What allows for the more complex morphology of humans relative to those of simple invertebrates such as nematodes?

The answer is; endoskeleton

An endoskeleton allows for smooth and steady growth unlike in an invertebrate with an exoskeleton that needs to shed their skeleton during irregular growth spurts. An endoskeleton is significant for leveraging (like an anchor) skeletal muscles hence enabling the animals to grow big and is able to support their own weight.