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Animal Unit Overview

  • Main Topics Covered:
    • Animal diversity and evolution
    • Body plans and physiology
    • Gas exchange mechanisms
    • Overview of organ systems
    • Movement and reproduction strategies
    • Immune responses and behavior
    • Interesting animal behaviors (e.g., sleeping, exercising)
  • Referencing Studies: Taboada et al. (2022), Science.

Key Quote

  • Charles Darwin (On the Origin of Species):
    • "…from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

Course Information

  • Unit: Animals
  • Subject: Animal Diversity and Evolution I
  • Date: Feb. 16, 2026
  • Instructor: Dr. Ana Longo
  • Course Code: BSC2011-Spring 2026

Chapter 22 Outline

  • 22.1: Distinct Body Plans Evolved among the Animals
  • 22.2: Large Multicellular Animal Groups Arose in Parallel
  • 22.3: Protostomes Have an Anterior Brain and a Ventral Nervous System
  • 22.4: Arthropods Are Diverse and Abundant

Learning Objectives

  • Key Skills to Acquire:
    • Explain differences in body symmetry, cavities, segmentation, appendages, and nervous systems and how these relate to movement and feeding.
    • Apply knowledge of phylogenetic relationships to describe the evolution and loss of major structural features.
    • Describe morphological innovations in arthropods and their influence on diversity.

Defining Characteristics of Animals

  • Criteria that Classify Animals:
    • Multicellularity: All animals are composed of multiple cells.
    • Heterotrophic Metabolism: Animals cannot produce their own food; they rely on consuming other organisms.
    • Internal Digestion: Processes involved in breaking down food occur within a digestive cavity.
    • Movement and Nervous Systems: Animals possess mechanisms for movement and nervous systems to coordinate it.
  • Evolutionary Note: All animals share a common ancestor and are classified as a monophyletic group.

Animal Phylogeny

  • Major Groups:
    • Sponges: Do not have true tissues
    • Ctenophores: Appeared next; possess nerve nets
    • Placozoans: Simple structure, no mouth or gut
    • Cnidarians: Possess specialized cells called nematocysts for capturing prey
    • Protostomes and Deuterostomes: Different embryonic development paths (discussed further)

Differing Early Development in Triploblastic Animals

  • Deuterostomes:
    • The blastopore develops into the anus; mouth forms later.
  • Protostomes:
    • The blastopore develops into the mouth; anus forms later.

Animal Body Plan Features

  • 1. Symmetry:

    • Asymmetrical: No plane of symmetry (e.g., sponges, placozoans)
    • Radial Symmetry: Body parts arranged around a central axis; can be divided into mirror images along several planes.
    • Bilateral Symmetry: Divisible into two equal halves along a single plane, characteristic of most animals.

  • 2. Body Cavity Structure (Coelom):

    • Tube-within-a-tube Model:
    • Inner tube: digestive tract
    • Outer tube: body wall (skin and muscles)
    • Coelom: fluid-filled space between the tubes
Types of Body Cavity Structures

  • Acoelomates:

    • Example: Flatworm
    • Characteristics: No enclosed body cavities; mesenchyme present.

  • Pseudocoelomates:

    • Example: Roundworms
    • Characteristics: Pseudocoel filled with fluid, lined with mesoderm; no mesoderm surrounds internal organs.

  • Coelomates:

    • Example: Earthworms
    • Characteristics: True coelom fully surrounded by mesoderm, housing internal organs.

  • Importance of Coelom:

    • Provides space for organ development and independent movement of organs.

Segmentation

  • Definition and Function:
    • Facilitates the specialization of different body regions
    • Allows for greater control of movements and body shape
  • Note: Arthropod diversity was influenced significantly by segmentation changes.

Appendages

  • Functionality:
    • Enhance movement (e.g., legs, fins)
    • Assist in various tasks such as chewing, capturing prey, sensory perception, and reproduction.

Nervous Systems

  • Diverse Structures:
    • Nerve Nets: Present in simple organisms like ctenophores and cnidarians.
    • Centralized Nervous Systems: Most bilateral animals, with complexities allowing coordinated muscle movements and sensory processing.

Early Animal Evolution

  • Four Non-Bilaterian Groups

    • Sponges, ctenophores, placozoans, cnidarians.
    • Sponges and ctenophores are the earliest lineages to diverge.
    • Discussion exists on the timing of these splits.

  • Choanoflagellate Protists:

    • Closely related to sponges; cells in colonies specialized and contributed to the rise of more complex animals.

Ctenophores (Comb Jellies)

  • Characteristics:
    • Possess radial symmetry and are diploblastic.
    • Movement facilitated by cilia arranged on eight comb-like structures called ctenes.
    • Prey adheres to sticky cells on tentacles for capture.

Placozoans

  • Description:
    • Asymmetric, diploblastic animals with only four distinct cell types.
    • Lacks mouth, gut, or nervous system.

Cnidarians

  • Examples Include: Jellyfish, sea anemones, corals, hydrozoans.
  • Anatomy:
    • Gastrovascular cavity serves multiple functions (digestion, circulation, gas exchange, hydrostatic skeleton).
  • Special Structures:
    • Nematocysts: harpoon-like stinging structures utilized for capturing prey.

Arthropods and Their Relatives

  • Diversity:
    • Include chelicerates (e.g., spiders, horseshoe crabs), myriapods (centipedes, millipedes), crustaceans, and hexapods (insects and relatives).
  • Chelicerates:
    • Modified mouthparts (chelicerae) for grasping and four pairs of walking legs.
  • Mandibulates:
    • Mouthparts (mandibles) for chewing and biting.
  • Crustaceans:
    • Diverse group includes various marine species like shrimps, lobsters, and crabs.
  • Hexapods (Insects):
    • Characterized by six legs, specific body segmentation (head, thorax, abdomen), and may have wings.

Evolutionary Impacts

  • Land Plants' Role:
    • Greatly influenced the evolution and diversification of animal life around -500 million years ago.
  • Pterygote Insects:
    • First flying animals; flight presented new lifestyle and feeding opportunities.
    • Homologous genes govern the development of insect wings and crustacean appendages.

Next Steps

  • Upcoming Video Presentation: Will be posted by the end of day 2/18/26.
  • Next Chapter Focus:
    • 22.5 Deuterostomes: encompasses echinoderms, hemichordates, and chordates.
    • 22.6 Life on land and its impact on vertebrate diversification.