Answer: Worm causing anisakiasis; either Pseudoterranova decipiens or Contracaecum spp.
This diagnosis is based on:
1. The size of the worm (note the size compared to the glands in the gastric mucosa
2. The characteristic location (embedded in gastric or intestinal mucosa
3. The presence of Y-shaped lateral cords on a broad base (see image below). The broad base of the lateral cords excludes Anisakis spp., although this is in the initial differential diagnosis.
For images of what this worm looks like in fish flesh, take a look at the previous weeks' cases:
Frozen fish: Case 140
Cooked fish: Case 16
Also, here is a case where the worm was removed from the fish. This is what might get submitted to the microbiology lab for identification: