Introduction On Animal Diversity Diversity
Introduction to Animals
Unit 4
General Features of Animals
Heterotrophy
Animals obtain energy and organic molecules by ingesting other organisms. This means they cannot produce their own food and must consume existing organic matter, exhibiting predatory, herbivorous, or parasitic feeding strategies.
Multicellularity
Many animals possess complex bodies composed of numerous cells, which are organized and specialized for diverse forms and functions, allowing for a division of labor among cells and tissues.
No Cell Walls
Unlike plants (cellulose) and fungi (chitin), animal cells lack rigid cell walls. This absence contributes to greater tissue flexibility, allowing for more diverse cell shapes and active movement.
Active Movement
Animals generally exhibit a remarkable ability to move rapidly and in complex ways, driven by muscular contraction and neural coordination, enabling activities such as hunting, escaping predators, and migrating.
Diversity of Form
Animals show significant variability in form and size, ranging from microscopic organisms (like rotifers) to large species, such as blue whales, reflecting a vast array of adaptations to different ecological niches.
Additional General Features of Animals
Diversity of Habitat
Animals are classified into approximately 35 to 40 phyla. The vast majority are found in marine environments, with significant numbers also thriving in freshwater and diverse terrestrial habitats like deserts, forests, and atmospheric zones, showcasing extensive adaptive radiation.
Sexual Reproduction
The majority of animals reproduce sexually, typically involving the fusion of male and female gametes. This often involves small, motile sperm fertilizing larger, nonmobile eggs, leading to genetic recombination and increased diversity. Many animals are diploid dominant throughout their life cycle.
Embryonic Development
Following fertilization, the zygote undergoes rapid mitotic cell divisions (cleavage) leading to the formation of a multicellular hollow ball of cells called a blastula. This is followed by gastrulation, which establishes the primary germ layers.
Tissues
In most animals, cells organize into distinct structural and functional units known as tissues (e.g., epithelial, connective, muscular, nervous tissue). These specialized tissues allow for highly complex bodily functions and integrated physiological processes.
Evolution of the Animal Body Plan
There are five key innovations that have occurred in the evolution of animal body plans, each contributing fundamentally to the diversification and complexity of animal life:
Symmetry: The arrangement of body parts relative to an axis.
Tissues: The organization of specialized cells into functional units.
Body cavity: Internal fluid-filled spaces providing support and facilitating organ development.
Various patterns of embryonic development: Distinct stages and processes during early growth.
Segmentation: The repetition of body units, enhancing flexibility and specialization.
Evolution of Symmetry
Sponges exhibit no definitive symmetry, being asymmetrical aggregates of cells. In contrast, most other animals demonstrate either radial or bilateral symmetry, defined by imaginary axes through the body.
Types of Symmetry
Radial Symmetry
Body parts are arranged concentrically around a central oral-aboral axis. This allows division into equal halves by any plane passing through the central axis (e.g., cnidarians like jellyfish, adult echinoderms). This type of symmetry is advantageous for sessile or slow-moving organisms that encounter their environment from all directions.
Bilateral Symmetry
The body has distinct right and left halves that are mirror images of each other. Only a single sagittal plane can bisect the organism into two equal, symmetrical halves (e.g., most mobile animals, including humans, insects, and worms).
Advantages of Bilateral Symmetry
Cephalization
The evolutionary development and concentration of sensory organs (eyes, antennae) and nervous tissue (brain) at the anterior (head) end of the body. This provides a distinct advantage for complex organisms by allowing them to efficiently detect and respond to their environment as they move forward.
Directional Movement
Bilateral symmetry is strongly associated with efficient, directed movement through the environment. The anterior end leads the way, equipped with sensory and feeding structures, while the posterior end often contains the anus and locomotor structures.
Specialized Tissues
The zygote is initially totipotent, meaning it has the potential to give rise to any and all body cell types. However, as development progresses through cell division and differentiation, cells specialize into distinct tissue types.
This process of cell specialization is generally irreversible in most animals, meaning a differentiated cell cannot typically revert to an earlier state or become another cell type.
The exception is sponges, whose cells retain some plasticity and can disaggregate and reaggregate to re-form a functional organism, reflecting their simpler tissue organization. All other animals possess distinct, well-defined tissues, with most cell types undergoing irreversible differentiation into specialized roles.
