systems.
• Cavities support organs, distribute materials, and facilitate intricate interactions, enhancing digestion and reproduction.

Types of Body Cavities in Bilateria

• Acoelomate: No body cavity.
• Pseudocoelomate: Body cavity between mesoderm and endoderm.
• Coelomate: Coelom within the mesoderm, providing proper organ support and circulation.

Circulatory Systems

• Open circulatory system: Blood fills body cavities (hemocoel) to bathe tissues.
• Closed circulatory system: Blood separated from other fluids for controlled circulation, improving efficiency.

Protostomes vs. Deuterostomes

• Coelomates differ in embryonic development:
  • Protostomes: Mouth develops from blastopore.
  • Deuterostomes: Anus develops from blastopore.
• Coelom formation:
  • Protostomes undergo schizocoely (splitting mesoderm).
  • Deuterostomes utilize enterocoely (pouches formed from endoderm).

Cleavage Types

• Protostomes exhibit spiral cleavage; cells are misaligned.
• Deuterostomes undergo radial cleavage; cells align parallel or perpendicular to the polar axis.

Evolution of Segmentation

• Segmentation (metamerism): Body divided into segments, influenced by mesoderm initially, and later reflected in ectoderm/endoderm.
• True segmentation is evident in annelids, arthropods, and chordates.
• Benefits of segmentation:
  • Each segment can develop its organ systems and replicate functions if damaged.
  • Enhanced locomotion through independent segment movement.