Unit 4
Animal Diversity
Animal Evolution
Animal evolution traces its roots back approximately 600 million years ago, primarily within the oceans. There are about 1 million extant (currently existing) animal species identified, with estimates suggesting that there could be between 3 to 30 million species yet to be discovered. Fundamental characteristics shared with prokaryotes include aspects of metabolism, the plasma membrane, and ribosomes. In contrast, common features shared with plants involve sexual reproduction and meiosis, while those with fungi involve heterotrophy—consuming living or deceased organisms, encompassing carnivores, herbivores, omnivores, and parasites.
Distinctive Animal Characters
To define what constitutes an animal, it is essential to identify autapomorphic (newly derived) characters absent in the last universal common ancestor (LUCA), bacteria, protists, fungi, and plants. Key characteristics include motility, diverse structural complexities associated with specialized tissues, and a diplontic life cycle where the diploid (multicellular) phase predominates, and the haploid (unicellular) phase represents gametes. Animals exhibit distinct embryonic developmental stages and fixed body plans, providing a framework for their diverse forms.
Complex Tissue Structures in Animals
Animals exhibit specialized tissue types that facilitate various functions, primarily related to obtaining and processing food. Specialized tissues evolved to enhance feeding mechanisms, illustrated by the presence of sensory organs. Unlike plant and fungal tissues, animal tissues are characterized by lacking cell walls and featuring communication via gap junctions. Vertebrates, with their connective tissues, provide structural support, while epithelial tissues safeguard internal organs and external surfaces. However, not all invertebrates possess organized tissues, as some are limited to specialized cells.
Specialized Systems in Animals
Animal systems are intricately specialized to meet essential needs such as food acquisition or evasion from predation, reproduction, and sensory responsiveness to environmental changes. Major specialized systems include:
Digestive System: Processes nutrients for energy and growth.
Sensory System: Detects environmental stimuli.
Nervous System: Coordinates actions and responses.
Muscular System: Facilitates movement.
Skeletal System: Provides structural integrity.
Classification of the Animal Kingdom
The Animal Kingdom is categorized into five monophyletic clades: Ctenophora (comb jellies), Porifera (sponges), Placazoa (simple amoeba-like organisms), Cnidaria (jellyfish and relatives), and Bilateria, which encompasses all other animals. Classification hinges on the presence or absence of backbones, distinguishing invertebrates (lacking a backbone) from vertebrates (possessing a backbone). Notably, all five clades include invertebrates, while only Bilateria features both invertebrates and vertebrates.
Body Plans in Animals
Animal classification is heavily influenced by body plans and embryonic development. Body plan considerations include symmetry (asymmetrical, radial, and bilateral) and embryonic development aspects such as the number of germ layers, presence or absence of a coelom, and the origins of the mouth and anus. These structural frameworks are pivotal for understanding how animals have adapted to various environments and roles.
Reproductive Strategies
Animals typically follow a diplontic life cycle where the multicellular diploid phase constitutes somatic cells, while the unicellular haploid phase includes gametes. Although all animals can engage in sexual reproduction, some also have modalities for asexual reproduction. Departures from this norm are observed in certain insect species, such as bees, ants, and wasps, where males may develop from haploid cells.
Zygote Development and Germ Layers
Following fertilization, zygotes undergo cleavage via mitosis, resulting in smaller cells known as blastomeres. This leads to the formation of a hollow structure known as a blastula, followed by gastrulation, which results in the archenteron (the early gut cavity) and the establishment of the embryonic germ layers: ectoderm, endoderm, and, in triploblastic species, mesoderm. It’s distinguished that some animals, particularly among the diploblastic phyla, may not possess a mesoderm, generating significant diversity in developmental pathways.
Hox Genes and their Role in Development
Hox genes, integral to determining body plans, are shared among most animal phyla (with exceptions noted in Ctenophora and Porifera). These genes direct the embryonic development pathway and dictate aspects such as body segment structure and appendage placements. The presence of multiple Hox gene sets in vertebrates (typically four) compared to invertebrates (one set) allows for increased morphological diversity, enabling a broader range of body structures.
