It is always a surprise for me, that also big ciliates are in the marine plankton. Generally they are very small. This ciliate was 250 µm long and it has many rows of cilia. Maybe somebody can help me for the identification.
I have confessed before in this forum .."that all ciliates look the same to me" ..so with no confidence, can i suggest that it may be a species of Prorodon.
I rarely see large ciliates (other than tintinnids) in plankton samples. I wonder if yours are migrating into the plankton from the benthic film on the quay where you sample ?
Very interesting! It does seem like a Holophryid of some sort. Kahl provides a picture of Prorodon marinus (sensu Faure-Fremiet...recorded under the name Pseudoprorodon, I think) that is similar in shape, and has an equally long oral basket. The "species" (a group of homonyms, likely) is said to be highly vacuolated (Carey), but the size range Kahl gives is somewhat smaller (140 um).
You might also consider the planktonic genus Gymnozoum, which has been acquiring new species, lately. The irregularity of that cytopharyngeal structure is suggestive of that genus, but the ciliation is probably too regular.