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Invertebrate Deuterostomes

Invertebrate Deuterostomes

Overview

  • This section covers invertebrate deuterostomes, focusing on echinoderms, hemichordates, and invertebrate chordates.

Echinodermata & Hemichordata

  • Chapter 16 is dedicated to Echinoderms.
  • Chapter 17 covers Hemichordata and Invertebrate Chordates.

Developmental Pattern of Deuterostomes

  • In deuterostomes, the first opening into the blastula becomes the anus.

Major Phyla within Deuterostomes

  • Ambulacraria: Includes Echinodermata and Hemichordata.
  • Echinodermata
  • Hemichordata
  • Chordata: Includes Tunicates and Lancelets

Animalia Phylogeny

  • Deuterostomia are a major group within Animalia, branching from earlier groups like Protists and Spiralia & Ecdysozoa

Deuterostome Phylogeny

  • Key phyla include Echinodermata, Hemichordata, and Chordata.

Echinodermata Characteristics

  • Calcium carbonate endoskeleton
  • Water vascular system
  • Loss of pharyngeal slits
  • Diffuse epidermal nervous system
  • Larvae with ciliary bands
  • Tripartite coelom
  • Radial cleavage, enterocoelous coelom formation.

Hemichordata Characteristics

  • Buccal diverticulum
  • Crown of ciliated feeding tentacles (in Pterobranchia)
  • Pharyngeal slits
  • Dorsal tubular nerve cord

Chordata Characteristics

  • Endostyle or thyroid gland
  • Tadpole larva
  • Postanal tail
  • Notochord
  • Pharyngeal basket (Urochordata)

Subphyla and Groups

  • Pterobranchia: Sessile and colonial. Loss of pharyngeal slits. Crown of ciliated feeding tentacles
  • Enteropneusta: Acorn worms.
  • Urochordata: Tunic. Loss of coelom.
  • Cephalochordata: Buccal apparatus.
  • Craniata: Enlargement of the neural tube forms a three-part brain. Endoskeleton including a cranium.

Phylum Echinodermata

  • Calcareous endoskeleton of ossicles.
  • Adults exhibit pentaradial symmetry; larvae are bilateral.
  • Water vascular system.
    • Tube feet & ampullae.
  • Complete digestive system (sometimes reduced).
  • Hemal system (originating from the coelom).
  • Reduced nervous system.
    • Nerve ring.
    • Nerve net.
    • Radial nerves.

Class Asteroidea (Sea Stars)

  • Typically have 5 arms radiating from a central disk.
  • Possess ambulacral grooves.
  • Madreporite located on the aboral surface.
  • Most are predatory, capable of everting their cardiac stomach to engulf prey, feeding on Mussels.
  • Regeneration capabilities allow for asexual reproduction.

Anatomy

  • Short digestive tract.
  • Stomach, anus, central disk, spines, gills, madreporite, radial nerve, gonads, ampulla, podium, tube feet, radial canal, ring canal, digestive glands.

Class Ophiuroidea (Brittle Stars & Basket Stars)

  • Characterized by long, mobile arms and a pentagonal central disk.
  • Madreporite is located on the oral surface.
  • Tube feet lack suction cups or ampullae.
  • Feeding habits include predation and scavenging; basket stars are filter feeders.

Class Echinoidea (Sea Urchins, Sand Dollars)

  • Rounded body shape, with the skeleton forming a test of closely fitted plates.
  • Mobile spines, sometimes with venom glands.
  • Long pedicellaria.
  • Aristotle’s lantern: chewing apparatus.
  • Diet consists of algae, corals, bryozoans, etc.

Class Holothuroidea (Sea Cucumbers)

  • Lack arms; oral tentacles are modified tube feet.
  • Elongate bodies with muscular body walls and reduced skeletal ossicles.
  • Most ingest detritus.
  • Antipredator defenses:
    • Toxins.
    • Cuverian tubules.
    • Evisceration.

Class Crinoidea (Sea Lilies, Feather Stars)

  • Extensive fossil record.
  • Arms with pinnules form a crown.
  • Calyx supports the crown and attaches to a stalk (in lilies) or rootlike cirri (in stars).
  • Suspension feeders.