Symmetry and Coelom Structure
Animal body symmetry significantly affects functional organization and ecological adaptations. Asymmetrical animals (e.g., Porifera) lack discernible symmetry, while radial animals (e.g., Cnidarians) exhibit symmetry around a central axis. Bilaterally symmetrical species display organization into left and right halves and often engage in directional movement due to cephalization.
Coelom formation also varies: a coelom provides mechanical cushioning and flexibility to organs. Triploblastic organisms without coeloms are termed acoelomates, while those with entirely mesoderm-surrounded coeloms are termed eucoelomates. Others, termed pseudocoelomates, possess coelomic spaces derived from both mesoderm and endoderm.
Developmental Pathways of Mouth and Anus
Developmentally, the fate of the blastopore (the opening of the archenteron) defines whether an animal is a protostome or deuterostome. In protostomes, such as annelids and mollusks, the blastopore develops into the mouth, while in deuterostomes—including echinoderms and chordates—it forms the anus. This distinction marks a significant evolutionary divergence in animal development and organization, contributing to the classification and evolutionary biology of animals.
Overview
Animal evolution began around 600 million years ago, mainly in oceans, with approximately 1 million known species and estimates of 3 to 30 million undiscovered species. Animals share characteristics with prokaryotes, plants, and fungi, including metabolism, sexual reproduction, and heterotrophy. Essential autapomorphic traits define animals, such as motility, complex tissues, and a dominant diplontic life cycle.
Animals have specialized tissues and systems for feeding, movement, and responsiveness to stimuli, including a digestive system, sensory system, nervous system, muscular system, and skeletal system. The Animal Kingdom consists of five clades: Ctenophora, Porifera, Placazoa, Cnidaria, and Bilateria, distinguished by the presence of backbones, with invertebrates lacking them and vertebrates possessing them.
Classification relies on body plans influenced by symmetry and embryonic development. Animals reproduce primarily through a diplontic life cycle, engaging in both sexual and asexual methods. Zygote development includes cleavage into blastomeres and formation of germ layers. Hox genes play a key role in body plan determination and are more diverse in vertebrates.
Body symmetry affects organization and ecology, with coelom formation providing organ flexibility. The fate of the blastopore during development distinguishes protostomes (mouth forms from blastopore) from deuterostomes (anus forms from blastopore), marking significant evolutionary differences in animal biology.
Animal Body
Animal Body Plan
Limitations and Types of Body Plans
An animal’s body plan influences its size, shape, and environmental interactions.
Asymmetrical Body Plan: Commonly found in aquatic, sessile filter feeders.
Radial Body Plan:
Typically aquatic and may be motile.
Examples of feeding strategies include filtering and active searching.
Lack cephalization, leading to non-directional movement.
Bilateral Body Plan:
Present in all vertebrates.
Characterized by cephalization, allowing for directional movement.
All are motile and actively search for food.
Defines anterior, posterior, dorsal, and ventral sides.
Bilateral Body Plan Dynamics
Aquatic Animals:
Constrained by water density, leading to a fusiform shape to minimize drag.
Exhibit convergent evolution traits to adapt to their environments.
Terrestrial Animals:
Constrained by gravity, necessitating adaptations such as wings, reduced body weight, and long tails to aid mobility.
Animal Structural Support
Exoskeleton
Definition: A hard outer covering providing protection and muscle attachment sites.
Composition: Typically made of chitin or calcium carbonate.
Muscle Attachment: Involves ingrowths called apodemes for movement.
Molting: Required for growth; limits the maximum size an animal can achieve.
Size Relationship: Doubling body size results in an 8x weight increase.
Endoskeleton
Definition: Internal structure made of bone or cartilage providing body support and facilitating movement.
Muscle Connection: Muscles attach to the endoskeleton.
Growth: Most endoskeleton animals exhibit determinant growth, increasing bone and muscle size with body size.
Balance: Speed and agility depend on the relationship between body size and muscle mass.
