Animal Body Plans: Symmetry and Cephalization

Animal Body Plans

• Body plans enables the classification of animals into phylogenetic frameworks.

Defining Animals

• Animals are distinct from organisms like choanoflagellates (single-celled eukaryotes).
• Animals exhibit multicellular, complex structures like plants and fungi.
• Animals are heterotrophic, obtaining energy from organic molecules unlike plants.
• Animals lack cell walls, facilitating movement.
• Animals undergo blastula development as a hollow ball of cells in early embryonic stages.

Classifying Animals by Symmetry

• Phylogenetic trees aid in classifying animals via shared traits and common ancestors.
• Asymmetry is a symmetry type: absence of symmetry (e.g., sponges).
• Radial Symmetry is a symmetry type: body parts arranged around a central axis (e.g., jellyfish, corals).
• Bilateral Symmetry is a symmetry type: a single plane divides the body into left and right halves (e.g., humans, insects).

Cephalization: The Development of Heads

• Cephalization concentrates sensory organs and the brain at the front of the body.
• Cephalization is seen in most bilaterians (e.g., flatworms, insects, vertebrates).
• Benefits of cephalization includes enhanced sensory perception, faster processing of information, and advantages for predators and prey.

Segmentation in Animals

• Segmentation is the division of the body into repeated units.
• Segmentation is found in arthropods (insects, spiders, crustaceans), annelids (segmented worms), and chordates (vertebrates).
• Segmentation allows specialization of different body regions, and greater mobility with adaptability.

Embryological Development and Germ Layers

• Diploblastic animals have two layers: cnidarians (endoderm + ectoderm).
• Triploblastic animals have three layers: bilaterians (endoderm + mesoderm + ectoderm).
• Mesoderm allows for development of muscles, circulatory systems, and complex organs.

Protostomes vs. Deuterostomes

• Protostomes are characterized by the blastopore becoming the mouth ("first mouth").
• Protostomes includes arthropods, mollusks, annelids, and flatworms.
• Deuterostomes are characterized by the blastopore becoming the anus ("second mouth").
• Deuterostomes includes chordates (vertebrates), and echinoderms (sea stars, sea urchins).

Molecular Evidence and Animal Phylogeny

• Early biologists used comparative anatomy and embryology to classify animals.
• Modern DNA, RNA, and protein sequencing confirm choanoflagellates are animals’ closest relatives.
• Sponges diverged early from other animals.
• The distinction between radial and bilateral symmetry is evolutionarily significant.

The Importance of the Coelom

• The coelom (body cavity) cushions organs and aids movement.
• Acoelomates lack a body cavity (e.g., flatworms).
• Coelomates have a true body cavity lined with mesoderm (e.g., vertebrates).
• Pseudocoelomates have a body cavity partially lined with mesoderm (e.g., nematodes).
• Molecular studies have shown that coelom-based classification is not always reliable.

Key Takeaways

• Animals have diverse body plans that are largely determined by symmetry, segmentation, and embryonic development.
• Bilaterians show cephalization, which aids movement, sensory processing, and predation.
• Segmentation provides modularity and allows for specialization.
• Protostomes and deuterostomes have different embryonic development pathways.
• Modern molecular data has refined our understanding of animal evolutionary relationships.

Animal Tissues and Organs Overview

• Animals have four main tissue types: epithelial, connective, muscle, and nervous.
• Tissues are collections of cells that perform specific functions.
• Organs are made of multiple tissue types (e.g., skin, heart, kidney).
• Sponges lack true tissues, while cnidarians have tissues but not true organs.

Epithelial Tissue

• Forms linings inside and outside the body (e.g., skin, gut, bladder, blood vessels).
• Composed of tightly packed cells connected by cellular junctions.
• Functions as a barrier, controls movement of substances, and absorbs/secretes materials.
• Lacks blood vessels and relies on connective tissue for blood supply.
• Classified based on layers (Simple-single layer, Stratified-multiple layers) and cell shape (Squamous-flat, Cuboidal-square/round, Columnar-tall).
• Special features includes keratinized layers (e.g., skin) and cilia (e.g., upper airways).

