Animal Development: Germ Layers and Body Plans

Diploblastic vs. Triploblastic

• Diploblastic organisms have two primary germ layers: ectoderm and endoderm
• Triploblastic organisms have three primary germ layers: ectoderm, endoderm, and mesoderm

Body Plans of Animals

• Asymmetrical animals lack any symmetry
• Radially symmetrical animals have body parts arranged around a central axis
• Bilaterally symmetrical animals have a distinct left and right side

Acoelomates vs. Coelomates

• Coelom refers to a body cavity derived from mesoderm tissue
• Acoelomates are triploblastic animals without a coelom, with mesoderm filled with tissue
• Coelomates (also known as eucoelomates) are triploblastic animals with a true coelom, surrounded by mesoderm tissue
• Pseudocoelomates are triploblastic animals with a coelom partially lined by mesoderm and endoderm

Protostomes vs. Deuterostomes

• Protostomes develop the mouth from the blastopore, considered primitive
• Deuterostomes develop the anus from the blastopore, considered novel

Porifera

• Sponges are asymmetrical and filter feeders
• They reproduce both sexually and asexually
• Adult sponges are immobile while larvae are motile
• They are neither deuterostome nor protostome, nor triploblastic nor diploblastic
• Sponges lack true tissue

Cnidaria

• Jellyfish display radial or biradial symmetry
• They have an incomplete digestive system and are diploblastic
• Neither protostome nor deuterostome
• Exist in two forms: polyp (sessile) and medusa (motile)

Platyhelminthes

• Flatworms are bilaterally symmetrical, triploblastic, and protostomes
• They are acoelomates with an incomplete digestive system
• They reproduce both sexually and asexually

Mollusca

• Mollusks are bilaterally symmetrical, triploblastic, and protostomes
• They have complete digestive systems and are divided into three classes:
  • Bivalvia: Clams, mussels, oysters
  • Gastropoda: Snails, slugs
  • Cephalopoda: Octopus, squid

Annelida

• Segmented worms are triploblastic, protostomes, and coelomates
• They exhibit bilateral symmetry with simple cephalization and segmentation
• They are motile with complete digestive systems

Nematoda

• Roundworms are triploblastic, protostomes, and pseudocoelomates
• They are bilaterally symmetrical with primitive cephalization
• They are motile with complete digestive systems and molt an exoskeleton
• They reproduce both sexually and asexually

Arthropoda

• Insects are triploblastic, bilaterally symmetrical, and coelomates
• They have complete digestive systems and are protostomes with jointed appendages and body segmentation

Arthropoda (Largest Phylum)

• They are divided into multiple classes:
  • Crustacea: Mostly aquatic
  • Myriapoda: Terrestrial
  • Chelicerata: First set of appendages are distinct
  • Trilobita: All extinct
  • Hexapoda: True insects

Echinodermata

• Starfish are bilaterally symmetrical in larval stage and radial or pentaradial in adult stage
• They are triploblastic, coelomates, and deuterostomes
• They are invertebrates with complete digestive systems

Chordata

• Chordates are bilaterally symmetrical, deuterostomes, and triploblastic coelomates
• They possess novel adaptations:
  • Notochord
  • Dorsal hollow nerve cord
  • Pharyngeal slits
  • Post-anal tail
  • Endostyle/thyroid gland

Chordata Invertebrates

• Cephalochordata (lancelets): possess notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail, and endostyle
• Urochordata (tunicates): possess pharyngeal slits and endostyle

Fishes

• Agnathans (jawless fish): possess a cranium, includes Hagfish (Myxini) and Lamprey (Petromyzontidae)
• Gnathostomes (jawed fish):
  • Osteichthyes (bony jawed fish): marine and freshwater, possess swim bladder for buoyancy, vertically flattened
  • Chondrichthyes (sharks, rays, skates): mostly marine, cartilaginous, well-developed sensory system
• All fish have a two-chambered heart and a circulatory system

Amphibians

• Include frogs, salamanders, and caecilians
• They exhibit a dual life transitioning between aquatic and terrestrial environments
• They are water-dependent for reproduction
• First tetrapods to evolve
• Undergo complete metamorphosis
• Divided into three orders:
  • Urodela (salamanders): Lungless
  • Anura (frogs and toads): capable of vocalization, have lungs and an eardrum
  • Apoda (caecilians): lack limbs, a derived trait

Reptiles

• Lizards, snakes, alligators, turtles, and others
• Novel adaptation of the amniotic egg
• Respiration in lungs
• Shedding of skin
• Divided into four orders:
  • Sphenodontia: Tuataras
  • Squamata: Lizards and snakes
  • Testudines: Turtles
  • Crocodilia: Alligators, crocodiles, etc.
• Ectotherms: Body heat comes from the environment

Amniotic Egg

• Advantages:
  • Increased access to terrestrial environments
  • Less competition for pools
  • Decreased predation
• Examples:
  • Reptiles
  • Birds
  • Mammals

Amniotic Egg & Evolution

• The evolution of the amniotic egg allowed organisms to reproduce without standing water, allowing for greater terrestrial expansion

Birds

• Novel trait of feathers
• Endotherms: regulate body temperature internally
• Complex sensory system
• Sexual reproduction
• Low body weight for flight
• Possess a cloaca: a body cavity connected to the digestive, urinary, and reproductive systems, eliminating the need for solid waste

Mammals

• Hair and mammary glands are novel adaptations for mammary glands
• Endotherms: regulate body temperature internally
• Secretory skin
• Adductor muscles in the jaw: facilitate up-down and side-to-side movement
• Four-chambered heart
• Kidneys
• External ears
• Brain with cerebral cortex

Three Major Lineages of Mammals

• Distinguished primarily by pregnancy and gestation:
  • Monotremes: Lay leathery eggs
  • Marsupials: Short placental gestation, born fetal and complete development occurs in a pouch
  • Placentals: Placenta connects fetus to mother

Offspring Production

• Viviparous: Egg sac remains attached to mother, embryo obtains nutrients from the mother, live births (most mammals)
• Oviparous: Fertilized eggs are laid outside the mother and obtain nutrients from the yolk (fish, reptiles, birds)
• Ovoviviparous: Fertilized egg develops inside the mother, embryo relies on the yolk for nutrients and hatches inside the mother, young are fully developed at birth (sharks, snakes, lizards)

This quiz covers the differences between diploblastic and triploblastic organisms, various body plans of animals, and the distinctions between acoelomates and coelomates. Additionally, it examines the developmental pathways of protostomes and deuterostomes, emphasizing their key features. Test your understanding of these fundamental concepts in animal biology.