The structures labeled as 1 are likely the gills or ctenidia. These are used for respiration and are located in the mantle cavity. The gills facilitate the exchange of gases (oxygen and carbon dioxide) with the surrounding water. The structures labeled as 2 are indeed part of the digestive system, specifically the digestive gland (sometimes referred to as the hepatopancreas). This gland plays a crucial role in digestion by secreting enzymes that help break down food particles and absorb nutrients. The red marks labeled 1, showing what seems to be a network or tissue structure, are indeed more consistent with the gills, as they are typically involved in gas exchange.

Thank you Fareeda Tahir for your response. I am sure that the red items 1 are not the gills or ctenidia. Patella species have no ctenidia. Instead they have pallial gills around the perimeter of the foot. They are labelled 4 in this image. The white ‘v’s show the route of colourless haemolymph depleted of oxygen passing from the body into the pallial gills. The blue ‘V’s show the route of oxygenated bluish haemolymph from the gills, through the efferent branchial vessel (5) to the nuchal cavity (7) where the heart pumps it to the organs in the body and head.

My question was whether the spheres are food balls. The digestive gland is very large in patella and is not the spheres. It is labelled 2 in the image at https://flic.kr/p/Bex7R6

Hi Ian:

From the position, it looks like the structure labelled 1 is a tubercle organ. These are sensory organs associated with the osphradium (Lindberg & Ponder, 2001). Haszprunar (1985) described them for Patella coerulea as "brilliant red spots" in the nuchal cavity.

References:

Haszprunar, G. (1985). The fine morphology of the osphradial sense organs of the Mollusca. 1. Gastropoda, Prosobranchia. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 307(1133), 457-496.

Lindberg, D. R., & Ponder, W. F. (2001). The influence of classification on the evolutionary interpretation of structure: A re-evaluation of the evolution of the pallial cavity of gastropod molluscs. Organisms Diversity & Evolution, 1(4), 273-299.

Let me know if you need the PDFs of the references.

Regarding the structure labelled 2, they look unusual. Can you take out one of those "spheres"?

Best regards,

Andre

Many thanks Andre for your answer; exactly what I hoped for the red spots. I shall acknowledge your help in the account. The photo was taken in 2017 and the specimen not preserved. Like archaeology, as one digs lower down, the higher levels get destroyed. I shall leave the spheres (2) unlabelled unless someone else can supply an answer before I complete the P. vulgata species account. Fretter and Graham (1962) mention the red spots on P. vulgata, so they are found on it as well on P. coerulea . I was able to download the PDFs from Researchgate, but thanks for the offer to send them. André Ampuero León

Best wishes

Ian