The three types of animals

The three types of animals 
Alternative title: Why you should never watch an alien movie with me

This may look long and boring, but it is really interesting.... Trust me, I wrote it. If it's too big you can read it in convenient chunks. In fact it may be better that way because it will give you a chance to really think about each part and how it applies to the things you know.

I often say to people who know me that there are three types of animals in the world. They are sponges, jellyfish and worms. Some people have often told me that this is a gross oversimplification,  but it isn't,  it is just an extreme simplification which is what it is supposed to be. It is as simple as it needs to be to illustrate the point. The point is to try to get people to look at the animal kingdom from a more objective perspective and thereby gain a greater understanding of life and the process of evolution on earth.

Imagine an alien life form. Chances are whatever you are imagining resembles some terrestrial lifeforms in many ways. you don't think so? Ask yourself this, was your alien bilaterally symmetrical? does it have a mouth on one and an anus on the other? does it have paired organs and appendages (eyes, antennae, arms legs etc..). These are all terrestrial characteristics and in fact characteristics of organism in the group I call worms. You may argue, "oh, but it just makes sense that things will end up like that...". It may make sense to you, but not to evolution. In the three lineages of animals in the world only one has ended up like. In all the lineages of plants, bacteria, fungi, archea and protozoans none of them have ended up like that. All of these organisms have had just as much time and opportunity to develop into bilaterally symmetrical animals as we have, but they have not done so. Add to that the fact that in the roughly 3.8 billion years life has existed bilateral symmetry has only been around for the last (very roughly) 1.2. What was life waiting for all that time? The answer is of course that these characteristics are not inevitable at all but are a once off adaptation to a particular circumstance which has been retained more out of convenience than anything else.  Most of what we consider to be an inevitable part of animal life is really just a result of common ancestry and if we are to ever find intelligent life on another planet there is no reason why it should look anything like what is here on earth.

An example of how common ancestry is more important than function
This would seem to indicate that we should take morphological similarity very seriously. Even apparent 'convergent' evolution, when examined more closely, seems to always turn out to be two radically different structures. Let me give you the example. The most typical example of convergence often given to school students is the presence of fins in dolphins and sharks, despite the fact that dolphins are actually mammals while sharks are in the same group as sting rays. Let's have a closer look at these fins:

Shark Pectoral fin:

Dolphin Pectoral fin:

Now lets take a look at some mammal 'fins':


Obviously the dolphin fin is more similar to a mammalian forearms than a shark fin. This becomes even more obvious when we start examining how other taxa propel themselves in the water. There is still a degree of similarity between a shark fin and a dolphin fin, but that is also a result of common ancestry, just a much more ancient ancestry than that of a dolphin and a human. If we started looking at the structures which propel squid, aquatic insects, crustaceans, marine flatworms, jellyfish, marine snails and the many other things which live in the oceans this point would become more and more obvious to you.

So back to the three types of animals. Often shocked sceptics will ask 'then which type of animals are we?' I usually reply that they already know the answer and if I tell you, you will know we could not fall into any other category. Not convinced? well it may interest you to know that both in terms of genetics and derived characteristics these acorn worms are more closely related to humans than they are to earthworms. So if worms exist, it makes no sense to exclude us from the group.


We are a type of very sophisticated worm. So the worms are the animals with bilateral symmetry. Our body plan tends to build things in pairs or along the mid line of symmetry. We have a mouth at one end, an anus at the other with a long narrow digestive tract in between. Our genitalia are usually close to our anus. Our sensory functions are often performed by organs concentrated in on or two locations. Our sense organs tend to cluster around near our mouth, which often forms a central nervous control system often called a brain out of convenience. We are big fans of appendages, which are almost always paired. We have a three layered body plan with an epithelial layer (skin- stuff), then musculature and organs, then a digestive tracts. Our digestive tract is separate from our circulatory system. All these characteristics have been retained in nearly all the descendants of whatever worm like creatures were the primal bilaterians, although some of these may only have been present as genetic homologues in the original. If you don't know what a genetic homologue is don't worry too much, I will soon write something on homology.

The worm group contains most things you will think of when you think of animals. It includes us and other things with spinal chords, insects and other arthropods, snails and squids, leeches and earthworms, flatworms, roundworms and many other worm like things. The most irregular group in the bilateria are probably the echinoderms which often have five branches of symmetry and the digestive tract is often folded in on itself so that the anus and the mouth make use of the same orifice. Close examination of these do reveal that they are a variation of the bilateria theme. The five layered symmetry can be thought of as one plain of medial symmetry with two paired plains of symmetry on either side.


