My hand draws back. I often sigh still for the dark downward and vegetating kingdom of the fish and reptile.
The beginning of line 9 draws us out of the reverie into the present day. The speaker admits that even now he daydreams of the past when he would visit the aquarium.
The description of the marine life as a "dark downward and vegetating kingdom" makes it seem mysterious, and even a little spooky (a good place for a Scooby-Doo episode, anyway).
The end of line 11 signals another jump in time. Now we're looking at a more recent past. Specifically, last March.
Lowell keeps it up with the alliteration. Notice "sigh still" in line 9. When you read this aloud it actually sounds a lot like a sigh. In line 10 he uses "dark" and "downward" together. The repetition of D sounds might conjure the dum dum dum DUM you hear when something bad or dramatic is about to happen in a movie (beeteedubs: those notes actually originally belong to Beethoven).
I pressed against the new barbed and galvanized
Check out how Lowell uses "press" as he did in the memory about the speaker as a young boy pressing his nose against the tank's glass in wonderment. He creates a tie between the past and present by using that same verb.
This last line leaves us hanging, though. What is pressing against? Thanks to this use of enjambment, we're left hanging until the next stanza. Let's hurry down there…