Homeostasis and Thermoregulation
Endothermic Animals:
Utilize metabolism to control body temperature (e.g., shivering, sweating).
Have adaptations such as fur and subcutaneous fat.
Require significantly more calories than ectotherms (90% more).
Ectothermic Animals:
Rely on environmental heat sources to regulate temperature (e.g., basking, seeking shade).
Require fewer calories (10% of what endotherms need).
Body Temperature Types:
Homeothermic: Maintain a constant body temperature.
Poikilothermic: Allow body temperature to fluctuate with environmental conditions.
Animal Bioenergetics
Energy Acquisition: Animals derive energy from consumed food (carbohydrates, proteins, lipids), converting these into:
Immediate Energy: ATP.
Long-term Energy Storage: Glycogen and fat.
Thermodynamic Efficiency: Energy conversions are not 100% efficient, resulting in heat loss.
Thermal Homeostasis: Endothermic animals utilize heat generated by metabolism for temperature regulation.
Metabolic Rate:
Defined as energy expended over time (joules or calories).
Differentiates into Basal Metabolic Rate (endotherms) and Standard Metabolic Rate (ectotherms).
Body Size and Energy Requirements
Surface Area to Volume Ratio:
Small endotherms have a higher ratio compared to larger ones leading to more rapid heat loss.
Metabolic Rates:
Higher in smaller endotherms; they use more energy per body weight.
Activity Level Influence: The more active an animal, the higher its BMR/SMR.
Diet is influenced by metabolic rates.
Environment and Energy Adaptations
Adaptation to Extremes:
Animals in harsh environments adapt through torpor (decreased activity/metabolism) to conserve energy.
Types of Torpor:
Hibernation:
Winter torpor; reduces body temperature, ceases food intake.
Estivation:
Summer torpor; occurs in response to extreme heat or limited water.
Daily Torpor: Utilized to avoid extreme temperatures during the day.
Animal Reproduction
Asexual vs. Sexual Reproduction
Overview:
All animals can reproduce sexually; some can also reproduce asexually.
Asexual Reproduction:
Offspring are genetically identical to the parent.
One individual can produce many clones quickly.
Favored in stable environments where the parent is successful.
Advantageous for colonizing new habitats (no mate required).
Sexual Reproduction:
Offspring are genetically diverse (not identical to the parent).
Increases genetic variation, improving survival in unstable environments.
Disadvantage: Requires both sexes to reproduce.
Types of Asexual Reproduction
Binary Fission:
Splitting into two equal organisms.
Example: Corals, anemones.
Budding:
Outgrowth from an adult organism that separates into a new adult.
Example: Hydra.
Fragmentation:
Regeneration of new individuals from body parts.
Example: Sea stars, flatworms, sponges.
Parthenogenesis:
Female eggs fuse without fertilization.
Example: Water fleas, bees, also occurs in reptiles and fish.
Sex Determination
Mammals:
XX = female, XY = male; Y chromosome plays a critical role.
Birds:
ZW = female, ZZ = male; W chromosome influences sex.
Temperature-Dependent Sex Determination:
Example: Turtles (cooler = male, warmer = female).
Example: Alligators/crocodiles (warmer = male, cooler = female).
Sequential Hermaphroditism:
Protogyny: Female first; Protandry: Male first (common in fish).
Haplodiploid Sex Determination:
Haploid individuals (males); Diploid individuals (females).
Example: Hymenoptera (bees, ants).
Types of Sexual Reproduction
Monoecious:
Individual has both male and female parts; can self-fertilize.
Example: Worms, slugs, snails.
External Fertilization:
Gametes released into water; seasonal or behavioral triggers.
Courtship spawning (one male, one female) vs. broadcast spawning (all at once).
High number of offspring but low survival rate.
Internal Fertilization:
Male deposits sperm inside female.
Protects embryos from predators and desiccation.
Few offspring but higher survival rate.
Internal Fertilization Mechanisms
Viviparous:
Embryo obtains nutrition from the mother, leading to live births (most mammals).
