Chordata

Chordata

-Bilateral symmetry is plesiomorphic feature inherited from bilaterians

-Segmentation of body is inherited from ancestral deuterostomes (in development, muscles developed as “blocks” of mesoderms)

-Triploblastic germ layers with coelom

-Includes invertebrates (Cephalochordata, Urochordata) and Vertebrata (95% of Chordata are vertebrates)

Derived Characteristics of Chordata

1. Notochord

2. Dorsal, hollow, nerve cord

3. Pharyngeal slits

4. Endostyle

5. Post-anal tail

Notochord

-Stiff, flexible rod of soft tissue supporting body axis

-Notochord transformed into vertebral column, positioned between nerve cord and digestive tract

-During embryonic development, notochord developed when organs are formed from endoderm, notochord formed from dorsal mesoderm

-Notochord provides structural support, body would be deformed during movement without it

Dorsal, Hollow, Nerve Cord

-Develops from plate of ectoderms, sinks/folds into neural tube, positioned dorsal to notochord

-Notochord/Mesodermal cells secrete signal molecules → Ectodermal neural plate formed → Neural tube develops into CNS

-Dorsal/Hollow nerve cord unique to chordates, while non-chordate animals have ventral/solid nerve cords

Pharyngeal Slits

-Pharynx is an anatomic region posterior to the mouth

-Embryonic arches develop in pharynx → Pharyngeal clefts develop into open slits

-In tetrapod’s, pharyngeal slits present during embryonic development, before being modified in head/neck

-Functions:

• Filter-feeding structures

• - Housing gills for gas exchange in aquatic vertebrates

Endostyle

-Organ on floor of pharynx that secretes mucus/iodine-binding proteins

-During pharyngeal slit development, endostyle groove formed → Cells differentiate into glandular cells

-In vertebrates, endostyle transformed into thyroid gland

Post-anal Tail

-Most adult chordates have muscular tail posterior to anus

• Length of digestive tract is shorter than full body length

-Post-anal tail contains skeletal and muscular elements

-Functions:

• Source of propulsive force

• Facilitator of balance, grip, etc

Urochordata - Larval Stage

-Urochordata are considered sister group to vertebrate

-Tunicate larvae are tadpole-like swimmers

-All 5 chordate synapomorphies present

Two types of Larvae

• Permanent filter-feeding larvae don’t go through metamorphosis

• Non-feeding larvae

  • Brief larval stage

  • Locate/attach to substrates and undergo metamorphosis

  • Non-feeding larvae metamorphize into colonies of pelagic adults

Urochordata - Adult Stage

-Adult tunicates converted by transparent coating called tunic, composed of polysaccharides and proteins

Filter Feeding Process

-Water/food particles drawn into pharynx through incurrent siphon

-Endostyle secretes mucus to catch food particles

-Water squirted through excurrent siphon

-Food passed to stomach for digestion

-Digested waste passes through anus, then excurrent siphon

Most tunicates are sessile filter feeders, pelagic tunicates often form colonies called “slaps”

Predator tunicates are ambush hunters

Cephalochordata

-Amphioxus/lancelets are small, sedentary, filter-feeding intervetebrates

• Lancelets only found in marine habitats

• Lancelets have all 5 synapomorphies

Feeding in adult lancelets

-Wriggle backward into substrates

-Water/food particles drawn into pharynx via ciliary actions

-Endostyle secretes mucus to catch food particles in pharynx

-Water exits body through atriopore

-Food/mucus passed into intestine, digested waste passes through anus