Invertebrates: Anatomy of a Hydra, Earthworm, and Sea Star (Starfish)

Introduction to Invertebrates

  • Importance of studying invertebrates

    • Understanding evolutionary significance, particularly in worms:

    • First animals with complete digestive tracts, similar to humans.

    • Exhibited sexual dimorphism (distinct male and female organisms).

    • Provides insights into diverse mechanisms of vital biological tasks:

    • Oxygen acquisition for aerobic respiration.

    • Circulation of oxygen within organisms.

  • Objective: Use various online resources to explore the morphology of invertebrates and investigate the anatomy of Hydra, earthworm, and sea star for comparison.

Classical Taxonomy of Invertebrates

  • Early Taxonomic Classification

    • Based on structural similarities in adult organisms and embryonic development stages.

    • Body plan description includes:

    1. Symmetry (distribution of features)

    2. Tissue arrangement

    3. Presence/absence of body cavities.

A. Symmetry

  1. Bilateral Symmetry

    • Body divided by a central plane into mirror-image halves.

    • Typically found in motile organisms with concentrated sense organs, usually located in the anterior (head) region.

    • Key terminology:

      • Anterior: head end

      • Posterior: tail end

      • Dorsal: back side

      • Ventral: belly side

      • Medial: close to midline

      • Lateral: away from midline

      • Proximal: closer to center

      • Distal: farther from center

  2. Radial Symmetry

    • Lacks distinguishable sides; can be divided into equal halves by any plane along a central axis.

    • Common in sessile or slow-moving organisms.

    • Unique terms for orientation:

      • Oral Surface: where the mouth is located

      • Aboral Surface: opposite the mouth

    • Radially symmetric animals exhibit multiple planes of symmetry (e.g., Crown-of-Thorns sea star).

    • Free-swimming larvae demonstrate bilateral symmetry, while adults show radial symmetry.

B. Tissue Structure

  • Definition of tissue: integrated collection of cells with a shared function.

  • Animals are multicellular, heterotrophic eukaryotes developing from germ layers:

    • Diploblastic: two layers (ectoderm, endoderm).

    • Triploblastic: three layers (adding mesoderm); found in bilaterally symmetric animals.

Phylum Cnidaria (Example: Hydra)

  • Overview: Cnidarians are diploblastic, radially symmetric, aquatic animals (mostly marine).

    • Includes over 9,000 species.

    • Exhibit two body configurations:

    • Polyp form: sessile, e.g., corals and sea anemones.

    • Medusa form: free-swimming, e.g., jellyfish.

  • Defining characteristic: Cnidocytes used for prey immobilization.

A. Morphology and Histology of Hydra

  • Body Plan:

    • Cylindrical column with a basal disc (adhesive end) at one end and a mouth at the other end.

    • Tentacles extend from below the mouth (hypostome).

  • Cellular Structure:

    • Epidermis: protective outer layer with contractile fibers.

    • Gastrodermis: inner layer responsible for digestion (contains gland and nutritive cells).

    • Mesoglea: acellular layer providing structural support.

B. Locomotion

  • Hydra can slide along surfaces, somersault, or float by contracting muscle fibers using the gastrovascular cavity as a hydrostatic skeleton.

  • Movement techniques include inching along through base disc or somersaulting.

C. Diet and Prey Acquisition

  • Hydras feed on plankton using tentacles that trigger nematocysts to capture prey via stinging.

  • Nutrients absorbed through the gastrovascular cavity for enzymatic digestion.

D. Excretion

  • Lacks a true excretory system; waste diffuses into surrounding water.

  • Indigestible waste expelled through the mouth (also serves as anus).

E. Circulation and Respiration

  • No circulatory system; relies on diffusion for gas exchange, aided by water currents.

F. Nervous System

  • Contains a decentralized nerve net; generalized responses to stimuli observed.

G. Reproductive Strategies

  • Asexual Reproduction: Budding when conditions are favorable.

  • Sexual Reproduction: Gonads develop under adverse conditions. Hermaphroditic, producing both ovaries and testes.

Phylum Annelida (Example: Earthworm - Lumbricus terrestris)

  • Importance: First animals to demonstrate true coelomic cavity and body segmentation (metamerism) allowing compartmentalization of functions.

A. Morphology and Histology

  • Segmented body showing division by septa; anterior (clitellum) and posterior regions.

  • Internal Structure:

    • Complete digestive tract lined with intestinal wall invaginated to form typhlosole for increased surface area.

  • Setae found on segments for movement and anchoring.

B. Locomotion

  • Earthworm movement via alternating contractions of circular and longitudinal muscles aided by a hydrostatic skeleton.

C. Diet and Nutrition

  • Earthworms are detritivores, recycling nutrients in the soil. Food intake involves grinding of soil in the gizzard.

D. Excretion

  • Nephridia function as filtration systems to remove wastes from coelomic fluid, similar to kidney function in vertebrates.

E. Circulatory System

  • Possess a closed system with hemoglobin dissolved in blood, using muscular blood vessels for circulation.

F. Respiration

  • Gas exchange occurs through skin as earthworms require moist environments for oxygen diffusion.

G. Nervous System

  • More centralized nervous system compared to Hydra; includes brain (ganglia) and ventral nerve cord.

H. Reproduction

  • Earthworms are hermaphroditic but require mating. Structures include testes, seminal vesicles, and ovaries. Fertilization occurs externally with the clitellum secreting a cocoon for eggs.

Dissection Procedures

  • Detailed steps on the dissection of earthworm:

    1. Use correct incisions to expose internal structures.

    2. Document the anatomical features, blood vessels, and nerve systems encountered.

    3. Cross-section of the intestine to observe the typhlosole and other associated structures.