### Shell

Shell drawn using conchoid function
en.wikipedia.org/wiki/conchoid_(mathematics)

```// You can find the Turtle API reference here: https://turtletoy.net/syntax
Canvas.setpenopacity(1);

// Global code will be evaluated once.
const turtle = new Turtle();

function line(x1,y1,x2,y2){turtle.penup();turtle.goto(x1,y1);turtle.pendown();turtle.goto(x2,y2);}

const subdiv = 100;
const max_angle = Math.PI/2*0.95;
const angle_range = max_angle*2;
const da = angle_range/subdiv;

let a = -2.5;

let min_dis = 1;
let max_dis = 90
let dis_range = max_dis-min_dis;
// The walk function will be called until it returns false.
function walk(i) {
for(let dstep=min_dis+1;dstep<max_dis;dstep++)
{
let theta1 = -max_angle+da*i, theta2 = -max_angle+da*(i+1);
let t = (dstep-min_dis)/dis_range;

d = (1-t*t*t*t*t*t*t*t)*dis_range+min_dis;
y1 = a+d*Math.cos(theta1), x1 = a*Math.tan(theta1)+d*Math.sin(theta1);
y2 = a+d*Math.cos(theta2), x2 = a*Math.tan(theta2)+d*Math.sin(theta2);
rt = t*0.5-1;

line(x1+t*2+Math.random(),-y1+Math.random(),x2,-y2);
line(x1+t*3+Math.random()*0.5,-y1+Math.random(),x2,-y2);
line(x1+t*5+Math.random(),-y1+Math.random(),x2,-y2);

line(x1+t+Math.random(),y1+Math.random(),x2,y2);
line(x1+t*1+Math.random(),y1+Math.random(),x2,y2);
line(x1+t*2+Math.random()*1.5,y1+Math.random(),x2,y2);

}

i++;
return i < subdiv;
}
```