Unit 2: Early Chordates

3 subphyla in the phylum chordata:

• 1) urochordata - tunicates and sea squirts

• 2) cephalochordata - amphioxus

• 3) vertebrate - craniates

Chordates

• animals that have a notochord in the embryo stage (at least)

  • Craniates - chordates with a neurocranium (brain case)

  • Vertebrates - chordates with vertebrae

Big 4 morphological characteristics of a chordate

• 1) notochord

• 2) hollow, dorsal nerve cord (and brain)

• 3) perforated pharynx

  • Pharyngeal gill slits or pouches

• 4) post-anal tail

Notochord

• the most distinctive character that all chordates have in common

• Longitudinal rod of support tissue derived from dorsal wall of embryonic gut

• All chordates have a notochord at some point during early development

• Cephalochordates and many vertebrates (but not urochordates) retain the notochord as adults

• A rod of living cells located immediately ventral to the central nervous system and dorsal to the alimentary canal, extending from the midbrain to the tip of the tail

• The part of the notochord in the head becomes incorporated in the floor of the skull

• In most vertebrates, the part in the trunk and tail becomes surrounded by vertebrae

• Hemichordates have a muscular proboscis with a small organ thought to be a homologous to a notochord

Dorsal hollow nerve cord

• it is derived from ectoderm by a process called neuralation

• A dorsal hollow central nervous consists of a brain and spinal cord and contains a central lumen - neurocoel

Perforated pharynx

• present, at least at some time during development and also usually in adults

• The pharynx is an expanded anterior portion of gut

• It may be perforated by numerous slits and allow water taken into mouth to be passed out of the body

• Pharyngeal arch (gill arch) - separates adjacent pharyngeal pouches

Ventral heart in circulatory system

• it may only be a ventral pulsating vessel (cephalochordates)

• Basic pattern:

  • The “heart” pumps blood forward into a ventral aorta, up through aortic arches, then into a dorsal aorta

Other chordate characteristics

• cephalization - principal sense and nervous organs are concentrated in the head (or head region)

  • Urochordates have no head

• metamerism - segmentation of some features of the body

Deuterostomes

• phylum echinodermata

• Phylum hemichordata - acorn worms

• Phylum chordata

Phylum Hemichordata

• tongue worms

• Balanoglossus and dolichoglossus

divide the phylum chordata into 3 subphyla:

• urochordata - tunicates

• Cephalochordata - amphioxus

• Vertebrata - bunch of classes

Classes of Vertebrata

• class Agnatha

• Class placodermi (all extinct)

• Class chrondrichthyes

• Class osteichthyes

• Class amphibia

• Class Reptilia

• Class Aves

• Class mammalia

Characteristics of Hemichordata

• have a vertebrate-type pharyngeal gill slits

• Have a larvae called a tornaria, which is bilaterally symmetrical

Circulatory System of Hemichordata

Consists of a dorsal and ventral longitudinal vessel

Reproductive System of Hemichordata

Consists of a segmented series of gonads

No vertebrate chordates

2 groups of extant non-vertebrate chordates:

• subphylum urochordata

• Subphylum cephalochordata

Subphylum Urochordata

• common name - tunicates

• Inhabit coastal areas of all oceans

• 2000+ living species

Adult characteristics of subphylum Urochordata

• shaped like sacs or stalks

• Sessile - do not move around

• The body is covered by a tunic

• Between the tunic and pharynx is a space - atrium

• Adults are filter-feeders

  • Water enters the pharynx by incurrent siphon, seeps through pores into the atrium, then leaves through the excurrent siphon

• food is trapped in a sticky mucus that moves from endostyle to pharyngeal bars and then to the esophagus

• Most adult tunicates really don’t look much like either cephalochordates or vertebrates

  • Pharyngeal gill slits are the only remaining chordate feature

Larvae Characteristics of Subphylum Urochordata

• appear more likely to belong to the phylum Chordata than adults do

  • Vertebrate characters are more apparent in the larvae

• Notochord - prominent, but only in the tail

  • Urochordata = tail + chord

• dorsal hollow nerve cord is present

• Pharynx with gill slits

• Post-anal tail is present

• Life span of larval stage - 2 days, then goes to bottom of sea and attaches to metamorphize into a sessile adult

Subphylum cephalochordata

• about 22 species in 2 living genera

• Good fossils back to 500 million years ago

• Small, fusiform (long and thin), fish-like, marine animals found throughout the world, particularly coastal areas with shallow warm water

• Best known for- lancelet - common name: amphioxus

Amphioxus has both primitive and specialized characteristics

• primitive

  • Nephridia

  • Pigmented eye spot

  • No heart

  • Epidermis - 1 cell thick

  • Musculature - segmented

  • Notochord for support

• specialized

  • Notochord extends anterior to the brain

  • Atrium and meta pleural folds

  • Feeding apparatus

Characteristics of Cephalochordata (amphioxus)

• myomeres - blocks of striated muscle fibers arranged on both sides of the body, separated by sheets of connective tissue

  • also found in urochordate larvae

• Metapleural folds

• notochord

  • Extends the full length of the body

  • Prevents the body from shortening when myomeres contract

  • It extends from the tip of the snout to the end of the tail, projecting beyond the region of myomeres

Respiratory System of Cephalochordata

• in amphioxus, the pharynx and gill slits are used for filter-feeding

  • They “breathe” through the skin

  • In vertebrates, the pharynx and gill slits are used for respiration

  • A major difference in cephalochordates and vertebrates

• amphioxus is small enough that respiration can occur by diffusion over the body surface

Amphioxus

Filter-feeding specializations of the head

• 1) buccal cirri - attached to the margin of the oral hood

  • These filter out large particles

• 2) wheel organ - ciliated tracts, some food particles are entrapped and sent back to pharynx

• 3) velum - screens food particles (velar tentacles)

Circulatory system of Amphioxus

• no true heart, but blood vessels conform to the basic vertebrate plan

• no red or white blood cells

• Ventral to dorsal circulation through gill arches powered by a ventral pump located behind the gill slits

• in vertebrates, it is necessary to have rapid movement of blood because that is our O2 source

• Since there is no need for amphioxus to transport oxygen by blood, there isn’t a need for rapid movement of blood

Nervous system of Amphioxus

• hollow dorsal nerve cord

  • Similar to other vertebrates

• no true brain at the anterior end

Excretory system of Amphioxus

• doesn’t conform to the basic vertebrate plan - no discrete kidney

  • Solenocytes/podocytes make up the excretory system and are also part of the nephrons in vertebrates amphioxus

  • An important difference is that the excretory organs of cephalochordates are derived from ectoderm instead of mesoderm (as in vertebrates)

Digestive system of amphioxus

• do not have a true liver

  • They have a hepatic diverticula or midgut cecum