The torso sausage
The torso-sausage is the core of any animal, and quite easy to construct. In this demo I actually drew the ribcage way too small, but ignoring that, look at how the sausage is like a water balloon that is being held up these two solid masses inside of it. The middle sags in the middle, and it remains relatively rounded and simple, conveying solidity and volume.
While a lot of animals will have visible masses and bulk along their spines (on the top), I still construct them in this manner, only adding the additional bulk after the fact similarly to the organic intersections from lesson 2.