Invertebrates: Anatomy of Hydra, Earthworm, and Sea Star

Chapter 4: Invertebrates - Anatomy of a Hydra, Earthworm, and Sea Star (Starfish)

INTRODUCTION

• Importance of Studying Invertebrates

  • Questions the value of studying organisms like worms.

  • Evolutionary Significance:

  • Worms were the first animals to develop a complete digestive tract, closely resembling human anatomy.

  • Introduced the concept of sexual dimorphism, which involves having distinct male and female organisms in the same species.

  • Objective of Study:

  • To compare different mechanisms animals use to perform essential life functions, specifically focusing on invertebrates like Hydra, earthworms, and sea stars, to understand their adaptations to environmental challenges.

  • Approach:

  • Utilize online resources to explore the morphology of various invertebrates and their habitats before an in-depth examination of anatomical structures.

  • Complete a comparison chart synthesizing information gathered during the study.

I. CLASSICAL TAXONOMY

• Early Taxonomic Classification:

  • Based on adult structural similarities and common embryonic features in development.

• Body Plan Characteristics:

  • Can be described via multiple dimensions:

  • 1. Symmetry: Distribution of body features in specific planes.

  • 2. Tissue Arrangement: How an organism’s tissues are organized.

  • 3. Cavity Presence: The existence or absence of a body cavity that houses internal organs.

II. TISSUES

• Definition of Tissues:

  • An integrated collection of cells functioning as a unit for a specific purpose, originating from embryonic layers.

• Development Process:

  • Majority of animals start from a fertilized egg (zygote) through a process called cleavage, leading to the formation of a blastula (hollow ball of cells).

  • Gastrulation:

  • Inward movement of blastula cells leads to the formation of the archenteron (primitive gut), establishing a body structure that includes ectoderm and endoderm.

  • Key Stages:

    • Blastula: Hollow stage post-cleavage.

    • Gastrula: Multilayered structure post-gastrulation.

    • Germ Layers:

    • Ectoderm: Skin and nervous system.

    • Endoderm: Digestive tract and organs.

    • Mesoderm: Connective tissues like the skeleton, muscles, and circulatory components.

• Diploblastic vs. Triploblastic:

  • Diploblastic animals (e.g., jellyfish, Hydra) have two germ layers (ectoderm & endoderm).

  • Triploblastic animals have three layers (adding mesoderm), leading to more complex structures.

  • Protostomes vs. Deuterostomes:

  • In protostomes, the blastopore becomes the mouth; in deuterostomes, it becomes the anus, leading to classification distinctions.

A. Protostome vs. Deuterostome Development

• Protostome Development:

  • Type: Schizocoelous

  • Embryonic Structure: Development leads to mouth formation from the blastopore.

• Deuterostome Development:

  • Type: Enterocoelous

  • Embryonic Structure: Anus formation from the blastopore; mouth develops secondarily.

III. BODY CAVITIES

• Human Body Cavity:

  • Divided by the diaphragm into thoracic and abdominal cavities.

• Cavity Presence in Animals:

  • Diploblastic Animals (e.g., Hydra): No true body cavity; ectoderm and endoderm layers are directly connected via mesoglea.

  • Coelomates (e.g., Earthworm):

  • Possess true body cavities (coelom) lined by mesoderm.

  • Coelom serves multiple functions: cushioning organs and acting as a hydrostatic skeleton.

• Examples of Body Cavity Classes:

  • Coelomate: Body cavity fully lined with mesoderm.

  • Acoelomates: No body cavity (e.g., flatworms).

IV. PHYLUM ANNELIDA (EXAMPLE: EARTHWORM [Lumbricus terrestris])

• Worm Diversity:

  • Common assumption links worms to earthworms, but they inhabit a variety of environments (marine, freshwater, parasitic).

  • Importance of worms in evolution: First to exhibit triploblastic tissue layers.

• Significance of Annelida:

  • Simplest group with a true coelomic cavity and distinct organ regions (digestive, circulatory, excretory).

  • Introduction of metamerism (body segmentation), advancing toward more complex structures.

A. Morphology and Histology

• External Features:

  • Lengthy, segmented body (metameres).

  • Anterior region differentiated by a clitellum, indicating reproductive specialization.

  • Dorsal vs. Ventral Side:

  • Dorsal: Darker and more rounded.

  • Ventral: Flatter and paler.

• Epidermis:

  • Consists of sensory and secretory cells producing a cuticle to prevent dehydration.

• Muscle Layers:

  • Circular Muscles: Contract perpendicularly to body length.

  • Longitudinal Muscles: Run parallel and provide structural integrity.

• Setae:

  • Bristle-like structures aiding in locomotion, often felt externally on the worm.

B. Locomotion

• Mechanism:

  • Achieved via alternating contractions of circular and longitudinal muscles, utilizing the hydrostatic skeleton effect.

• Movement Example:

  • When setae anchor into soil while circular muscles contract, the anterior segments push forward.

  • Alternatively, the contraction of posterior segments drags the anterior end through the soil.

C. Diet and Acquisition

• Feeding Habits:

  • Earthworms are detritivores; they consume decomposing organic matter, crucial for nutrient recycling.

• Feeding Process:

  • Soil enters the buccal cavity, passes through the pharynx to the esophagus, then into the crop for temporary storage.

  • The gizzard mechanically grinds the food, enhancing digestive capabilities.

• Nutrient Assimilation:

  • Nutrient uptake occurs in the intestine, wherein chloragogue cells play a role akin to a vertebrate liver, processing carbohydrates, proteins, and nitrogenous wastes.

D. Excretion

• Nephridia:

  • Present in all segments except the terminal ones; responsible for waste filtration and urine expulsion.

• Excretion Process:

  • Waste materials enter nephridia filters from coelomic fluid, are processed and then expelled out via the nephridopore.

E. Circulation

• System Type:

  • Earthworms have a closed circulatory system with blood contained within vessels.

  • Blood flow is facilitated by the dorsal blood vessel, which acts as the main pumping unit.

• Vessel Network:

  • Connective pseudo-hearts function to maintain blood circulation throughout the body, ensuring effective nutrient delivery and waste removal.

F. Respiration

• Respiratory Mechanism:

  • Gas exchange occurs through the epidermis; requires moist conditions.

• Oxygen and Carbon Dioxide Exchange:

  • Oxygen diffuses into blood, carbon dioxide diffuses out across the skin surface.

• Temporary Challenges:

  • Overly moist conditions can be fatal due to drowning; needs to surface after heavy rains to avoid suffocation.

G. Nervous System

• Cephalization:

  • Earthworms exhibit the concentration of sensory and neural structures at the anterior end, a notable advancement over simpler organisms.

• Neuronal Structures:

  • Comprises a pair of ganglia (suprapharyngeal and subpharyngeal) connected by circumpharyngeal connective nerves and a ventral nerve cord extending throughout the body.

H. Reproduction

• Reproductive Strategy:

  • Earthworms are monoecious, possessing both male and female organs, yet cross-fertilization is required.

• Mating Process:

  • Two worms align ventrally, exchanging sperm; cocoon formation and fertilization occur externally, leading to juvenile worm development despite one fertilized egg per cocoon surviving.

IV. DISSECTION

• Purpose:

  • To explore and understand the internal anatomy of the earthworm.

• Dissection Steps:

  • Utilize the "pop-top" method for anterior openings and expose organs sequentially based on the provided guidelines.

• Focus Areas:

  • Examine muscle fiber structure, identify nervous system components, and trace the digestive tract from mouth to anus.

• Visual Inspection:

  • Note the structures highlighted in the accompanying figures, consider the functional significance of each organ discovered during dissection.