Notes on Deuterostomes and Chordates

Chapter 31: Deuterostome Animals

Major Characteristics of Deuterostomes

  • Radial symmetry as adults: Adult organisms show a radial symmetry but are classified under bilateral symmetry, with a special case called pentaradial symmetry.

  • Embryonic Development: All deuterostomes follow a deuterostomic pathway for embryonic development which includes:

    • Radial cleavage

    • Blastopore becomes the anus

    • Coelom develops from mesodermal pockets budded off from the endodermal cavity of the gastrula.

  • Common Ancestor: They share a common ancestor that was bilaterally symmetrical and possessed pharyngeal slits.

Groups of Deuterostomes

  • Two major groups within deuterostomes:

    • Phylum Echinodermata: Includes starfish, sea urchins, and sea cucumbers.

    • Phylum Chordata: Encompasses organisms known as chordates, including vertebrates and some invertebrates.

Echinodermata Overview

  • Diversity:

    • Around 7,500 species, primarily slow-moving or sessile marine animals.

  • Characteristics:

    • An endoskeleton composed of calcareous plates.

    • The epidermis covers the internal skeleton and may have spines or bumps.

    • Water Vascular System: A network of hydraulic canals branching into tube feet used for locomotion, feeding, and gas exchange.

    • Although echinoderms possess spoke-like structures, their body arrangement can lead to a unique pentaradial symmetry.

  • Classes of Echinoderms:

    1. Asteroidea (Sea Stars and Sea Daisies)

    • Features: Central disk with outward radiating arms, tube feet on ventral side.

    • Feeding: Everts stomach into bivalve's mantle cavity, secreting digestive juices and can regenerate lost appendages.

    1. Ophiuroidea (Brittle Stars)

    • Distinct central disk, thin arms; move by thrashing.

    1. Echinoidea (Sea Urchins and Sand Dollars)

    • Lack arms, have five rows of tube feet; specialized mouth structures for feeding.

    1. Crinoidea (Sea Lilies and Feather Stars)

    • Either sessile or moveable; suspension feeders with arms surrounding a dorsal mouth.

    1. Holothuroidea (Sea Cucumbers)

    • Reduced endoskeleton; ejects gut parts to confuse predators but can regenerate.

Chordate Diversity and Taxonomy

  • The Phylum Chordata has around 68,000 species and is divided into:

    • Subphylum Cephalochordata (e.g., lancelets)

    • Subphylum Urochordata (e.g., tunicates)

    • Subphylum Vertebrata (vertebrates)

Defining Characteristics of Chordates

  • All chordates share four distinctive features, some of which are present only in embroyonic stages:

    1. Notochord: A flexible rod providing skeletal support.

    2. Dorsal, Hollow Nerve Cord: Unique to chordates, forms central nervous system.

    3. Pharyngeal Gill Slits: Develop from the pharynx allowing for water flow and aiding respiration.

    4. Muscular, Post-anal Tail: Extends posteriorly to the anus, aids in locomotion.

Chordate Groups and Characteristics

  • Cephalochordata (Lancelets):

    • Most basal group; adults retain all four chordate characteristics, filter feeding by burying in sand.

  • Urochordata (Tunicates):

    • Larval stage retains chordate features, but adults undergo metamorphosis losing notochord and tail, becoming sessile filter feeders.

  • Vertebrata:

    • Defined by possession of a backbone. Vertebrates display complex organ systems, an internal skeleton, and advanced nervous and circulatory systems.

Vertebrate Classes

  • Class Myxini (Hagfishes):

    • Jawless, cartilaginous skeleton, uses sensory tentacles for feeding.

  • Class Petromyzontida (Lampreys):

    • Jawless, teeth-like structures for parasitic feeding, also cartilaginous.

  • Gnathostomes- jawed vertebrates:

    • Features jaws derived from ancestors' gill structures, fostering more complex feeding and locomotion capabilities.

  • Class Chondrichthyes (Sharks, Rays):

    • Cartilaginous skeletons, adapted to predatory lifestyles, with modified teeth and distinct evolutionary traits.

  • Class Osteichthyes (Bony Fish):

    • Possess a bony endoskeleton; includes ray-finned and lobe-finned fishes.

Tetrapod Evolution

  • Lobe-finned ancestors adapted to land around 365 MYA, leading to the development of limbs and the evolution of tetrapods, marked by skeletal structure transition from fins to limbs.

Reptiles and Birds

  • Class Reptilia: Ectothermic, laying eggs with shells, adapted for dry environments.

  • Class Aves: Birds derived from theropod dinosaurs, adapted for flight with feather and bone modifications.

Mammals

  • Characteristics:

    • Hair, mammary glands producing milk, differentiated teeth, and endothermy.

  • Groups:

    1. Monotremes: Egg-laying mammals (platypus).

    2. Marsupials: Pouched mammals (kangaroos).

    3. Eutherians: True placental mammals, providing live birth with complex reproductive systems.