There are more things in the Jellyfish group than you imagine. By Jellyfish in this context I mean all members of  the Cnidaria and Ctenophora (Jellyfish, sea anemones, coral, comb jellies, Hydrozoans, box jellyfish, sea wasps etc...) which is my definition of the radiata. I suspect this group will also include Dendrogramma in a new phylum but more work remains to be done in this area before we hastily jump to conclusions.

These organisms all have a radially symmetrical body plan with three layers including an epithelial layer (sort of skin) and an internal mesoglea of tissue which does not form into organs and a gastrovascular system which fulfils both the role of digestion and circulation. They have sensory tissue in their epithelial tissue, but they do not form organs and have no central nervous system. They are used mostly for localised responses to stimuli such as stinging anyone who touches them.

While there are a lot of differences between jellyfish and worms there are a few similarities such as symmetry in general, a three layered body plan, and cells differentiating into tissue even though the tissue does not form organs. This is also probably a result of common ancestry as the worms and the jellyfish are sister groups.  In fact the genes which control body plan in the worms (we call them Hox genes) have fairly recently been found to be present in Jellyfish in some form. There is some debate as to whether the group is monophyletic or paraphyletic, but clearly I think they are monophyletic or this whole thing is stupid.

People talk about the jellyfish being less complex than the worms, and I guess there is some truth to this due to their lack of organs but complexity can occur in several different ways and can be a matter of perspective. I challenge people to look at the lifecycle of some Cnidarians and tell me it is more simple than ours, and the chemical interactions in the mesoglea and the methods by which jellyfish 'know' things are for a large part not understood. They are certainly differently complex, but there are some very complex members of this group as there are some very simple members of our group. Even within our own phylum there are organisms such as the urochordates.


This type of animal is the most basal of my three types of animals and only includes the Phylum Porifera.These are animals with no fixed body plan, no tissue or organs and undifferentiated cells. In Porifera various cells in the body will perform different functions depending on their position in the body. Cells communicate their positioning through a complicated series of cell to cell chemical exchange. As a result different species of sponge will grow into different shapes which are roughly similar. Cells can even perform functions which are useless on their own but collectively that have an effect which benefits the whole organism. This does not occur as a result of any central coordination but by each cell doing what it is programmed to do when it exists in a certain location on the body. In this way they can form channels and pump currents of water through them so food gets sucked in and filtered out. The nutrients from this food can be passed from cell to cell across the sponge.

A cell can be taken from one part of the sponge, inserted into another and it will start performing the function of the cells in its new location. Given ideal conditions an entire sponge can be regrown from a single one of its original cells. This raises some interesting philosophical questions such as weather a sponge is an individual organism or is it a colony of organisms, or what is the difference anyway? There is a growing body of work suggesting that Porifera are paraphyletic and that any universal similarities in the group are a result of common ancestry, but I don't buy it (yet).


O.K. so you know how I said there are three types of animals over and over again? Well there might be a fourth, the Placozoa. The Phylum only has one recognised species, Trichoplax adherens. It is a free living blob of cells which lives at the bottom of the ocean. It has no body plan and no organs or tissue like a sponge. It is, however, encased in a layer of cells which might be homologous with epithelial cells, suggesting that it is more closely related to Jellyfish and worms although it certainly does not belong to either of these groups. These epithelial cells contain cilia which enable it to move around absorbing micro-biotic food. To be honest I don't really know what to do with these guys.

Questions for alien movies:

There are a lot of things common to living things on earth which are a result of common ancestry which ought not occur in Aliens, because evolution is not a process which chooses the best possible outcome or even the best outcome given the circumstances, it only chooses the best solution out of the very limited number of solutions available. Even the sexual/asexual reproduction dichotomy on earth is just a result of the structure of our DNA and the way it recombines. So next time you see a movie with aliens in it, ask yourself what the odds are that the aliens would look so terrestrial. Do they need to have symmetry let alone the same type of symmetry as us? Is the bottom necessarily the best place for waste disposal? Do the senses need to be mostly based around the head? Do you really need organs for intelligence or can you do it some other way? Is skin or exoskeleton really necessary? Is a fixed body plan really necessary for organs? Why can't they eat dirt? Considering the number of amino acids that are possible, what are the odds they will be able to eat living things from earth? Or what are the odds we will be able to eat anything from another planet?  Why does everything always need to breathe? I could go on, but this is already getting pretty long.

These are the questions that have ruined many a sci-fi movie for me and those who watch them with me. I hope they will now ruin them for you. Now you know my true motive for writing this. I'm angry at the world because I can't watch movies with aliens without objecting to almost every thing.  Now you will all have to share in my pain (if anyone actually read this) because once these concepts have been considered, you can't unthink them.

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