Connective Tissue

• Supports, protects, and connects different parts of the body.
• Few cells, but an extensive extracellular matrix (proteins + polysaccharides).
• Types includes basal lamina (thin layer beneath epithelial tissue), dermis (strong, flexible layer under the skin), structural tissues like bone, cartilage, tendons, ligaments, and specialized tissues like adipose (fat) and blood.
• Functions includes structural support, cushioning, blood supply, and movement.

Muscle Tissue

• Made of fibers that contract using actin and myosin.
• Three types in vertebrates: skeletal muscle (attaches to bone, enables voluntary movement), cardiac muscle (found in the heart, contracts to pump blood), and smooth muscle (found in gut, blood vessels, controls involuntary movements).
• Present in bilaterians, but not cnidarians.

Nervous Tissue

• Processes and transmits information using neurons.
• Includes brain, spinal cord, and peripheral nerves.
• Functions to receives sensory input, processes information, and sends signals to muscles and organs.
• Neurons communicate via synapses using electrical and chemical signals.

Organs and Organ Systems

• Organs consist of multiple tissue types and perform specialized functions.
• Examples of organs include intestine (has all four tissue types), the heart as a pump, lungs/gills for gas exchange, and kidneys to excrete waste and regulate water balance.
• Organ systems are groups of organs working together (e.g., digestive system).

Bio-Inspired Robotics

• Scientists use animal structures to design robots.
• Robotic fish help understand fish movement and behavior.
• Applications includes mapping ocean floors, monitoring underwater cables, and studying marine life.
• Octobot is inspired by octopus.
• Octobot is fully soft-bodied and powered by chemical reactions with hydrogen peroxide gas.
• Created using 3D printing.

Homeostasis

• Homeostasis keeps internal conditions stable, ensuring optimal biological function.
• Negative feedback loops regulate homeostasis by counteracting changes.
• Thermoregulation is a key example, with body temperature controlled by the hypothalamus.
• Claude Bernard & Walter Cannon contributed to the understanding of homeostasis.

What is Homeostasis

• Homeostasis actively maintains stable internal conditions in cells and organisms.
• Keeps physiological parameters (e.g., body temperature, blood pH) within a narrow range.
• Ensures proper enzyme function, protein folding, and cellular reactions.
• Maintains ion balance for neuron function and action potentials.
• An example is cell membranes regulating ion concentrations to maintain homeostasis.
• An example is organisms in an acidic environment, actively pump protons to keep internal pH neutral.

Homeostasis in the Body

• Physiological parameters regulated by homeostasis: temperature, heart rate, blood pressure, blood sugar levels, blood pH, ion concentrations, and water balance.
• First described by Claude Bernard in the 1800s.
• Term "homeostasis" was coined by Walter Cannon in 1932.

How Homeostasis Works: Negative Feedback

• Negative feedback is the primary mechanism for homeostasis.
• Process:
  • Stimulus: Change in environment (e.g., cold temperature).
  • Sensor: Detects change (e.g., thermostat or hypothalamus).
  • Effector: Responds to counteract change (e.g., heater or shivering).
  • Response: Returns system to set point (stable condition).

Thermoregulation in Humans

• Cold temperature: Hypothalamus detects drop and the body responds by shivering (muscle contractions generate heat) and blood vessel constriction to reduce heat loss.
• Hot temperature: Hypothalamus detects rise and the body responds by sweating (evaporation cools the body) and blood vessel dilation (increases heat loss).

Why Homeostasis Matters

• Prevents extreme fluctuations that could disrupt biological functions.
• Allows organisms to adapt to environmental changes (e.g., drought response in animals).
• Maintains body function across different conditions (e.g., sleeping vs. awake).

Animal Phylogeny & Early Evolution

• Animals are closely related to choanoflagellates (single-celled eukaryotes).
• Choanoflagellates ingest bacteria and sometimes form simple multicellular groups.
• Their similarities to sponges suggest early animals were simple multicellular organisms.