Oviparous:
Eggs laid outside the body; embryo uses yolk for nourishment (most fish, reptiles, birds).
Ovoviviparous:
Eggs develop inside the female and hatch in utero.
Example: Some sharks, snakes; embryos can cannibalize siblings.
Spermatogenesis
Definition:
Production of sperm in male testes.
Process:
Spermatogonia divide by mitosis, producing one primary spermatocyte (undergoes meiosis).
Meiosis I produces two secondary spermatocytes.
Each secondary spermatocyte undergoes meiosis II, yielding four spermatids.
Spermatids differentiate into sperm cells in the seminiferous tubules.
Spermatogonia remain inactive at birth until puberty activation by hormones.
Oogenesis
Definition:
Production of eggs in ovaries.
Process:
Oogonia undergo mitosis, producing two primary oocytes.
Primary oocytes arrest in Prophase I until puberty, when hormones trigger completion.
Meiosis yields one secondary oocyte (arrested in Metaphase II) and one polar body.
Secondary oocyte completes meiosis only upon fertilization; produces one zygote and another polar body, which degenerates.
Invertebrates
Invertebrates Overview
Invertebrates represent 97% of all species in the Kingdom Animalia.
Primarily classified within the phylum Arthropoda.
Lack a backbone and cranium.
Habitats include land, freshwater, and oceans.
Focus on 8 invertebrate phyla:
Porifera (sponges)
Cnidaria (jellyfish, corals, sea anemones)
Platyhelminthes (flatworms)
Mollusca (mollusks)
Annelida (segmented worms)
Nematoda (roundworms)
Arthropoda (arthropods)
Echinodermata (echinoderms)
Key adaptations discussed for each phylum include:
Gas exchange mechanisms
Food ingestion and digestion
Waste removal (solid food waste and metabolic waste)
Nervous and sensory organ development
Reproductive strategies
Phylum Porifera (Sponges)
Tissues: Lack true tissues; do not develop a gastrula, endoderm, or ectoderm; possess specialized cells for “tissue-like” functions.
Contain 12 different cell types.
Morphology:
Body Plan: Asymmetrical, cylindrical shape.
Internal cavity known as spongocoel.
Water flows into the spongocoel via ostia (pores) and exits through the osculum (top opening).
Choanocyte cells create water currents with flagella.
Physiology:
Lack digestive, nervous, respiratory, and circulatory systems.
Food is digested intracellularly; absorbed by cells in the spongocoel.
Gas exchange through diffusion; all cells can absorb O2 and release CO2.
Reproduction is both sexual and asexual.
Phylum Cnidaria (Jellyfish, Anemones)
Body Plan:
Exhibits radial symmetry; consists of diploblastic layers (ectoderm and endoderm).
Possesses an incomplete digestive system where food enters and exits through the same opening.
Defining feature: cnidocytes (stinging cells with nematocysts).
Morphology:
Two body types: Polyp (sessile) and Medusa (motile).
Epidermis from ectoderm; gastrodermis from endoderm; non-living mesoglea between them.
Lacks true organ systems, though contains specialized cells.
Physiology:
Digestion begins extracellularly in the gastrovascular cavity with the help of enzymes.
Nutrient absorption occurs in both the gastrovascular cavity and epidermis.
No true nervous system; simple nerve networks facilitate movement and responses.
Reproduces mostly sexually with some forms of asexual reproduction.
Classes within Cnidaria:
Anthozoa: Coral and sea anemones (only sessile).
Scyphozoa: True jellyfish (medusa form).
Cubozoa: Box jellies (medusa form).
Hydrozoa: Generally possess both polyp and medusa forms (e.g., Hydra and Portuguese man o' war).
Phylum Platyhelminthes (Flatworms)
Body Plan:
Exhibits bilateral symmetry and possesses triploblastic organization; acoelomate body structure.
Has an incomplete digestive system; mouth also functions as the anus.
Physiology:
Simple cephalization with nerve tissues concentrated in the head area.
Free-living species function as predators or scavengers.
An excretory system is networked with tubules featuring flame cells to excrete waste.
