I need some hints on how to draw this figure

Question

This is the figure I hope to draw in Tikz.

I need to draw 4 components.

1. Two Banana-like objects facing each other.
2. Two curves with a certain curvature connecting the two Banana-like objects
3. A circle, placed at a certain location.
4. An arrow connecting the bottom of the circle and passes through one of the curve.

I can figure out how to draw 3 and 4 myself, but not in perfect coordinate as I don't know how to set up 1. But I really need some helps or hints on how to draw 1 (especially) and 2. Thank you very much.

Answer 1

Updated version

I just noticed that the filling for the bananas is done using different shades of gray:

The code:

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[>=latex]
\coordinate (A) at (0,0) ;
\coordinate (B) at (0,-5) ;
\coordinate (C) at (4,0) ;
\coordinate (D) at (4,-5) ;

\filldraw[top color=gray!20,bottom color=gray!70,middle color=gray!30]
  % the banana to the left
 (A) to[out=-110,in=110] 
   coordinate[pos=0.07] (auxlu)
   coordinate[pos=0.93] (auxll)
 (B) to[out=140,in=-140] 
 (A);
  % the banana to the right
\filldraw[top color=gray!70,bottom color=gray!20,middle color=gray!30] (C) to[out=-70,in=70] 
   coordinate[pos=0.07] (auxru)
   coordinate[pos=0.93] (auxrl)
 (D) to[out=40,in=-40] 
 (C);
\draw[thick,looseness=1.3] 
% the upper curve
  (auxlu) to[bend right] (auxru) 
% the lower curve
  (auxll) to[bend left] (auxrl); 
% the circle
\node[circle,draw=green!60!black,inner sep=0.5cm] 
  at (1.2,-2.5) (circle) {};   
% the arrow
\draw[<->,thick]
  (circle.south) -- ++(-45:3cm);
\end{tikzpicture}

\end{document}

Initial version

The code (with explanatory comments):

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[>=latex]
\coordinate (A) at (0,0) ;
\coordinate (B) at (0,-5) ;
\coordinate (C) at (4,0) ;
\coordinate (D) at (4,-5) ;

\filldraw[fill=gray!20]
  % the banana to the left
 (A) to[out=-100,in=100] 
   coordinate[pos=0.05] (auxlu)
   coordinate[pos=0.95] (auxll)
 (B) to[out=120,in=-120] 
 (A)
  % the banana to the right
 (C) to[out=-80,in=80] 
   coordinate[pos=0.05] (auxru)
   coordinate[pos=0.95] (auxrl)
 (D) to[out=60,in=-60] 
 (C);
% the upper curve
\draw 
  (auxlu) to[bend right] (auxru); 
% the lower curve
\draw 
  (auxll) to[bend left] (auxrl); 
% the circle
\node[circle,draw=green,inner sep=0.5cm] 
  at (1.8,-2.5) (circle) {};   
% the arrow
\draw[<->]
  (circle.south) -- ++(-45:3cm);
\end{tikzpicture}

\end{document}

Remarks

• For the "banana" shaped figures, the out=, in= operations.

• For the curved paths the bend operation (the out=, in= operations could also have been used).

• The bend cirves are drawn by joining some auxiliary coordinates placed upon the inner path for the "bananas" with the help of pos=.

• The circle is a \node with circle shape.

• The arrow was drawn using the anchors for the node.