Sponges (Phylum: Porifera)

• Sponges are the simplest living animals, with no true tissues.
• Body plan: Vase-like structure with pores for water flow.
• Key parts include choanocytes (inner surface) as flagellated cells for feeding & gas exchange.
• Mesohyl is the gelatinous middle layer with amoeba-like cells for skeleton formation, reproduction, and nutrient transport.
• Sponges are filter feeders where choanocytes create water currents to trap food.
• Sponges perform intracellular digestion where food metabolized inside individual cells.
• Sponges have no dedicated reproductive organs.
• Sexual reproduction occurs when choanocytes produce sperm, which fertilize eggs in another sponge’s mesohyl.
• Skeleton types: silica spicules (glass-like), calcium carbonate spicules (hard, reef-building), and protein-based skeletons (bath sponges).

Cnidarians (Phylum: Cnidaria)

• Have radial symmetry, diploblastic structure (two germ layers: ectoderm & endoderm).
• Mouth surrounded by tentacles with stinging cells (nematocysts).
• Cnidarians include sea jellies, corals, sea anemones.
• Body Forms: Polyp (sessile, mouth/tentacles face upward) and medusa (free-swimming, mouth/tentacles face downward.)
• Cnidarians gastrovascular cavity has one opening for food & waste.
• These organisms uses enzyme digestion allows for large prey consumption.
• Has a simple nerve net (no brain, but some have light-sensitive cells).
• Muscle-like cells allow movement.
• Colonial Forms: Corals are sessile, calcium carbonate skeletons, often reliant on symbiotic algae.

Ctenophores (Phylum: Ctenophora)

• Resemble cnidarians but with key differences, known as "Comb jellies."
• Key features include radial symmetry, nerve net, and epidermal cilia for movement.
• Have a digestive system with an anal pore for waste removal (unlike cnidarians).
• Use sticky cells instead of nematocysts to catch prey through predatory feeding.
• Historically grouped with cnidarians, some data suggest they branched before sponges.

Placozoans (Phylum: Placozoa)

• The simplest multicellular animals with only one named species, Trichoplax adhaerens.
• No specialized tissues (only a few cell types).
• They are flat, with a millimeter-scale body using upper/lower epithelium.
• This organism moves with cilia, performing external digestion.
• This organism is genetically similar to cnidarians and bilaterians, despite its simplicity.
• They may provide insight into early animal evolution.

Animal Phylogeny Uncertainties

• Bilaterians (animals with bilateral symmetry) form a monophyletic group.
• Placozoans may be the sister group to cnidarians & bilaterians.
• Ctenophores' placement is debated.
• Traditionally thought to be close to cnidarians.
• Some DNA evidence suggests they are the earliest branching animals, possibly even before sponges.
• If true, their nervous system evolved independently.

Protostome Animals

• Protostomes and deuterostomes are the two main groups of bilaterian animals.
• Protostomes blastopore becomes the mouth.
• In euterostomes the blastopore becomes the anus.
• Protostomes can be further divided into Lophotrochozoa and Ecdysozoa.

Lophotrochozoa Characteristics

• Includes mollusks (clams, snails, squid) and annelid worms (earthworms, marine worms).
• Lophophore are tentacle-lined organ used for feeding.
• Has a trochophore larva which is a larval stage with cilia, found in mollusks and some annelids.

Annelids (Segmented Worms)

• Known for their segmented bodies, allowing specialized structures for different functions.
• Earthworms live in soil and ingest organic material from sediment.
• Marine Annelids include polychaete worms are common in oceans.
• Specialized Annelids, Leeches feed on blood, and vestimentiferan worms live at hydrothermal vents and rely on chemosynthetic bacteria.
• Has a closed circulatory system for moving oxygen and nutrients.
• Earthworms reproduce with a clitellum for egg secretion.

Mollusks

• Mantle: Structure for breathing, excretion, and shell formation.
• Gastropods (Snails & Slugs): Head with mouth, radula (feeding structure), and muscular foot for movement.
• Cephalopods (Squid, Octopus): Highly adapted predators with tentacles, jet propulsion for swimming, and exceptional vision.
• Bivalves (Clams, Mussels): Two shells hinged together, filter food from water.
• Use jet propulsion for swimming.

Ecdysozoa

• Undergo ecdysozoans molts their tough external cuticle during growth.
• Cuticle provides protection and can form appendages used as tools, weapons, or wings.
• During the molting process the cuticle is shed and replaced with a larger one as the animal grows.

Nematodes (Roundworms)

Has a simple structure that includes a mouth, intestine, anus, and basic organs (nervous system, muscles, gonads).