Lacks circulatory and respiratory systems, relying on the epidermis for gas exchange (O2 in, CO2 out).
Reproductive strategies tend to be monoecious (hermaphroditic), with internal fertilization producing eggs.
Key Characteristics:
Simple nervous system with two nerve cords and cerebral ganglia for sensory functions.
Nutrient absorption occurs in the gut after extracellular digestion.
Circulatory Systems in Invertebrates
Open Circulatory System:
Found in many invertebrates; blood (hemolymph) bathes organs directly.
Heart pumps hemolymph into body cavity, mixes with interstitial fluid.
Nutrient exchange occurs between hemolymph and tissues; hemolymph returns via ostia.
Closed Circulatory System:
Present in few invertebrates and all vertebrates; blood remains within vessels.
Heart pumps blood through arteries; returns via veins, with capillaries facilitating nutrient and gas exchange.
Phylum Mollusca
General Overview
Includes species like snails, mussels, octopus, etc.
Second most diverse phylum with numbers following Arthropoda (which is #1).
Body Plan
Bilateral symmetry with cephalization.
Triploblastic and eucoelomate.
Develops as protostomes.
Contains a complete digestive system with a mouth and anus.
Composed of a muscular foot for locomotion, a visceral mass (main body), and a mantle that covers the visceral mass.
The mantle produces a calcium carbonate shell, which is reduced in species such as squid and octopus.
Respiratory System
Aquatic species possess gills.
Terrestrial species utilize the mantle for respiration.
Digestive System
Complete with food ingestion via the mouth, processing in the gut, and expulsion through the anus.
Circulatory and Nervous Systems
Both systems vary depending on the class of mollusks.
Classes of Mollusca
Bivalvia
Gastropoda
Cephalopoda
Class Bivalvia
Characteristics
Includes marine and freshwater species.
Body enclosed in a two-part shell.
Adductor muscles close the shell; abductor muscles open it.
No cephalization, but some species have eye spots.
They are filter feeders, taking in/exhaling water.
Food is captured by gills and moved to the mouth via cilia.
Respiration
Occurs through gills, with oxygen diffusing into them.
Circulatory System
Open circulatory system.
Contains a heart and aorta.
Waste Excretion
Metabolic waste is expelled by a pair of nephridia.
Reproduction
Sexual reproduction with external fertilization through broadcast spawning, producing larvae.
Examples include clams, mussels, and oysters.
Class Gastropoda
Characteristics
Found in marine, freshwater, and terrestrial environments.
Some have shells; others do not.
Utilizes a muscular foot for locomotion.
Demonstrates true cephalization with head, eyes, and tentacles.
Exhibits simple nervous system.
Includes both predators and herbivores.
Feeding
Radula “tongue” scrapes food from surfaces.
Respiration
Aquatic species have gills, while terrestrial species use the mantle.
Circulatory System
Open circulatory system with a heart serving the visceral mass and foot.
Waste Excretion
Metabolic wastes are expelled via nephridia.
Reproduction
Sexual reproduction varies:
Terrestrial species are monoecious with internal fertilization and no larval stage.
Marine species often dioecious with external fertilization, producing larvae.
Examples
Snails, slugs, and conchs.
Class Cephalopoda
Characteristics
Nautiluses (have shells), while squids, octopuses, and cuttlefish possess internal shells.
Exhibit abilities in color-changing for camouflage and mating.
All are carnivorous predators with a jawed beak.
Show true cephalization with high intelligence and developed nervous system.
Circulatory System
Only Mollusca class with a closed circulatory system.
Morphology
Muscular foot is modified into tentacles and arms.
Use siphon as a jet propulsion mechanism for locomotion.
Respiration
Two developed gills with a heart for each.
Waste Excretion
Metabolic wastes are removed by nephridia.
Reproduction
Sexual, producing eggs with no larval stage.
Examples
Nautilus, cuttlefish, squid, and octopus.
Phylum Annelida
General Overview
Includes segmented worms inhabiting marine, terrestrial, and freshwater environments.
