What does the author mean when she writes "I could see him pulling his old turtle head back into his stupid turtle shell" in Because of Winn-Dixie?

This quote is a metaphor, a
figure of speech in which a comparison between two unlike things is made without using
like or as for the purpose of creating a
deeper meaning.  Here, the comparison is between Opal's father and a
turtle.

In this scene, the preacher (Opal's father) is
listing for her ten things about her mother for her to remember.  When he mentions that
number ten is "that she loved you very much," Opal responds, "But she left me."  When
her father replies, "She left us," Opal sees him as a timid turtle, who rather than
facing his problems head on, simply retreats into his shell and hides from things.  Opal
wishes her father would be more courageous, more of a leader, and simply stronger as a
person.  The betrayal and confusion over her mother leaving, and this view of her father
is a premise for the entire story, which is why Opal feels so lonely, and why Winn Dixie
makes such a wonderful companion for her.