Agnatha {jawless fish} were first fish.
size
Jawless fish are mostly small but can be up to one meter long.
body
Jawless fish have cylindrical bodies, with no fins and no jaws. Distinct trunk is between head and tail. Head is independent of trunk. Pharynx separates digestion and respiratory pathways.
backbone
Cartilage backbone supports larger size and more speed.
digestion
Jawless fish prey on small organisms by scavenging and parasitism. They have a sucking disc around mouth. Some agnatha are vertebrate parasites. Jawless fish are not filter feeders.
circulation
Heart has one aorta leaving one ventricle and one vena cava entering one auricle.
circulation: blood
Jawless-fish have hemoglobin with one protein sequence.
skin
Outer-skin epidermis layer is protective and smooth. Inner-skin dermis layer contains blood vessels, skin glands, and neurons.
nervous system
Jawless fish have three brain parts: forebrain for smell, midbrain for sight, and hindbrain for hearing. Telencephalon has olfactory bulb. Optic tectum is for sight. Cortex has three cortical layers. Cerebellum associates with hindbrain for sensorimotor coordination. All vertebrates have similar brainstem serotonin-neuron patterns. Spinal cord distributes nerves to body and collects sense signals.
senses
Vestibular system has one or two semicircular canals and helps balance and vision. Frontal eyes, with no eye muscles and no lens, are for pattern detection and make retinoic acid. Jawless fish can detect prey and mates. Parietal eyes can detect sunlight level. Nostrils aided smell.
senses: pain
Jawless fish seem to feel pain.
behavior
Jawless fish can control sucking.
development
Neural crest is at neural-groove edges.
life cycle
Most jawless fish spawn in fresh-water streams, develop into larvae, metamorphose to adults, and swim back to ocean.
Biological Sciences>Zoology>Kinds>Fish>Jawless
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Date Modified: 2022.0224