Ch 2 – Origin of Chordates

Kingdom Animalia

All animals.

Subkingdoms

Parazoa (sponges) vs. Eumetazoa (true tissues).

Bilateria

Animals with bilateral symmetry.

Developmental Lineages

Protostomia (mouth first) vs. Deuterostomia (anus first).

Superphylum

Ambulacraria (includes echinoderms and hemichordates).

Phylum

Chordata.

Multicellularity

Begins as a blastocyst (a ball of cells).

Cleavage

Cell division from the zygote (radial or spiral).

Gastrulation

The process where cells grow inward (invagination) to form the gut.

Germ Layers

Formation of ectoderm (outer), endoderm (inner), and mesoderm (middle).

Blastopore Fate (Protostomes)

Becomes the mouth.

Blastopore Fate (Deuterostomes)

Becomes the anus.

Cleavage Pattern (Protostomes)

Spiral (cells are offset).

Cleavage Pattern (Deuterostomes)

Radial (cells are aligned).

Coelom Formation (Protostomes)

Schizocoelic (mesoderm splits).

Coelom Formation (Deuterostomes)

Enterocoelic (gut outpockets).

Skeleton (Protostomes)

Ectodermal.

Skeleton (Deuterostomes)

Mesodermal.

Notochord

A slender, hydrostatic rod dorsal to the coelom that provides axial support and signals nerve cord formation.

Pharyngeal Slits

Openings in the pharynx used for filter feeding or evolved into respiratory gills.

Endostyle or Thyroid Gland

A glandular groove in the pharynx floor (endostyle) or a hormone-producing gland (thyroid); both involve iodine metabolism.

Dorsal Hollow Nerve Cord

Derived from ectoderm via invagination; it forms the central nervous system and surrounds the neurocoel.

Postanal Tail

A posterior elongation extending beyond the anus used for locomotion.

Pharynx

The region of the digestive tract used for filter feeding and containing pharyngeal slits.

Invagination

The inward folding or growth of a layer of cells (e.g., during gastrulation or nerve cord formation).

Neural Plate

The specialized area of ectoderm that folds to create the dorsal hollow nerve cord.

Neurocoele

The fluid-filled central canal found within the dorsal hollow nerve cord.

Myomere

Serially repeated blocks of muscle running the length of the body and tail.

Echinodermata

Unsegmented adults with pentaradial symmetry and a calcium carbonate endoskeleton; no head/brain.

Protochordates

An informal group of marine animals that use cilia and mucus for filter feeding; includes hemichordates and non-vertebrate chordates.

Hemichordata

"Half chordates" that are marine benthic worms; they possess pharyngeal slits and a dorsal nerve cord in the collar.

Enteropneusta (Acorn Worms)

Solitary, benthic "worms" living in sand or mud with 1-200 pharyngeal slits.

Chordata

Defined by the five characteristics

Cephalochordata (Amphioxus/Lancelets)

Marine burrowing filter feeders; have a wheel organ for food processing and Hatschek's pit for mucus secretion.

Urochordata (Tunicates)

Marine animals with a saclike body covered by a tunicin "tunic"; larvae have all 5 chordate traits, but adults often lose them.

Patterning

The process of specifying general regions of an embryo to determine the position of body parts.

Body Patterning Difference

The chordate body plan is effectively a hemichordate body plan "flipped over" (inverted).

Inversion

Ventral gene action in non-chordates (like hemichordates) is dorsal in chordates.

Structure locations

Pharyngeal arches were dorsal in hemichordates but are ventral in chordates.