What was Tom doing in February?

J: Hi, Tom!
T: Judy. Haven’t seen you in weeks. Where have you been?
J: In Florida.
T: What, going on holiday! While the rest of us are studying on the campus in February cold?
J: Not exactly. I spent most of my time under water.
T: I don’t understand.
J: I was on a special field trip. I went with my marine biology class.
T: So you went diving. What were you looking for, sunken treasure?
J: You might say so. The sea’s full of treasures. All kinds of strange fascinating organisms. Our class concentrated on studying plankton.
T: I thought planktons were too small to be seen.
J: That’s common misconception. The term plankton covers a wild variety of freely flowing plants and animals, from one-cell organisms to larger ones, such as the common jellyfish.
T: Jellyfish may be large enough to be seen. But can you see the very tiny planktons?
J: By concentrating, I was able to see the outlines of lots of different plankton plants and animals.
T: That sounds like an interesting trip. But I think if I’ve been in Florida in February, I’d much rather spend my time just swimming and lying in the sun.