Body Plan: Bilateral with minimal cephalization.
Tissues: Triploblastic, eucoelomate, and develop as protostomes.
Body Segmentation
Exhibits metameric segmentation, showing repeated features both internally and externally.
Types
Polychaetes (marine) and oligochaetes (terrestrial) which can be filter feeders, predators, or parasites.
Annelida Morphology
Skin
Protected by chitinous cuticle.
Movement
Chitinous bristles called setae aid in locomotion.
Digestive System
Complete system includes mouth, pharynx, esophagus, crop, gizzard, intestine, and anus.
Circulatory System
Closed system consisting of dorsal and ventral blood vessels.
Respiration and Waste Excretion
Gas exchange occurs across the epidermis, with waste excreted by nephridia.
Nervous System
Well-developed with a ventral nerve cord and ganglia.
Annelida Reproduction
Reproduction Types
Some are monoecious, with both sexes in a single organism having permanent gonads allowing for self-fertilization or cross-fertilization.
Some dioecious with male or female forms having temporary gonads.
Reproduction Specifics
Polychaetes: external fertilization with larvae that metamorphose into adults.
Can also reproduce asexually via fragmentation.
Oligochaetes: internal fertilization leads to developing eggs from which young worms hatch.
Phylum Nematoda
General Overview
Includes roundworms with a bilateral symmetric body plan.
Triploblastic, pseudocoelomate, and develop as protostomes.
Characterized by a cuticle exoskeleton which is molted as they grow.
Morphological Traits
Can be free-living or parasitic with minimal cephalization possessing only primitive nerves.
Systems
Absence of circulatory, respiratory, or excretory systems; gas exchange occurs through the epidermis along with waste excretion.
Reproductive Strategies
Diverse reproductive strategies including monoecious, dioecious, and parthenogenetic.
Phylum Arthropoda
General Overview
The largest phylum with high diversity (includes insects, spiders, lobsters, etc.).
Comprised of five subphyla with various habitats (terrestrial, aquatic, aerial).
Body Plan: Bilateral, triploblastic, eucoelomate; develop as protostomes with a complete digestive system.
Morphological Features
Includes jointed appendages for various functions; body segmentation into head, thorax, and abdomen.
Contains a chitinous exoskeleton that is molted.
Arthropoda Morphology
Internal Systems:
Central cavity (hemocoel) and heart for fluid movement.
Excretion varies by class (e.g., green glands in crustaceans).
Reproductive Methods
Most species are dioecious with sexual reproduction; some monoecious and some can reproduce parthenogenetically.
Subphyla of Arthropoda
Trilobita
All extinct marine species.
Chelicerata
Mostly terrestrial, includes horseshoe crabs, spiders, mites.
Myriapoda
Terrestrial with numerous legs: millipedes and centipedes.
Crustacea
Mainly aquatic like shrimp and crabs but some are terrestrial.
Hexapoda
True insects, exhibit complete or incomplete metamorphosis.
Phylum Echinodermata
General Overview
Exclusively marine species such as sea stars and sand dollars.
Exhibits bilateral symmetry in larval stage and radial symmetry as adults, often pentaradial.
Tissues: Triploblastic, eucoelomate, and develop as deuterostomes with ossicles forming an endoskeleton.
Morphology
Body covered with an epidermal layer over ossicles.
Each arm includes digestive glands and tube feet for mobility and feeding.
Echinodermata Systems**
Nervous System
Simple structure lacking a brain; possesses a central nerve ring.
Reproduction
Anticipated to be dioecious with external fertilization alongside bilateral larvae produced.
