In praise of invertebrate brains

A philosophical question (sorry NDT!): Can the brain understand the brain?

Of course, what I really mean is whether we humans will be aver able to understand our own brain. Of course, as far as the topic of the brain is concerned, we have a vertebrate-biased frame of mind. Nothing wrong with that. However, I am sure that more than a few people will agree that it is a good idea to study our neural predecessors if we are serious about understanding the origins of our brain.

However, how far “back” do we have to go? In other words, what specific neuronal forerunners should we take a look at to fundamentally understand what we want to?

To many scientist, the “obvious” thing to do would be to study “lower” mammals or other “lower” vertebrates. That is all right and useful too, but the thing is that the differences between any two given vertebrate brains is a matter of degree, not kind. For example, virtually every possible anatomical feature of a human brain will be found in the chimp’s brain, or on the gorilla’s and so forth. You do not even have to limit yourself to primates, pretty much the same holds true for rats, mice, dolphins, whales, eagles, you get the point. Sometimes people call these commonalities the “brain’s basic plan”.






Undoubtedly, the study of vertebrate brains in general will help us understand our own. Nonetheless, it is important to realize that there is more than one way to make a brain, that maybe a “brain is as the brain does”. In this sense, we must not forget that nature has regaled us with many other examples of animals that without any doubt, possess a brain or something very like it.

Please keep in mind that evolution works on what is “there”, building upon previously present structures, from organs to tissues to cells to biochemistries. It stands to reason that other types of organisms with markedly different types of brains will give us insights and useful information in our quest to understand ourselves.

Take a look at this particular brain:


These are not kidneys. This is a representative diagram of the type of brain present in octopi and squid. These cephalopods are considered rather smart invertebrates, with sophisticated communication strategies. With the mollusks cousins which include snails and slugs, these guys have a brain wrapped around its esophagus, as in the digestive system…

Octopi (sorry, octopuses!) brains are fascinating; you can read two very good general science posts here and here. Just by it very different organization in its structure it is very possible that previously undiscovered physiologies are waiting to be known.

This is just an example, if you,”keep going down” you will find even more remarkable examples. Take for instance, certain jellyfish. They most certainly do not have a “brain” in the traditional sense; its nervous systems does not even seem to be centralized, yet it is far from unorganized. Interesting critters too… Also, other members of the cnidarian family like hydra and some sea anemones display a distinct nerve net:



We can even go “lower” to sponges and related organisms, who were not deemed worthy of classifying them as having a proper nervous system. Rather, what they have is termed a “neuroid”
system. A pretty good, clear review of simple invertebrate nervous systems can be downloaded from here.

And, what about my favorite invertebrate brain? You know that I am talking about planarians. Boy, are they interesting! A good, yet somewhat technical review of planarian brains can be found here. These little guys have a bona fide bilobar brain, with not one, but two nerve cords. Despite being such simple organisms, they display a surprisingly complex set of behaviors that have gained them the nickname of neuroscience darlings

In my book ‘The First Brain’ (TFB) I explore many aspects of planarian biology in terms of history, pharmacology and of course, neuroscience. In fact I am using my little friends as an excuse to talk about neuroscience and the process of scientific research. It is intended to be read by the interested layperson, but I truly hope that scientists find something useful in it as well.

Invertebrate neuroscience in general and planarian research in particular have the potential to advance our knowledge of nervous systems. One of the points that I try to make is that planarian brains may well be one useful resource for understanding our own… Please tell me what you think!


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  1. Do you know anything theory wise about thecrossed over neurons in cordites? We mutated such that our heads are on backwards. I suppose it doesn’t compare to the donut shaped example you published.


  2. This is a great review of some of the evolutionary history associated with brain development in animals. It’s interesting that in squid, the main nerve ring surrounds the esophagus – so the amount of food they can consume is directly limited by the size of their brain! Very interesting topic, thanks.


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