Lesson 12: Chordata (Cephalochordata and Tunicates)

Chordata & Invertebrates Similarities

• bilateral symmetry 

• anterioposterior axis 

• coelom 

• tube-within-a-tube body plan 

• metamerism 

• cephalization 

5 Classic Chordate Characteristics

1. dorsal hollow nerve cord

2. notochord

   • first 2 only true synapomorphies 

3. pharyngeal pouches/slits 

4. endostyle/thyroid gland 

5. muscular, postanal tail 

Notochord

• rodlike, semirigid tissue enclosed in a sheath

• extends the length of the body, lying between the gut and the nervous system

• main purpose is to stiffen the body

  • providing skeletal scaffolding for the attachment of swimming muscles

  • support

• always found at some embryonic stage

  • 1st part of the endoskeleton to appear in the embryo

• in non vertebrate and jawless chordates it persists throughout life

  • however, it is displaced by the vertebrae in most vertebrates. remains as a disc

Dorsal Hollow Nerve Cord

• dorsal to digestive tract

• anterior end= brain

  • via neurulation

• passes through the neural arches of vertebrae 

  • or just runs dorsal to notochord if no vertebrae 

Neurulation

• the process in early vertebrate development where the neural plate, a flat sheet of cells on the back of the embryo, folds to form the neural tube

• ectodermal origin 

Pharyngeal Pouches/Slits

• from the in pocketing of the ectoderm and envagination of endoderm of pharynx 

• perforated pharynx function as filter-feeding apparatus in non-vertebrate chordates 

• post embryonic secondary development in some vertebrates

  • auditory tube, middle ear, glands, larynx…

• fishes added a capillary network with thin gas-permeable walls

  • leads to the evolution of gills 

Endostyle/Thyroid Gland

• longitudinal ciliated groove ventral to pharynx 

• some cells in endostyle secrete iodinated proteins homologous with iodonated hormone-secreting thyroid gland of adult lampreys and the remainder of vertebrates 

• secretes mucous for food capture in the filter-feeding non vertebrate chordates 

Postanal Tail

• postanal tail + musculature = motility

  • mainly for larval tunicates and amphioxus to swim 

• efficiency increased in fishes, but became smaller or vestigial in later lineages 

Earliest Known Chordate

• Pikaia

  • 510 million years ago

• remnants found in burgess shale, BC

• similar to living cephalochordate

  • though originally classified as a polychaete

Cephalochordata

• the poster chordate (lancelets) 

• amphioxus

  • pointy at both ends

• burrow in porous mud and sand 

  • seldom swim 

• expose mouth to seawater, and filter out particles 

  • water enters the mouth driven by cilia in the buccal cavity and pharynx 

  • passes through pharyngeal slits, where food is trapped in mucus secreted by the endostyle 

  • branchiostoma: gill mouth

• closed circulatory system

  • but no heart

• only moderate cephalization

  • no distinct brain or cranium

• body surface respiration

Urochordata

• tunicates

  • named after the cellulose tunic layer

• most sessile as adults 

  • but some are free living

• two classes 

  • ascidiacea 

  • appendicularia 

• no cephalization 

• two-directional heart 

Ascidiacea

• sea squirts

  • class of tunicates

• may be compound, colonial, or solitary 

• one of aquacultures worst enemies 

  • common in atlantic canada, and wreck havoc on aquaculture sites via biofouling 

• includes the thaliacea (salps)

Thaliacea

• the salps

  • apart of the class ascidiacea, tunicates 

• free swimming

  • can be solitary or colonial 

• use water current for both feeding and gas exchange, as well as for locomotion 

  • form of jet propulsion 

• feed using a mucus net 

Appendicularia

• “small appendage” 

• larvaceans; ghost larva

  • pelagic, free-swimming, tiny larva 

• show all 5 chordate characters in the adult form 

• secrete a mucous house to suspension feed 

Chordata Organization

Chordata Symmetry

Chordata Body Cavity

Chordata Development

Chordata Segmentation