A myelin sheath is not found in the nerve fibres of
A
Fish
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B
Frogs
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C
Egg laying mammals
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D
Cyclostomes
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Solution
The correct option is D Cyclostomes Myelin is considered a defining characteristic of the (Gnathostome) vertebrates only. Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason, are often referred to as Cyclostomes. They are the most primitive of the vertebrates. The axons of their neurons are unmyelinated (like those of all invertebrates).