Classes
Asteroidea - Sea Stars
Ophiuroidea - Brittle Stars
Echinoidea - Sand Dollars
Crinoidea - Sea Feathers and Lillies
Holothuroidea - Sea Cucumbers (appearance of functional bilateral symmetry)
Summary of Classification
Metazoa (animals)
Eumetazoa (with specialized tissues)
Bilateria (bilateral symmetry, triploblastic)
Acoelomates (without a coelom)
Protostomia
Ecdysozoa
Nematoda (roundworms)
Arthropoda (arthropods)
Platyhelminthes (flatworms)
Rotifera (rotifers)
Ectoprocta
Brachiopoda
Annelida (annelids)
Lophotrochozoa
Mollusca (mollusks)
Deuterostomia
Chordata (chordates)
Echinodermata (echinoderms)
Acoela
Vertebrates
Vertebrates Overview
Phylum Chordata
Most recognized of all species within the Kingdom Animalia
Body Plan & Embryonic Development:
All are bilateral deuterostomes
Germ Layers: Triploblastic eucoelomates
Complete digestive system present
Excretory System: Kidneys for metabolic waste removal
Nervous System: Presence of a brain and central nervous system
Respiratory System: Varies by group (gills, lungs, skin)
Defining Characteristics of Vertebrates
Five critical characteristics of all vertebrates:
Notochord:
Flexible rod structure from mesoderm tissue
Located dorsal to the digestive tube
Provides support and muscle attachment
Replaced by vertebral column in true vertebrates
Hollow Dorsal Nerve Cord:
Derived from ectoderm
Develops into brain and spinal cord
Pharyngeal Slits:
Openings that allow connection to the pharynx
Developmental role varies among species (gills for fish, parts of the ear/tongue in land vertebrates)
Post-anal Tail:
Tail extending beyond the anus; used for locomotion in aquatic species and balance in terrestrial species
Reduced to the coccyx in humans and great apes
Endostyle/Thyroid Gland:
Mucus-producing tissue in the pharynx
Similar to thyroid hormones; develops into thyroid in most vertebrates
Chordate Clades
Invertebrate Chordates:
Cephalochordata (lancelets)
Adults retain all five characteristics
Urochordata (tunicates)
Larvae retain all five characteristics, adults have only pharyngeal slits and endostyle
Subphylum Vertebrata (Craniata)
Largest Group of Chordata
Cranium: Bony, cartilaginous, or ligamentous structure covering the brain and jaw structures
All members possess five defining characteristics of Chordata
Evolutionary Adaptation: Vertebral column replaces the notochord in adults
Classification within Vertebrates
Agnathostomes: Jawless vertebrates (Myxini and Petromyzontidae)
Gnathostomes: Jawed vertebrates, further divided into:
Fishes
Tetrapods: Animals with four limbs (amphibians, reptiles, birds, mammals)
Two groups:
Amphibians
Amniotes (reptiles, mammals, birds)
Fishes: Subphylum Vertebrata
Largest group with greatest species diversity
Two types:
Agnathostomes (jawless fish)
Include lampreys and hagfish
Characteristics: lack hinged jaw, paired lateral fins, scales, and internal ossification
Gnathostomes (jawed fish)
Novel features: true hinged jaws and paired fins for propulsion and movement
Include two classes: Chondrichthyes and Osteichthyes
Class Chondrichthyes
Characteristics:
Cartilaginous skeleton (sharks, rays, skates)
Predominantly marine and carnivorous
Covered with placoid scales; possess a well-developed sensory system
Respiratory system: breathe through gills
Reproduction:
Internal fertilization; can be viviparous (live birth), oviparous (eggs hatch outside), or ovoviviparous (eggs develop inside and then birthed alive)
Class Osteichthyes
Characteristics:
Bony skeleton; covered with overlapping scales
Present in marine and freshwater species; varied dietary habits (carnivorous, herbivorous, omnivorous)
Respiration through gills; possess a swim bladder for buoyancy
Reproduction:
Sexual reproduction; external fertilization with minimal parental care
Fish Circulatory System
Structure: 2-chambered heart (one atrium and one ventricle)
Circulation Path:
Blood flows from heart to gills for oxygenation (gill circulation) and then to body (systemic circulation)
Deoxygenated blood returns to the heart.
Amphibians
Definition: Amphibians, meaning "dual life," are a class of Chordata that exhibit both aquatic and terrestrial life stages.
Examples: Frogs, salamanders, and caecilians.
