Answer to Parasite Case of the Week 604: Plant material, not a parasite. Although the large size and round shape in section superficially resembles a cross-section of a large roundworm (i.e., Ascaris lumbricoides), there are some key features that allow us to identify this as plant material.
As nicely stated by Florida Fan, "There does not seem to be a layer of polymyarian muscle cells as well as the other anatomical components such as a digestive tube or reproductive organs and no excretory organs either" which would be seen in Ascaris lumbricoides. He also adds: "when magnified enough we can see polygonal cells often observed on vegetable matters. The other observation is that there is a fine brown line inside which I believe to contain melanin pigment, a character we do not find in helminths."
These are excellent observations on the key features for distinguishing plant material - in this, likely a cereal bran - from round worms. The polygonal cells are a characteristic feature of plants, and can be seen in the outer layer of the structure in this case
This outer layer would likely have been birefringent with polarized light, which is another helpful feature for differentiating plant material from helminth body layers. Dr. Mary Parker, a botanist, kindly provided the following explanation for this case: "I am pretty sure this is plant material, probably a few layers of the bran of a cereal grain. One of the layers of cereal bran contains a water-insoluble orange/red pigment called phlobaphene which give the grain its colour. This pigment layer is most obvious at 5, 8, and 12 o'clock on your section image. Botanists call a cereal grain a caryopsis because it is technically a seed with the fruit wall adhering to the outside (I explain this to my students as one pea in a pod with the pod adhering to the pea surface). Cereal bran therefore consists of layers derived from the fruit wall and the seed testa, and it is one of the testa layers that is pigmented. The digestive process does separate these layers."
For comparison, here is a cross-section of A. lumbricoides, showing the thick acellular outer cuticle, underlying hypodermis, and tall polymyrian muscle cells. Note also the sections of internal organs.
Thanks again to Dr. Gilligan for donating this interesting case!