Inside the tortoise shell
This line follows closely the theme of the song. Aesop continually refuses to directly address his problems in the way a turtle uses its shell to defend against predators. This keeps the turtle safe for the time being, but does not address its weaknesses: soft skin, inability to fight for anything, etc.
Basically, these defense-mechanisms prevent further growth as a human being, something heavily tied to the human spirit of expansion. Instead of regaining his power as a man and fighting against these problems, his actions are completely controlled by his metaphorical predators..whatever they may be..
This is another common aspect of the human psyche; those that go through pain are more likely to shy away from the things that originally caused them this pain, whether it be relationships, sports, or anything else.
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