Yet, concerning the eyes there is an obvious difference.20 Vertebrates, including humans, use a camera-like arrangement, whereas the crabs employ the compound eye that is typical of most arthropods. Has the principal of convergence broken down? Not at all, because it is among eyes that we find some of the most compelling examples. The existence of camera- and compound eyes reminds us, of course, that solutions to biological evolution need not be unique, but are simply very strongly constrained.21 Even so, in at least the case of eyes it is possible to argue that the camera-eye is inherently superior to the compound arrangement: both are viable, but when it comes to association with sentience the camera-eye may prove to be the winner.
When considering convergences between camera-eyes it is almost inevitable that comparisons will be drawn between the camera-eyes of vertebrates and those of the advanced cephalopods,22 notably the squid and octopus, which are one group of the molluscs (Fig. 7.2). This is the stuff of textbooks, but there are well-known differences. Most notable are those between the relative position and detailed structure of the light-sensitive layer, the retina, which arise as a result of the different embryologies in vertebrates and molluscs. In the molluscs the retina is derived from the outer layer (known as the ectoderm, and effectively equivalent to our skin). As it infolds to make
Optic nerve H Retina
| Pigmented layer □ Nuclear layer
figure 7.2 Convergence of the camera-eye, including the classic comparison between the octopus (cephalopod) and human (vertebrate), as well as the alciopid polychaete (annelid). (Redrawn from various sources.)
the eye-cup, the associated nerve cells extend into the body to make their connection with the brain. In contrast, the vertebrate retina is effectively an outgrowth from the central nervous system. The net result of this process is that, in contrast to the molluscs, in the vertebrates the nerve cells come to overlie the retina. The exit point of these nerves to the optic tract, which then leads to the brain, results in a 'hole' in the retina, better known as the 'blind spot'. Accordingly the arrangement of nerve cells and retina in molluscs and vertebrates is reversed, the cephalopods having arguably the more 'sensible' design of nervous layer beneath sensory retina.
The cephalopod lens is also formed in a rather different way.23 It has a rather remarkable layered structure,24 and being inflexible cannot change its shape to assist in the process of focusing.25 It is still not clear exactly how the cephalopods manage to focus, but most probably it is by moving the entire lens backwards or forwards.26 The style of construction of the lens may also explain why at least some cephalopods are able to correct for spherical aberration. (This is a phenomenon in which the light rays, on passing through the lens, fail to focus at a single point, resulting of course in a blurred image.) Even so, correction for such aberration seems to be less refined than in the fish. In the fish eye the refractive index (which is a measure of how much the light ray is bent as it passes from one medium to another) varies through the thickness of the lens. In cephalopods, however, spherical aberration may be dealt with by using non-spherical lenses, which in turn is possible because of their different mode of formation.27 These differences perhaps represent relative disadvantages of cephalopods as compared with fish.
Control of eye movements is, of course, critical. This is achieved by the so-called extra-ocular muscles that are attached to the exterior of the eyeball. Through what is known as the oculomotor control system, routed via the brain, this arrangement is linked to the organs of balance and so provides a coordinated arrangement, referred to as the vestibulo-oculomotor reflex.28 As already noted there are interesting convergences in the systems of balance, which in cephalopods are represented by the statocysts and in vertebrates by part of the inner ear. Given the nature of the oculomotor system it is not surprising to find convergences in the brain pathways between some cephalopods, such as the octopus, and the vertebrates.29 The degree of similarity between these systems is, in one sense, hardly surprising, but there are differences: for example, comparison of the oculomotor reactions, which allow the eye to swivel in its socket by the contraction of the extra-ocular muscles, shows the vertebrate arrangement to be superior.30
All these differences are important, but it is still the case that the similarities between our eyes and those of a cephalopod are very striking. It is rather less well known that a similar camera-eye has evolved independently in several other groups. The most notable example is in a group of marine annelids, close relatives of the more familiar earthworms.31 The group in question is known as the alciopids.32 These eyes are strikingly similar to those of the vertebrates and cephalopods (Fig. 7.2), and because the annelids are also relatively closely related to the molluscs the retina has the same arrangement as in the latter group.33 There is, moreover, another and striking convergence in the alciopid eye. This is in the form of the so-called accessory retinas, which are light-sensitive patches located nearer to the front of the eye. These, too, are strongly convergent on similar structures found in some deep-sea fish and cephalopods.34 Returning to the molluscs, but this time considering the gastropods (or snails) we find that in this group a camera-like eye seems to have evolved independently at least three times.35 Specifically these are in the pelagic heteropods,36 the familiar shore-snail known as the winkle (Littorina),37 and a large herbivorous tropical snail called Strombus.38
