This is about as loose as I get.
Larger hermit crabs are red. I ran into one once in the surf, just before sunrise. I was the size of a 1lb. lobster. It was red with a fleshy tail, and without a shell. It looked like lobster, but lobsters are red after they are cooked. It took me awhile to find out what this red clawed thing was on the internet. Finally I saw a poster showing the parts of a hermit crab. Bingo.
I used to find shells hard to draw. I think it was more of a perspective thing. I have grown in seeing shapes, sizes and relationships better. This comes with practice, practice, practice.