Thanks Vasco! As per my conclusion that planarians show "substantial evidence", I would estimate the probability of sentience to be on the lower side, especially since I would weight the first four physiological criteria lower than the last four behavioral ones, and we lack research on at least three of them. Maybe 15-20% ? I don't really know how I would estimate this properly. Instinctively, I would rank planarians lower than insects but higher than nematodes (their behaviors seem less chaotic, but I'm no nematode expert). I would be more interested in finding out if these animals somehow have a "lighter" form of sentience, as I'm not fully convinced sentience (as for current definitions) is either absent or present. Planarians can move around without a head, but they definitely do better with one. So, is there some sort of cognitive emergence only possible in the brain ? I have no idea. I looked at the estimates from the book and it checks out as slightly lower probablity than insects. I would be interested to have the estimates from those that read this post too!

Planarians can definitely be considered numerous as we can find some almost anywhere. 

I could not find any estimates for total numbers of individuals or colonies. Here again, instinctively, I would assume that they are still way less numerous than nematodes or even insects. Because they are very small (usually around 1-2 cm depending on the species) they move very slowly and feed on detritus or other very small invertebrates (and thus they don't proliferate so much), and although they are very small, are known for regenerating and being able to multiply exponentially, they do not seem to form large colonies in the wild. You can find a few under some rocks or dead leaves if you search for them in most river, but I would not consider this "abundant".

edit: I corrected my mixed up train of thought.

Thanks, Bob! From experience and footnote n°19, mucus seems to be relatively homogeneously produced but it's actually difficult to tell the production apart from different areas of their ventral side. We usually stain the glass they were gliding on so we can see that they produced more mucus when in noxious environments, but since mucus is spread along the path of the animal, it's not clear if any part produces more than the other. When cut, we can usually see an accumulation of mucus at the site of the wound, as it has a protective role, but to me it would be better explained as some sort of inflammatory response to cell death and exposure to the environment rather than a regulatory process from the nervous system.

We have some evidence that they can react region-specifically to UV lasers where only the exposed body portion contracts/shrinks on itself, but here also we could argue that muscle contraction can be the consequence of local nerve activation (in this case: by ROS produced from UV exposition) rather than from central regulatory processes.

Maybe the best bet to search for self-representation would be in the way they extrude their pharynx to search their surroundings for food; but current evidence shows that these mechanisms are almost entirely based on chemotaxis, especially since their pharynx can basically keep searching for food and munching while cut from its host (yes, it's weird). However, there is still definitely some sort of communication between the pharynx and the brain to decide when to stop and when to continue moving.

I never tried drawing a white spot on their head and placing them in front of a mirror - maybe we should try!

Hi Vasco. I am new on this forum, so please excuse my lack of knowledge on the specifics of your work over the years. I see that you produce high quantities of seemingly high quality work on undervalued topics such as this one. I myself am very much interested in the sentience and welfare of such “primitive” organisms. Here are a few of my impressions on this post:

• Although you take care of summarizing your research and clearly displaying your methodology, your work is still very enigmatic at first glance. Because your conclusion feels counter-intuitive and may spark defensiveness from animal advocates readers, not being able to understand clearly how you came to these conclusions make them feel untrustworthy.
• The reasoning that links your work with your conclusion feels… odd. I’m feeling like there are parts missing. For example, from what I understand of this post, you recommend increasing land use to reduce soil animals lifespan because you estimate they have negative lives. However, you thus recommend changes in food consumption towards diets that include more animal-based foods like beef, because they require more agricultural land than any other foods. Why do you consider eating beef to be the most cost-effective option to increase land use ? As I believe most vegans (or vegetarians) developed a strong deontological reasoning for not eating animal-based products, implying that eating beef might be the best utilitarian way to reduce suffering does not feel right: increasing land use is, so why should they tolerate factory farming, or even extensive farming at all ? Are there not any other opportunities to increase land use ? For example, instead of arguing for changes in food consumption, we could argue in favor of considerably increasing the space allocated to farmed animals so that more of the land would be used, farmed animals’ welfare would be increased; and because land is limited, the actual consumption of animal-based products by humans could actually decrease;  human health would thus be improved as well. You even considered the possibility of “paving all the Earth’s land”, but nothing in-between. Here’s another scenario: increasing the consumption of beef and land use implies huge greenhouse gas emissions, water pollution, etc -> climate change increases the probability of huge natural disasters and reduces the yield of agricultural production -> billions are spent in damages and health issues that could have been best invested in climate-resilient agricultural practices and wild animals welfare issues. I feel like your conclusion lacks a more systematic view on these issues; and I understand that this work is about utilitarianism, but caring about the welfare of a few species (even in way higher numbers) while recommending more consumption of other poorly-treated species (even in way smaller numbers) feels odd, to me atleast.
• I do not get why you consider “targeted research on whether soil animals have positive or negative lives” to be one of the most cost-effective ways of increasing animal welfare in the same way you consider increasing animal-based consumption is, because the latest suggestion implies that the former one already has been made. If research shows that soil animals have positive lives, then your recommendation for more animal-based foods consumption is actually counter-productive, which you acknowledge. I understand that you estimated around 55% probability that they have negative lives, which may be sufficient to consider the horrific possibility that such sheer numbers of animals live horrible lives and work on this question - but it also feels way too close to a 50% chance to actually recommend anything to me. It feels wrong to consider as cost-effective two recommendations that could potentially invalidate each-other: more research would already answer if we should increase land use or not, so recommending eating more beef does not belong here yet.
• Your research is very interesting, but I feel like strongly endorsing such concrete and disruptive recommendations (i.e. increasing the consumption of beef) under such uncertainties and hypotheses feels odd. You even argue that eating cheaper plant-based foods allows for more donations, increasing cost-effectiveness even more; which seems to contradict your recommendation. Plus, you mention the “at a margin” problem, as most of the world already eats huge quantities of beef, way above healthy recommendations, so I don’t get why you would recommend eating more beef at all. I feel like such a recommendation would need way more arguments, especially in a systemic analysis of the whole food production chain. That increasing land use could increase global animal welfare is an interesting point - but that we should eat more beef is not a conclusion that I would endorse with this post’s arguments.

Thank you for sharing your work here. I do not expect you to answer thoroughly to every point I made if you do not think your time is worth it - I just felt like sharing a few thoughts on an interesting topic and participating in the forum - but I will gladly read your responses (or anyone's) if you have some.