• Some parasitic nematodes is hookworms are used in tropical medicine.

Arthropods:

• Discussed in the next section and focuses on insects and other arthropods
• Arthropods are the most diverse group of animals.
• Their evolutionary success is attributed to the jointed legs, which provide adaptability for various functions (e.g., paddles, pincers, stilts, etc.).
• Jointed legs have allowed them to colonize land and support larger bodies over uneven terrain, with applications influencing NASA's planetary exploration vehicles.
• External skeleton is made of chitin, a lightweight polysaccharide, which also supports fungi.

Main Groups of Arthropods

• Insects: The most diverse group, constituting about 80% of all known animal species.
• Chelicerates: Includes spiders, scorpions, and relatives.
• They are mainly carnivores and use venom to subdue prey and were the first animals to invade land over 400 million years ago.
• Myriapods: Composed if centipedes and millipedes and produce cyanide for defense.
• Crustaceans: Includes lobsters, shrimp, and crabs where the larvae are called nauplius, have a single eye and swim using their antenna.

Key Features of Arthropods

• A hallmark feature of Jointed Legs which provides versatility and supports the evolution of diverse appendages.
• Have a exoskeleton consists of chitin which provides strength, protection, and support for various functions.

Evolution and Adaptations in Insects

• Critically adapted to desiccation-resistant eggs, wings, specialized respiratory systems, and metamorphosis.
• To avoid desiccation the eggs can be laid in environments such as the canopy forest floor.
• The insects first wings were evolved around 340 million years ago.
• Use spiracles connected to tracheae for gas exchange, unlike aquatic arthropods that use gills.
• Undergo incomplete metamorphosis (e.g., grasshoppers) or complete metamorphosis (e.g., butterflies, beetles). which involves a dramatic transformation from a larval form to an adult, often involving the dissolution of body parts.

###Metamorphosis Process

• In complete metamorphosis, the insect undergoes a pupa stage where its tissues dissolve and emerge as an adult.
• Involves the diversification of insect which is controlled by hormones.

###Vertebrates: Key Characteristics and Diversity

• Vertebrates are characterized by jointed skeletons.
• They share a cranial with chordates for food intake.
• Have an internal skeleton composed of calcium phosphate.
• They possess a coelom and a closed circulatory system.
• They possess pharyngeal slits found during embryonic stages.

###Anatomical Features

• Vertebrates possess a cranium.
• Vertebrates possesses a mandible.
• Present in some species, but absent in hagfish.

###Evolutionary Groups within Vertebrates

• Hagfish and Lampreys is an early vertebrate branch.
• Lampreys possesses a vertebral column, while hagfish do not.
• Hagfish feed on marine worms and decaying animals
• Lampreys are parasitic.

###Cartilaginous Fishes (Chondrichthyes) Consists of sharks, rays, and chimaeras with a skeleton made of cartilage and teeth consisting of calcium phosphate.

###Bony Fishes (Osteichthyes):

• Are the comprised of about 28,000 species.
• Adaptations exist for survival in diverse aquatic environments.
• Tetrapods have evolved to lungs.

###Transition to Land

• They can breathe air and survive in dry conditions by burrowing in mud.
• Are a group of four limbe animals that includes amphibians and mammals
• Legs, rib cage, and changes in the skull all were adaption for terrestrial life.

###Amphibians

• Use a life cycle with a an aquatic larval stage (gills) and a terrestrial adult stage (lungs).
• Avoid egg desiccation.

Adaptations for Amniotes

Have an evolved amniotic egg which promotes terrestrial life styles.

• Includes: reptiles, birds, and mammals and requires an internal Fertilization prior the egg forms.

###Amniotic Egg

• Is composed of four membranes that supplies nutrient, waste exchange, and management.
• Composed of adaptations for tetrapods who uses internal development.
• Tetrapods exist in mammals and reptiles.

###Mammals

• Feed with milk from mammary glands.
• Monotremes are found in Australia.
• Placental contains the mammals that have evolved from earlier nocturnal creatures, approximately 210 million years ago.
• Rodents have teeth that are constantly developing and teeth are used to degrade tough materials found in terrestrial terrestrial environments.
• All mammals have hair bodies.