It can be a Ctenophora indeed but I didn't see any tentacles.
The second one was very slowly moving and changing a little of shape.
The third one is a kind of mussel.
Number four is the syphon of the mussel.
Byomic BYO500T microscope /CIOC XDS-1 invert.
Byomic ST-340 stereomicroscope
Olympus BHM Metallurgical Microscope
Canon EOS 7D Mark II / Canon MP-E65
Cognisys StackShot www.ngvm.nl