Internal organs can be hollow or parenchymatous. The first include the esophagus, stomach, small intestine, large intestine, ureter, etc. Their main peculiarity is that they have a cavity limited by a wall inside them. They can be conventionally depicted as in the animation☝🏻 But parenchymatous organs do not have any cavity inside themselves - they have a connective-tissue skeleton, the basis of the organ supporting its functional (working) tissue. Such organs include liver, kidneys, lungs, etc.