This data paper is important because it sets an important precedent for sharing comparative imaging datasets, which remains far too rare. The manuscript reads well and puts the data in a proper perspective.

However, there were some serious problems with the data made available to me, as detailed below.

Major Compulsory Revisions

1. Missing datasets I initially checked only the folders for taxon names starting with the letter P. In doing so, I noticed that several of these were empty. Looking around a bit further without checking all folders, I found the following list of taxa where no data was accessible to me:
2. Diadema savignyi
3. Echinarachnius parma
4. Echinocardium cordatum
5. Echinodiscus auritus
6. Laganum laganum
7. Mespilia globulus
8. Micropyga tuberculata
9. Nacospatangus alta
10. Notocidaris gaussensis
11. Peronella lesueuri
12. Peronella orbicularis
13. Plesiodiadema indicum
14. Plexechinus planus
15. Podocidaris sp
16. Pourtalesia jeffreysi
17. Pourtalesia wandeli
18. Psammechinus miliaris
19. Psychocidaris ohshimai
20. Strongylocentrotus purpuratus

21. Urechinus naresianus

22. Incomplete datasets I then went by the "Scan" column of the table in the supplementary file and looked for folders whose name started with "3D", without being able to find any.

What I did find (e.g. for Paracentrotus lividus) were folders labeled "2D_Magnevist" that contained RAW and TIFF folders. The "Data requirements" section of the manuscript states that the 2dseq file can be opened by dragging it onto the ImageJ menu panel. However, this does not work properly - presumably because of some metadata mismatch.

Similarly, the manuscript states that the TIFF image stacks should be opened using the 'File:Import:Image Sequence' command chain. This did not work for me either, although a simple drag and drop of the TIFF folder opened a menu asking whether to open all images in the folder, and with the "virtual stack" option therein, the TIFF stack opened properly.

I expect that the next revision contains the complete datasets as well as functional descriptions of how to handle the data contained therein.

Minor Essential Revisions

1. The manuscript states "More specific information on sample preparation and equipment or the application of contrast agents and different scanning protocols has been published previously [6].” but reference 6 is behind a paywall and not available through any public repositories, so if this dataset is to be useful, it will have to provide the necessary information.

2. In the manuscript: "MRI metadata files deposited online together with all raw and derived image data [7].”

It would be useful to explain in which of the provided files the metadata can be found that is listed in the “Experiment” column of the table in the supplementary file. Likewise, it would be useful to cite or link to an explanation of the various file types contained in the folders.

1. The manuscript: "In various instances, specimens were scanned twice using the same scanning protocol, i.e., once before and once after the application of a contrast agent (Magnevist, Bayer HealthCare, Leverkusen, Germany).”

It is not clear which specimens these were, nor is there a description of the protocol used for administering Magnevist.

1. The manuscript: "several scans show a significant presence of artifacts.”

These should be noted in the table in the supplementary file, perhaps best in a dedicated column.

1. Nowhere in the manuscript is the location of the scanners stated. Table 1 would be a good place to do that.

2. Typo in supplementary file: "Misaki Marine Biological Staion"

Level of interest An article of outstanding merit and interest in its field

Quality of written English Acceptable

Statistical review No, the manuscript does not need to be seen by a statistician.