Nor does the list of convergently evolved camera-eyes quite end there: there are two more examples, each in their own way quite surprising. The first example comes from the much more primitive cubozoans. These are a type of jellyfish (hence the alternative name of cubomedusae) renowned both for their highly toxic stings and for their remarkable eyes. The eyes are similar in construction to other camera-eyes, with a large lens located in front of the retina39 but separated by a layer of cells that may help in focusing.40 Cubozoan jellyfish belong to a primitive group of animals, the cnidarians, which also includes the sea anemones and corals. While primitive eye-spots are known in other cnidarians,41 at first sight the sophistication of the cubozoan eyes, which typically total eight arranged around the margin of the swimming bell, is quite surprising. Cubozoans, however, are active and highly agile swimmers, have obvious visual acuity,42 and uniquely for cnidarians engage in bouts of copulation.43 What is particularly interesting is the relative simplicity of the nervous system, which consists of a nerve net linked to a series of four pacemakers, a neural architecture that is effectively imposed by the jellyfish body plan.44 Thus there is no brain,45 yet complex eyes and sophisticated behaviour.
Seeing without a brain has certainly attracted notice, although as we shall see below (p. 165) other organisms have an eye that evidently can focus an image without even the benefit of a nervous system. But the case of the cubozoans deserves wider scrutiny. As one worker has remarked, 'If cubomedusae have developed unique image forming eyes, is it possible that they have also developed a unique method of processing the information?'46 In this context it is also worth noting that nerve nets are by no means confined to primitive animals. They also occur in the echinoderms (the group represented by the starfish, brittle-stars, and sea urchins) and their relatives, the somewhat more obscure hemichordates (which, as it happens, are also quite closely related to the chordates, the group to which we belong). Interestingly, there is now evidence for unexpected complexities of function in the echinoderm nerve net.47 Not only that, but there is now evidence that part of the brittle-star skeleton has been converted into a sort of compound eye, convergent on the arrangement seen in the trilobites (p. 159). Whether anything like an image is seen by the brittle-star is very conjectural, but here too the suspicion is that these brainless animals possess hidden levels of sophistication.
More specifically, knowing how it is that cubozoans see might even have a bearing on extraterrestrial alternatives. What is evident in these animals is that the nervous system shows levels of autonomy, so that at times it can act as an integrated network yet in other circumstances show a greater degree of independence and be capable of dealing with directional inputs. As Richard Satterlie and Thomas Nolen remark,48 'The presence of four pace-makers in cubomedusae, as well as their semi-independent coupling, allows significant modulation of swimming by complex sensory systems and fast, directionally accurate behavioral responses in a radially arranged, non-cephalized nervous system. Functionally, one could view [this] as a form of condensation of neural networks, analogous to cephalization in bilateral animals'.49 What they are saying, albeit in somewhat technical language, is that coordinated and rapid activity may not automatically necessitate a brain with an attendant battery of sensory organs. Just as there are several ways of building eyes, each also convergent, so perhaps arising from a common substratum there is the possibility of several sophisticated neural arrangements evolving, and perhaps even distinct pathways to advanced intelligence marked by such features as plasticity of behaviour, memory, temperament, and even sleep?50 These possibilities are explored in subsequent chapters, but we can at this point note that it is surely an exciting prospect to explore both the nature of the building blocks (e.g. proteins) and the levels of complexity that lead both to obvious differences and to striking similarities in the intelligences of cephalopods, insects, and vertebrates. And does an underlying neural commonality undermine the principle of convergence? Not at all. In the next chapter we shall look at the extraordinary sophistication of the social insects: here, if anywhere, is the nearest analogy to an extraterrestrial intelligence. There may well be several types of intelligence, but the number will be restricted and the pathways to each type strangely similar. And arguably there will be deeper, if not universal, similarities: in principle all will understand algebra.
