That's correct! Roundworms (also known as nematodes) have the following characteristics:

* Muscle tissue: They have longitudinal muscles that allow them to move in a wriggling motion.

* Gut with one opening: They have a digestive tract with a single opening that serves as both mouth and anus. This is called a complete digestive system.

This is in contrast to simpler organisms like flatworms which have a gastrovascular cavity with only one opening.