The simplest animals

Image of a sponge. Courtesy of National Wildlife Federation.
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*** SUMMARY of Conclusions reached References

General Considerations

The current scientific definition of animals is described in detail in an Appendix. In short: animals are usually identified as eukaryotic organisms that are made up of many cells, feed on other organisms, reproduce sexually and follow certain stages (notably a blastula) in their embryonic development. In fact, most of these features are not unique to animals, while the last is not found in some animals (sponges). The distinguishing feature of animals is currently thought to be the presence of a special kind of extracellular matrix (the substance between cells) that is composed of four types of molecules that are created inside, but exist outside, the cells of animals. Extracellular matrix plays a key role in the development of animals. It guides the development of mobile cells in the developing embryo, and also helps to control the transition of cells from one type to another. All animals and only animals share this complex pattern of development, mediated by their extracellular matrix (Morris, 1993).

The sensory capacities of animals are summarised in an Appendix. Among the various senses animals possess, touch and chemical senses appear to be universal among animals (Shaw, 2002). However, it has been argued above (Conclusion S.5) that the ability to sense stimuli does not necessarily indicate the presence of mental states.

Most but not all animals can move. A few groups of animals, (e.g. most sponges, corals, sea lilies, barnacles and some tube-dwelling worms), are sessile, that is, they remain attached to a solid substrate, while many parasites may become lodged within the bodies or tissues of their hosts (Messing, 2000).

In short, there does not seem to be any feature possessed by all animals, and only by animals, which is relevant to the possession of mental states. Given that organisms belonging to the other kingdoms of life appear to lack mental states, we should expect to find that some but not all animals possess mental states.

Porifera (sponges)

I shall begin my survey of animal minds with the better-known sponges or Porifera, which are among the simplest animals (discussed at further length in the Appendix).

Before we discuss these animals' abilities, the first point that needs to be made is that they are bona fide individuals (see the Appendix for a discussion on this point).

Ten of the estimated 10,000 species of sponges are now known to be able to crawl - albeit at the rate of a few millimetres a day. The relevance of this point to studies of animal cognition is that sponges' responses may be too slow for us to observe by normal methods. Dennett discusses the possibility (which he later rejects) that plants may be just "very slow animals ... whose sentience has been overlooked by use because of our timescale chauvinism" (1997, p. 87), but the same point could also be made for sponges.

Although sponges have internal skeletons, they possess no definite organs, no circulatory systems, and no definable nervous systems.

Sponges have senses: they can respond to light and poison, and can open and close pores. However, it has already been argued (Conclusion S.5) that sensory capacities - which are found in all cellular organisms - do not, by themselves, indicate the occurrence of mental states. Only if sponges show signs of flexibility and novelty in their patterns of responding to their environment is it useful to ascribe mental states to them. The question we have to ask is: can they learn?

According to Rose and Rankin (2001, p. 63), habituation "has been demonstrated in all organisms tested, including single celled protozoa." Hooper (personal email, 26 May 2003) reports there have been experiments performed on habituation in Porifera. However, it has been argued (Conclusion S.8) that non-associative habituation does not satisfy the requirements for "true" learning, as described by Kilian and Muller (2001) above: in these cases (unlike associative learning), the organism does not learn to do anything new or different.

There is no evidence to date of associative learning in sponges. I have not been able to find any reports in the literature of experiments designed to test it, despite the fact that several easy experiments suggest themselves. For instance, it would be interesting to see whether sponges can be conditioned to contract in response to the presence of light, subsequent to its being paired with poking (an unconditioned stimulus which automatically causes contraction).

In short, it appears that the behaviour of sponges can be adequately described using a mind-neutral, goal-centred intentional stance. There does not seem to be any reason to suppose that these animals possess mental states of any kind, although studies designed to exclude this possibility have not been performed to date.

Aside from Porifera (sponges) and their close relatives Placozoa, there are two other small phyla of animals (Rhombozoa and Orthonectida) which appear to have evolved independently of the lineage leading to the remaining 33 phyla of eumetazoa, or "true" animals. They will not be discussed in this thesis.