Evolution of the Body Cavity
Most animal embryos develop three distinct primary germ layers during gastrulation, classifying them as triploblastic animals:
Ectoderm: The outermost germ layer, which forms the outer body coverings (epidermis), the nervous system (brain and spinal cord), and sensory organs.
Mesoderm: The middle germ layer, which develops into the skeleton (bones, cartilage), muscles, the circulatory system, excretory organs, and the reproductive system.
Endoderm: The innermost germ layer, which forms the lining of the digestive tract (gastrovascular cavity) and associated glands (liver, pancreas), as well as the lining of the respiratory system.
All triploblastic animals exhibit bilateral symmetry. In contrast, cnidarians are diploblastic (comprised of only ectoderm and endoderm, with a non-cellular mesoglea in between), while sponges lack true germ layers altogether, highlighting their more primitive evolutionary position.
Body Plans
Body cavity: Refers to the internal fluid-filled space surrounded by mesodermal tissue that forms during embryonic developmental processes. The presence and type of body cavity are crucial for organ development and movement.
Three Basic Kinds of Body Plans:
Acoelomates: Organisms that lack a fluid-filled body cavity. The space between the ectoderm and endoderm is completely filled with mesodermal tissue (parenchyma), which limits internal organ complexity and relies on diffusion for transport (e.g., flatworms).
Pseudocoelomates: Animals with a body cavity (pseudocoelom) that is positioned between the mesoderm and endoderm, meaning it is not entirely lined by mesodermal tissue. This pseudocoelom provides a hydrostatic skeleton and space for organs but less organization than a true coelom (e.g., roundworms/Nematoda).
Coelomates: Organisms that possess a true body cavity (coelom) entirely encompassed within mesoderm. The coelom is lined by a mesodermal membrane called the peritoneum, which also suspends and supports the internal organs, allowing for independent movement of organs and more complex physiological systems (e.g., annelids, mollusks, chordates).
Circulatory Systems
The evolution of a true body cavity (coelom) greatly facilitated the advancement of complex organ systems, including the development of a circulatory system. This system is vital for enhancing nutrient flow and efficient waste removal throughout the body, especially in larger animals.
Open Circulatory System: Blood (hemolymph) is pumped from vessels into open spaces called sinuses (hemocoel), where it directly bathes the internal organs and tissues before returning to the vessels. This system is less efficient for fast-moving or large animals due to lower pressure (e.g., arthropods, most mollusks).
Closed Circulatory System: Blood is continuously circulated and confined within a network of vessels (arteries, capillaries, veins) that remain separate from the interstitial body fluids. This allows for higher blood pressure, more efficient delivery of oxygen and nutrients, and better regulation of blood flow (e.g., annelids, cephalopod mollusks, vertebrates).
Different Patterns of Development
The basic Bilaterian developmental pattern includes a series of rapid mitotic cell divisions known as cleavage. These divisions result in the formation of a multicellular, hollow ball of cells called the blastula, which contains a fluid-filled cavity called the blastocoel.
The blastula then undergoes a process called gastrulation, involving invagination (inward folding) of cells, leading to the formation of a two-layered body plan with:
Blastopore: The initial opening to the exterior formed during gastrulation.
Archenteron: The primitive gut cavity formed during gastrulation, which will eventually develop into the digestive tract.
Groups of Bilaterians
Bilaterians, characterized by bilateral symmetry and triploblasty, can be categorized into two major evolutionary groups based on their embryonic development patterns:
Protostomes: In these animals, the mouth forms first, either from or near the blastopore. The anus develops later from another distinct region of the embryo.
Deuterostomes: In these animals, the anus develops first from the blastopore, followed by the formation of the mouth from a different region of the embryo.
Differences Between Protostomes and Deuterostomes
Cleavage Pattern of Embryonic Cells:
Protostomes: Typically exhibit spiral cleavage, where new cells formed during division are offset and located in the furrows between previous cells, resulting in a rotational arrangement.
Deuterostomes: Display radial cleavage, where new cells develop directly on top of or beneath previous cells, aligning symmetrically and forming neat tiers.
Developmental Fate of Cells:
Protostomes: Feature determinate development, meaning the developmental fate of each embryonic cell is irrevocably set early in development. If an early cell is removed, the embryo will not develop completely.