Evolution & Physical Traits
Evolutionary Transition: Represent the first vertebrate tetrapods with a notable evolutionary trait of 4 complex limbs.
Limbless Species: Hind limbs are absent in caecilians, showcasing evolutionary adaptation.
Skin Characteristics: Have moist, permeable skin that aids in water absorption.
Respiration
Species-Specific: Different species utilize various methods (lungs, skin, gills).
Unique Features:
Skin can function in gas exchange.
Some (like axolotls) possess gills.
Advanced Sensory Systems:
Color vision and well-developed hearing in frogs and toads, not in all amphibian species.
Thermoregulation
Ectothermic Nature: Body temperature relies on environmental heat rather than metabolic processes.
Behavioral Adjustments:
Basking in sunlight for warmth.
Seeking shade or water for cooling.
Reproduction
Water Dependency: Formation of eggs in aquatic environments fosters external fertilization (sperm and eggs released into water).
Metamorphosis:
Complete Metamorphosis: Characteristic life cycle with larval stage (tadpoles) transitioning through metamorphosis to adult stage.
Some retain larval traits as adults (e.g., axolotls).
Larval Stage
Characteristics of Larvae:
Gills, long-finned tails, lack of limbs, varying dietary habits (filter feeding to predation).
Changes During Metamorphosis:
Loss of tail and gills, development of four limbs, jaw, digestive system, eardrum, and lungs.
Circulatory System
3-Chambered Heart Structure:
Composed of two atria and one ventricle.
Deoxygenated blood enters the right atrium, pumped to respiratory organs for oxygenation, then back to the left atrium to be pumped into the body.
Double Circuit: Blood flows from heart to respiratory organs, back to the heart, and then throughout the body.
Reptiles
Characteristics:
Includes lizards, snakes, and turtles.
Major innovations such as the amniotic egg for terrestrial reproduction and scaly skin to limit water loss.
Physiology
Respiration: Exclusively via lungs.
Growth: Continuous throughout life, shedding skin as needed.
Ectothermic: Depend on external sources for body heat, regulating temperature through behavior.
Amniotic Egg Structure
Adaptation:
Protective shell enclosing the embryo.
Components include:
Albumen: Protects and nourishes the embryo.
Chorion: Membrane surrounding the embryo.
Yolk Sac: Nutrient source for the embryo.
Amnion: Fluid-filled cavity protecting the embryo.
Allantois: Facilitates gas exchange.
Circulatory System
Reptilian Heart Structure:
Also a 3-chambered heart but with a partial septum to minimize mixing of oxygenated and deoxygenated blood.
Certain species like alligators and crocodiles have a primitive 4-chambered heart.
Birds
Lineage: The only surviving clade descended from dinosaurs, exhibiting distinct traits such as feathers.
Thermoregulation: Endothermic with elevated body temperature necessary for flight.
Morphology
Physical Adaptations for Flight:
Wings, feathers, and low body weight.
Unique skeletal features like lighter bones and absence of a urinary bladder.
Digestive/Excretory Adaptations:
Cloaca for waste and reproductive functions.
Reproduction
Internal Fertilization: Eggs with a hard calcium shell, requiring incubation and parental care.
Mammals
Key Traits:
Unique adaptations include hair and mammary glands.
Regulate body temperature through metabolic heat.
Skin & Adaptations
Secretory Functions:
Sebaceous, eccrine, and apocrine glands aid in moisture retention and temperature regulation.
Jaw structure: Adductor muscles permit nuanced jaw movement for feeding.
Circulatory System
Heart Structure:
4-Chambered heart completely separates oxygenated and deoxygenated blood.
Circulation: Pulmonary (to lungs) and systemic (to body) circulation pathways.
Pulmonary System
Air Pathway: From nasal cavity to trachea, leading into lungs.
Branching Structure: Bronchial tree leading to bronchioles for efficient gas exchange.
Types of Mammals
Monotremes: Egg-laying mammals that provide milk.
Marsupials: Short gestation, develop in pouches.
Placentals: Longer pregnancies supported by a placenta for nutrient and gas exchange.