These look great, first of all! The lines are confident and smooth, and really capture the animals likeness. One piece of advice I can give is to make thicker, darker strokes where shadow is. In the photo, the camels jaw is mostly covered in shadow, so drawing it with a heavy line can help to convey that. Halftones can be represented with the thickness you have now, and areas in lots of light can be thinner and/or lighter. You did this well with the snail!