Thoreau believes that people should work because
a. it will give them time to think about important things.
b. they love what they do.
c. it will keep them out of trouble.
d. god said so in the bible.

"the day we found the sharks' teeth" i would say highlight the element of the plot as it sets its boundaries and centers the adventures of them being on a boat and cleaning spit of sand.

the two answers are

get thee back into the tempest and the night's plutonian shore

on the pallid bust of pallas just above my chamber door