The Monster's Web: Basehart disappears part way through
I think Richard Basehart disappears part way through the filming of this episode.
All the flying sub scenes were filmed in one go, so you see Basehart in those scenes but you don't see his face in any other scenes. When you see Basehart enter the flying sub, it is someone else with a cap on his head to hide the fact that it isn't Basehart. Also the scenes, where his head is heavily bandaged. Its his voice but they dubbed that in later.
Basehart wasn't in the following two episodes, the Menfish nor the Mechanical Man.
I think he may have been taken ill.
Basehart despite being the lead character never appeared in every episode of Voyage to the Bottom of the Sea. Only Crane did.