Deuterostomes: Have indeterminate development, where the developmental fate of cells remains flexible for several cleavage divisions. Each early embryonic cell is totipotent and can develop into a complete organism if separated, explaining how identical twins can form.
Segmentation
Evolution of Segmentation:
Segmentation, the division of the body into a series of repeated units or segments (serial homology), has emerged multiple times independently in animal evolution (e.g., annelids, arthropods, chordates), providing two main evolutionary advantages:
Allows for the existence of redundant organ systems in adults (e.g., repeated excretory organs or ganglia in annelids), meaning damage to one segment may not be fatal.
Facilitates more efficient and flexible movement. The independent movement of individual segments, often driven by separate muscle contractions, allows for precise and sophisticated locomotion.
Classification of Animals
Multicellular animals (Kingdom Animalia or Metazoa) are broadly categorized into distinct phyla based on fundamental body plan characteristics.
Animals can be divided into two primary branches:
Parazoa: A basal group of animals lacking definitive true tissues, organs, and symmetry; exemplified by Phylum Porifera (sponges).
Eumetazoa: A vast group containing all animals with organized true tissues, distinct embryonic layers, a nervous system, and a defined shape and symmetry. Within Eumetazoa:
Cnidaria: Diverges early in the evolutionary tree, preceding the split into Bilateria. They are diploblastic and radially symmetrical.
Bilateria: Further categorized into Deuterostomes and Protostomes, all of which are triploblastic and bilaterally symmetrical.
Key Animal Phyla Overview
Table 32.1: Common Phyla, Examples, and Characteristics
Arthropoda (e.g., beetles, crabs, spiders, lobsters):
Characteristics: Possess a rigid chitinous exoskeleton that must be shed (molted), a highly segmented body, jointed appendages, and adapted to diverse habitats including aquatic, terrestrial, and aerial environments. They are the most species-rich phylum.
Approximate Number of Named Species: 1,200,000
Mollusca (e.g., snails, octopuses, clams, slugs):
Characteristics: Coelomate body typically covered by a calcium carbonate shell (though absent in some like slugs and octopuses), a muscular foot for locomotion, a visceral mass containing organs, and often a unique rasping tongue-like organ called a radula for feeding.
Approximate Number of Named Species: 150,000
Chordata (e.g., mammals, fish, birds, reptiles, amphibians):
Characteristics: Defined by the presence of four key features at some stage of development: a notochord (flexible rod), a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Many undergo significant metamorphosis.
Approximate Number of Named Species: 56,000
Platyhelminthes (e.g., flatworms, tapeworms, flukes):
Characteristics: Unsegmented, acoelomate (lacking a body cavity), bilaterally symmetrical, and dorsoventrally flattened. They are often parasitic forms, but free-living species exist. Respiration occurs via diffusion.
Approximate Number of Named Species: 55,000
Table 32.2: Additional Phyla, Examples, and Characteristics
Nematoda (e.g., roundworms, hookworms, C. elegans):
Characteristics: Pseudocoelomate, unsegmented, with a cylindrical body and a complete tubular digestive tract. Many are parasitic, while others are free-living in soil or water.
Approximate Number of Named Species: 61,000
Annelida (e.g., earthworms, leeches, polychaetes):
Characteristics: Segmented body with repeated units, coelomate (true body cavity), and a complete digestive tract. They exhibit specialized segments and often bristles (setae) for locomotion.
Approximate Number of Named Species: 32,000
Echinodermata (e.g., sea stars, sea urchins, sand dollars):
Characteristics: Possess secondary pentamerous radial symmetry (larvae are bilateral), a characteristic water-vascular system with tube feet used for locomotion and feeding, and an endoskeleton made of calcareous ossicles.
Approximate Number of Named Species: 12,000
Cnidaria (e.g., jellyfish, corals, sea anemones, hydras):
Characteristics: Diploblastic (two germ layers), radially symmetrical, and utilize specialized stinging cells called nematocysts for prey capture and defense. Most have a sac-like body plan with a single opening.
Approximate Number of Named Species: 10,000
Bryozoa (e.g., sea mats, moss animals):
Characteristics: Exclusively colonial aquatic animals, often sessile, composed of numerous microscopic zooids. They possess a lophophore – a ciliated feeding organ that surrounds the mouth.
