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!).