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Hemichordata and Invertebrate Chordates
Phylum Hemichordata
Bilaterally symmetrical.
Body divided into three sections, a proboscis, a collar and a trunk.
Characteristics of Hemichordata:-
All live marine environments.
Body cavity a truecoelom divided intothree cavities.
Complete digestive tract, feeds on fine particles in the water.
Dorsal, sometimes tubular, nerve cord.
Class Enteropneusta
Burrow in sandy and muddy substrates
Ciliated epidermis and glandscover acorn worms
Marine worms
Common name ‘acorn worms’
Maintenance FunctionsVentral MouthLateral Pharyngeal slits,few to several hundred
Cilia and mucus assist acorn worms in feeding
Ciliary tracts converge near the mouth and form a mucoid string that enters the mouth
Enteropneusta extends its posterior end out of the burrow during defecation.
Eww!!!
Nervous system• Ectodermal in origin
• Lies at the base of the ciliated epidermis
• Consist of dorsal and ventral nerve tracts
• No major ganglia
• Sensory receptors are unspecialized
Respiration
• Simple diffusion of metabolic waste
• Cilia associated with Pharyngeal slits circulate water into mouth and out of body
• Gas exchange as water passes through pharyngeal slits
Circulatory system• Colorless blood moves nutrients
and wastes• Dorsal and ventral contractile
vessel• Blood moves anteriorly in dorsal• Posteriorly in ventral vessel• Branches from theses vessels
lead to open sinuses (Partially open circulatory system)
• Anterior flowing blood moves through glomerulus (excretory organ)
• Waste filtered into proboscis coelom and out through pores found in wall of proboscis
Reproduction and Development
Dioecious!!
External Fertilization pheromones
Evolutionary ties b/w hemichordatres and echinoderms
Ciliated larvae
Class Pterobranchia
pteron- wing
branchia- gill
Deep marine water
Some live in shallow waters
Body divided into three regions
Size 0.1 - 5mm
Individuals called zooids
Most live in secreted tubes in asexually produced colonies
Maintenance Functions
Filter feeders
Cilia on tentacles trap and transportFood to mouth
Respiration and excretory exchange by diffusion
Reproduction and Development
Asexual budding is common and responsible for colony formation
Also some posses one or two gonads
Most species are dioeciousExternal fertilizationPlanula-like larva (cnidarian) Settles to a substrate, forms cocoon and
metamorphoses into an adult
Phylum Chordata
Characteristic of the chordatesDeuterostome embryo development
Notochord, phayngeal slits or pouches, dorsal tubular nerve cord, and postanal tail
Coelomate animalsBilateral symmetryEndoskeletonTwo major groups: nonvertebrate chordates
and vertebrates
Subphylum Urochordata
• Uro, tail and chorda, cord
• Class ascidians – Tunicates or sea squirts
• Solitaire or colonial• Sessile adults• Attach to solid substrates• two siphons that permit
seawater to circulate through the body
• Oral siphon also the mouth
• Atrial siphon
Body wall
• Tunic- gown• Connective tissue
like covering• Tough secreted by
the epidermis• Composed of
proteinsSalts, cellulose
• Stolons extensions of tunic help root
Maintenance Functions
• Longitudinal and circular muscles below the body wall epithelium
• Nervous system largely confined to body wall
• Forms nerve plexus with a single ganglion between oral and atrial opening
• Sensitive to mechanical and chemical stimuli around siphon but no complex sensory organs
Reproduction and Development
• Tadpole larva• Attaches to substrate by
adhesive papillae located below the mouth
• During development internal structures rotate 1800 bending the digestive tract into a U-shape
Reproduction and Development
• Monoecious
• Self fertilization and cross fertilization
Subphylum Cephalochordata
• Elongated laterally flattened nearly transparent
• Lancelets• Size up to 5 cm tadpole like
animals• All four chordate
characteristics persist throughout life
• Shallow waters• genera Branchiostoma
(Amphioxus)
Reproduction and Development
• Dioecious• Gonads shed gametes into
the atrium • Leave the body through
the atriopore• External fertilization • Bilaterally symmetrical
larva• Larva free swimming• Larva Settle to substrate
then metamorph into adults
Maintenance Function
• Filter feeders
• Buried in sandy substrate
• Mouth pointed upward
• Cilia on lateral surfaces of gill sweep water into mouth
• Food sorted in the cirri
•Edible particles move along cilia to the gut
• No true heart• Contractile waves in
the walls of major vessels propel blood
• Blood contains amoeboid cells and bathes tissues in open spaces
• Excretory tubules are modified coelomic cells closely associated with blood vessels.
• Coelom reduced- restricted to canals near the gill bars, endostyle and the gonads