Approximate Number of Named Species: 10,000
Animal Phylogeny
Overview
Kingdom Animalia (Metazoa) is believed to have diverged into several major branches:
Parazoa: Represents the earliest diverging animal lineage, characterized by animals lacking definitive true tissues, organs, and symmetry; primarily exemplified by Phylum Porifera (sponges).
Eumetazoa: Contains all animals with organized true tissues, distinct embryonic layers (diploblastic or triploblastic), a nervous system, and a defined shape and symmetry.
Cnidaria: Diverges early in the Eumetazoan evolutionary tree, representing the earliest group with true tissues but preceding the split into bilaterally symmetrical animals. They are typically diploblastic and radially symmetrical.
Bilateria: This clade includes all animals that are triploblastic and bilaterally symmetrical, further categorized into Deuterostomes and Protostomes, representing the vast majority of animal species.
Phylum Porifera: Sponges
Characteristics:
Comprise about 26,000 marine species and 150 freshwater species. They are among the most abundant animals in deep ocean ecosystems, playing a crucial role in filtration.
Sponges are sessile filter feeders, drawing water through pores to extract food particles.
Sponge Characteristics
Primarily lack definitive symmetry and display a wide variety of growth forms, often irregular or encrusting. Larval sponges are free-swimming and ciliated, aiding dispersal, while adult sponges are sessile (immobile) and attached to a substrate.
Cell Types: Possess a truly multicellular structure, but without forming true tissues or organs. The body is organized into three main functional layers:
Outer Epithelium (Pinacoderm): Made of flattened cells called pinacocytes, which form the protective outer covering. Water enters the body through numerous small pores (ostia) and exits through a larger opening (osculum).
Mesohyl: The middle, gelatinous, non-cellular matrix located between the outer and inner layers. It contains various amoeboid cells (amoebocytes or archaeocytes), spicules (hard skeletal elements made of calcium carbonate or silica, providing structural support), and spongin (a tough, flexible protein fiber, also for support).
Choanocytes (Collar Cells): Flagellated collar cells that line the internal cavity (spongocoel) or canals. Their beating flagella create water currents for feeding and gas exchange, and their collar-like microvilli trap food particles, which are then phagocytosed.
Sponge Reproduction
Asexual Methods: Sponges can reproduce asexually through fragmentation (pieces breaking off and regenerating), budding (outgrowths forming new individuals), or forming gemmules (dormant, resistant buds that can survive harsh conditions).
Sexual Process: Sponges are typically hermaphroditic (can produce both sperm and eggs), often sequentially. Choanocytes or amoebocytes can differentiate into sperm, which are then released into the water. Sperm from one sponge are captured by choanocytes of another, transferred to amoebocytes, and then to eggs within the mesohyl for internal fertilization. The resulting planktonic larva (e.g., amphiblastula) settles on a substrate and undergoes metamorphosis into an adult sessile form.
Eumetazoa
Defined by the presence of true tissues with distinct embryonic layers that give rise to specific structures. This organization allows for more complex body plans and specialized functions compared to sponges:
Endoderm forms the gastrodermis (the inner digestive tissue).
Ectoderm forms the epidermis (outer covering) and the nervous system.
Mesoderm (present only in bilateral animals) develops into muscle tissues, circulatory systems, and internal organs.
Symmetries: Eumetazoans possess either radial or bilateral symmetry, contributing to their diverse lifestyles and modes of interaction with the environment.
Phylum Cnidaria
Comprised primarily of marine species (some freshwater, like Hydra); they exhibit a diploblastic organization with two distinct tissue layers (ectoderm and endoderm) separated by a gelatinous mesoglea. They have distinct tissues but lack true organs, and their nervous system is not concentrated.
Nervous System: Composed of a diffuse, lattice-like arrangement of nerve cells forming a nerve net. This simple system allows them to respond to touch, gravity, and light from any direction but lacks a centralized brain.
Prey Capture: Cnidarians are carnivorous predators that utilize specialized stinging structures called nematocysts for feeding purposes, subduing or killing prey.
Body Forms in Cnidaria
Cnidarians typically exist in one of two basic body forms, though many species exhibit both forms during their life cycle (polymorphism):
Polyp: A sessile, cylindrical form with a mouth and tentacles oriented upwards, typically attached to a substrate (e.g., sea anemones, Hydra).