So camera-eyes are remarkably diverse in their distribution, and they form an obvious contrast to the compound eyes that are so typical of the arthropods. In due course we shall see that in exceptional circumstances a compound eye can be transformed into a sort of camera-eye. Yet this is not the only route: another example of a sort of camera-eye is encountered in some of the spiders, notably those adept at net-casting, including a type known as Dinopis. This is a hunter of a special sort, suspending itself from a web but itself clasping a net of sticky and elastic silk. When prey is detected, the spider extends its net, rapidly drops and then pushes the net against its prey. Then, as it ascends, the net folds around the hapless victim.51 The spider is nocturnal, and like other members of this group has eight eyes. In Dinopis two of the eyes, located on the posterior median part of the head, are strikingly large.52 (This explains why these spiders are often called the ogre-faced spiders.) In cross-section each eye consists of a cup-like retina and overlying lens, the lens being embedded in an iris. Interestingly, the texture of the lens changes in consistency through its thickness, from jelly-like at the front to being harder further back. Evidently this, in combination with the retina, acts to correct for spherical aberration, in a manner similar to that of the fish lens (see above). The spectacular eye of this spider does not provide a good image; its function is rather to collect light, which is a useful attribute given the spider's crepuscular activities. A somewhat similar camera-like arrangement is found in the antero-median eyes of jumping spiders, but there the emphasis is very much on image formation,53 to the extent that they can recognize video pictures of their prey (and mates).54 In addition, in these spiders the retina has a quite extraordinary stacked structure, which even incorporates a telephoto system of a kind.55 It has also been proposed that this multi-layered retina makes colour vision possible, and evidence exists for spectral sensitivity.56 What is not in doubt is that for their size these spiders have a quite remarkable visual acuity. The occurrence of simple eyes, effectively camera-like, in these spiders is curious. This is because the majority of arthropods, the phylum to which the spiders belong, employ compound eyes, as evidently did the ancestors of the spiders. As we shall see (pp. 160-162) the nearest approach to a simple eye in the other arthropods is based on a radical reconfiguration of the standard compound eye. It remains an open question as to why this spider route was not taken more often.
The hallmark of the animals that evolved camera-eyes, be they squid, vertebrate, heteropod snail, alciopid polychaete, cubozoan jellyfish, or even spider, seems to be that all are active, mobile, and typically predatory. At first sight, the two other examples I have already briefly mentioned, those of the snails Strombus and Littorina, seem to be marked exceptions. Strombus is generally slow-moving and herbivorous. In remarking on the strombid eye Mike Land makes no attempt to conceal his puzzlement when he writes, 'What the eye is used for, other than simple taxes [i.e. responses to stimuli] for which it seems over-adequate, remains a mystery'.57 Yet there is considerable evidence for electrical, and by implication visual, activity in the eye of Strombus (note 38). Perhaps we have underestimated this seemingly unassuming gastropod? Strombids are a very successful group in the tropics, and appear to have displaced similar forms such as the pelican-shells (aporrhaids) into colder waters.58 Not surprisingly, members of both groups are anxious to avoid attack, especially by fish, crabs, and drilling snails, but the strombids are particularly adept. As Kaustuv Roy notes, when escaping they 'generate a series of very rapid leaps away from the predator ... [showing] some of most specialized and effective escape responses known among gastropods'.59 Perhaps a sophisticated camera-eye is more useful to the strombids than might at first be thought?
What of the other snail, Littorina? There is little doubt that the winkles can see better than most other gastropods. This acuity is evident from a number of tests that show, for example, that various species of Littorina can orientate themselves at night, perhaps by recognizing silhouettes,60 and are adept at navigating both by day and beneath starlight.61 It is perhaps this snail's habitat, which is typically a gently sloping tidal flat,62 that provides the explanation, inasmuch as there is a premium in being able to recognize particular shapes, especially plant stems. This is because as the tide rises, and the predators move in, Littorina can climb the stems to relative safety.63 Such optical acuity is probably an important component in explaining the immense success of the humble winkle.
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