Phylum Hemichordata

  • Share pharyngeal slits & dorsal nerve cord with Chordata.
  • Body plan: Proboscis, collar & trunk.

Enteropneusta (Acorn Worms)

  • Tube dwellers that collect food on a mucus-covered proboscis, sweeping it to the mouth with ciliated tracts.
  • Produce copious amounts of poop/sand.

Pterobranchia

  • Possess a collar with arms & ciliated tentacles.
  • Form colonies through asexual reproduction.

Deuterostome Phylogeny (Detailed)

  • Echinodermata
    • Holothuroidea: Elongation on oral/aboral axis, reduction of skeletal plates into ossicles.
    • Echinoidea: Fusion of skeletal plates into a rigid test. Loss of suckers
    • Ophiuroidea: Extension of ambulacral grooves along the side of the body
    • Asteroidea: Oral surface to substrate, tube feet with suckers
    • Crinoidea: Numerous extinct taxa. Arms with ciliated grooves for suspension feeding
  • Hemichordata
    • Pterobranchia: Crown of ciliated feeding tentacles.
    • Enteropneusta
  • Chordata: Three subphyla. Dorsal tubular nerve cord, notochord, postanal tail, tadpole larva, endostyle or thyroid gland

Phylum Chordata

  • Defined by four key characteristics:
    • Notochord
    • Pharyngeal slits
    • Dorsal tubular nerve cord
    • Post-anal tail
  • Endostyle or thyroid gland
  • Ventral “heart”

Derived Characters of Chordates

Notochord

  • A longitudinal, flexible rod between the digestive tube and nerve cord.
  • Provides skeletal support throughout most of the length of a chordate.
  • In vertebrates, a more complex, jointed skeleton develops; adults retain only remnants of the embryonic notochord.

Dorsal, Hollow Nerve Cord

  • Develops from a plate of ectoderm that rolls into a tube dorsal to the notochord.
  • Develops into the central nervous system: the brain and spinal cord.

Pharyngeal Slits

  • Grooves in the pharynx called pharyngeal clefts develop into slits that open to the outside of the body.
  • Functions:
    • Suspension-feeding structures in invertebrate chordates.
    • Gas exchange in vertebrates (except tetrapods).
    • Develop into parts of the ear, head, and neck in tetrapods.

Muscular, Post-Anal Tail

  • A tail posterior to the anus.
  • Often reduced during embryonic development.
  • Contains skeletal elements and muscles.
  • Provides propelling force in many aquatic species.

Chordate Evolution

  • ANCESTRAL DEUTEROSTOME → Notochord

  • Chordates → Vertebrae

  • Vertebrates → Jaws, mineralized skeleton

  • Gnathostomes → Lungs or lung derivatives

  • Osteichthyans → Lobed fins

  • Lobe-fins → Limbs with digits

  • Tetrapods → Amniotic egg

  • Amniotes → Milk

  • Key groups along the evolutionary path:

    • Echinodermata
    • Cephalochordata
    • Urochordata
    • Myxini
    • Petromyzontida
    • Chondrichthyes
    • Actinopterygii
    • Actinistia
    • Dipnoi
    • Amphibia
    • Reptilia
    • Mammalia

Subphylum Urochordata (Tunicates)

  • Adults feed via water circulating through a pharyngeal basket; oral & atrial siphons.
  • Tough, secreted tunic.
  • Monoecious; tadpole-like larvae (chordate characteristics) that settle and transform.
  • Most are sessile, some colonial, and some planktonic (salps).

Anatomy & Life Cycle

  • Larva: Notochord, dorsal hollow nerve cord, tail, muscle segments, intestine, stomach, atrium, pharynx with slits
  • Adult: Incurrent siphon to mouth, excurrent siphon, anus, intestine, esophagus, stomach, pharynx with numerous slits, atrium, tunic

Subphylum Cephalochordata (Lancelets)

  • Small filter feeders that mostly bury tail-first in the sand.
  • Muscular notochord.
  • Dioecious, with external fertilization.
  • Studied as a “basic chordate” model (but NOT an ancestor!).