Medusa: A free-living, umbrella-shaped (bell-shaped) form with a mouth and tentacles oriented downwards, adapted for swimming (e.g., jellyfish).
Common Characteristics: Both forms share a single opening (the mouth/anus) leading to a blind-ended gastrovascular cavity. This cavity serves as a site for digestion, gas exchange, waste discharge, and gamete formation. The body wall consists of an outer epidermis (from ectoderm), an inner gastrodermis (from endoderm), and a non-cellular mesoglea layer in between.
Support Structure in Cnidarians
The gastrovascular cavity functions as a hydrostatic skeleton; the fluid-filled cavity, pressurized by muscular contractions, enables shape maintenance, allows for movement (e.g., bending, extending tentacles), and generates force.
Many polyps, particularly corals, create rigid exoskeletons from materials like chitin or calcium carbonate. These external skeletons provide structural support and protection, forming massive coral reefs.
Cnidarian Life Cycle
Some species exhibit only polyp or medusa forms, while many alternate between both generations (alternation of generations) during their life cycle. Cnidarians are typically diploid throughout both phases:
The medusa form (sexual stage) produces gametes (sperm and eggs) through meiosis; sexes are often separate (gonochoristic).
Following fertilization, the zygote develops into a free-swimming, ciliated planula larva. The planula settles on a suitable substrate and undergoes metamorphosis into a polyp form. This polyp may then produce further medusae or polyps asexually through budding.
Digestion in Cnidarians
A major evolutionary advancement in Cnidarians involves extracellular digestion. Food is initially broken down by digestive enzymes secreted into the gastrovascular cavity.
Following initial breakdown, gastrodermal cells lining the cavity engulf the smaller food fragments via phagocytosis, where intracellular digestion (within food vacuoles) completes the process.
Nematocysts
Nematocysts are highly specialized, complex stinging structures secreted within unique cnidarian cells called nematocytes (or cnidocytes). They are primarily used for prey capture and defense.
The discharge mechanism involves an explosive eversion of a barbed, often venomous, harpoon-like thread. Their rapid discharge (one of the fastest biological processes known) is triggered by mechanical and chemical stimuli, although the exact full physiological mechanism remains somewhat mysterious.
Cnidarian Classes
Anthozoa: Includes sea anemones and most corals; they exist exclusively as polyps (lacking a medusa stage) and can be solitary (anemones) or colonial (most corals). Their gastrovascular cavities are often compartmentalized by septa. Many corals promote mutualistic symbiosis with photosynthetic dinoflagellates (zooxanthellae), which are crucial for the health and growth of coral reefs.
Cubozoa: Known as box jellies, these marine organisms are notable for their cube-shaped medusa and highly potent venom, which can cause severe pain, paralysis, and even human fatalities. They are also recognized for their relatively complex eyes (rhopalia) and strong swimming abilities.
Hydrozoa: Comprised of hydroids, fire corals, and the solitary freshwater organism Hydra. This class uniquely includes freshwater organisms, and many species exhibit both polyp and medusa forms, though the polyp stage is often dominant and colonial (e.g., Obelia).
Scyphozoa: Commonly known as true jellyfish, where the medusa form is prominent and typically larger than in hydrozoans. They possess muscular rings within their bell, allowing for rhythmic pulsations that enable efficient propulsion through water.
Staurozoa: Star jellies, a small class of cnidarians that resemble medusae but are permanently attached to substrates by a stalk at the mouth’s opposite side. They are typically found in cold, polar, and deep-water environments.
Review
Identify five general features common to animals:
Heterotrophic
Multicellular with complex body plans and specialized cells.
Lack cell walls, contributing to flexible body structures.
Active movement, often driven by muscles and nerves.
Diversity of form in both shape and size, adapting to various environments.
Define a Body Plan and the five key innovations relating to its evolution:
A body plan refers to the fundamental structural and developmental characteristics of an organism. The five key innovations that have shaped animal body plan evolution include symmetry, defined true tissues, a body cavity, distinct embryonic development patterns (protostome/deuterostome), and segmentation.
Name the two types of symmetry in animals: Radial vs Bilateral. Be able to categorize any given species appropriately based on these two fundamental body arrangements.