Chapter 23 Intro to Chordates/Vertebrates

agnatha vs gnathostome

no jaws vs jaws

fins vs limbs

fish vs tetrapods

cartilage vs bones

chondrichthyes vs osteichthyes

anamniotes vs amniotes

none vs yes to embryo with fluid filled sac

Notochord

Supportive rod that stiffens the whole body & provides a site for muscle attachment
Composed of fluid-filled cells enclosed in a fibrous sheath surrounded by elastic sheath (hydrostatic organ)
Flexible, but not compressible
VENTRAL to CNS
Persists throughout life in agnatha (amphioxus/hagfishes) and jawless fishes/ lampreys
Replaced by vertebrate column in gnathostomes/vertebrates (remnants remain in intervertebral discs)
[becomes spine for support in human]

Dorsal Tubular Nerve Chord

- Nerve cord is dorsal to digestive tract
- Hollow, tubular shape results from development
- Anterior end enlarges to form brain
- Protected by neural arches and a cranium in vertebrate chordates
[becomes CNS, sits on TOP of spine]

Post anal tail

Postanal tail, with musculature attached to notochord, provides flexible motility ability
Evolved for propulsion in water – later fins where added to it to improve efficiency
Lost in apes and humans (only left as a vestigial appendix: coccyx)
Segmented muscles = myomeres (in vertebrates & some protochordates)
[TAIL BONE useless for humans]

Endostyle

Endostyle occurs in protochordates and lamprey larvae; secretes mucus to trap food, some cells secrete iodinated proteins.
Thyroid gland is the derivative of endostyle for gnathostome; produces iodinated hormones (wraps around throat)
[essentially iodinated protein/hormone secretion]

Pharyngeal pouch/slit

- In aquatic organisms, some pouches break through to form slits
+Perforated pharynx evolved as a filter feeding structure (and still functions as such in protochordates)
+Evolved into gills for respiration in aquatic vertebrates
- In others (tetrapods) pockets do not break through and form arches instead
- Give rise to Eustachian tube, middle ear cavity, tonsils, parathyroid glands
[essentially pharynx that allows us to take in air for lungs and food/water for esophagus and maintain pressure on land]

formation of pharyngeal grooves

inpocketing outside of ectoderm

pharyngeal pouch formation

outpocketing of the endodermal lining of the pharynx

neural crest

highly migratory; contribute to head and facial region
Develops into:
Cranium
facial and anterior ventral skull cartilage and bones
tooth dentine
some cranial nerves
ganglia
some endocrine glands (ex. adrenal medulla)
Schwann cells
various tissues of the head and neck

ectodermal placodes

form various sensory structures
develops into:
olfactory epithelium
lens of the eye
inner ear epithelium
some ganglia and cranial nerves
lateral line mechanoreceptors
electroreceptors

Why were the ‘upgrades’ seen in chordates so important, and how?

supporting higher activity level & metabolic rate of vertebrates; also provide protection, caused by the flexibility of Hox genes in development and patterning of the body axis (4 sets vs invertebrates having 1)

Anatomical adaptations

endoskeleton: cartilage → to bone creates cranium for protection, vertebrae, centra (replace notochord for strength), and neural spines for muscle attachment
exoskeleton: (develops from skin)
bone: early fish had bony, dermal armor → scales
keratin: terrestrial vertebrates have keratinized epidermal structures (Scales, hair, feathers→ nails and claws)

physiological adaptations

Respiration: Pharynx to pump water, gills cause function of pharynx → gas exchange
Circulation: multichambered heart + erythrocytes with hemoglobin = increased gas, nutrient and substance transport
Digestion: move food with muscles rather than cilia, develop liver and pancreas for more nutrient absorption
Excretion: glomerular kidneys= remove waste, regulate body fluid + ion concentration

nervous +sensory adaptations

DISTINCT head, face, and specialized sense organs
anterior end of DTNC=tripartite brain (fore,mid,hind- brain) protected by cranium
Paired sensory organs for distance reception, rest of DTNC becomes spinal chord
caused by NEURAL CREST and ECTODERMAL PLACODES