Ciliated protozoan (Paramecium multimicronucleatum), scanning electron micrograph (SEM).

      There are tens of thousands of species of protozoans. Some are covered by a coat of flailing threads or cilia, which with a coordinated beat drive the creature through the water. Others, including the amoeba, move by bulging out fingers from the main body and then flowing into them. Many of those that live in the sea secrete shells with the most elaborate structure of silica or calcium carbonate. These are among the most exquisite objects that the microscope-carrying explorer will ever encounter. Some resemble minuscule snail shells, some ornate vases and bottles. The most delicate of all are of shining translucent silica, concentric spheres transfixed by needles, gothic helmets, rococo belfries and spiked space capsules. The inhabitants of these shells extend long threads through pores with which they trap particles of food.

      Other protists feed in a different way, photosynthesising with the aid of their packets of chlorophyll. These can be regarded as plants; the remainder of the group, which feed on them, as animals. The distinction between the two at this level, however, does not have as much meaning as such labelling might suggest, for there are many species that can use both methods of feeding at different times.

      Some protists are just large enough to see with the naked eye. With a little practice, the creeping grey speck of jelly which is an amoeba can be picked out in a drop of pond water. But there is a limit to the growth of a single-celled creature, for as size increases, the chemical processes inside the cell become difficult and inefficient. Size, however, can be achieved in a different way – by grouping cells together in an organised colony.

      One species that has done this is volvox, a hollow sphere, almost the size of a pinhead, constructed from a large number of cells, each with a flagellum. The striking thing about these units is that they are virtually the same as other single cells that swim by themselves and have separate existences. The constituent cells of volvox, however, are coordinated, for all the flagella around the sphere beat in an organised way and drive the tiny ball in a particular direction.

      This kind of coordination between constituent cells in a colony was taken a stage further, probably between 800 and 1,000 million years ago – some time in October in our calendar – when sponges appeared. Sponges can grow to a very considerable size. Some species form soft shapeless lumps on the seafloor two metres or so across. Their surfaces are covered with tiny pores through which water is drawn into the body by flagella, and then expelled through larger vents. The sponge feeds by filtering particles from this stream of water passing through its body. The colonial bonds between its constituents are very loose. Individual cells may crawl about over the surface of the sponge like amoebae. If two sponges of the same species are growing close to one another, they may, as they grow, come into contact and eventually merge into one huge organism. If a sponge is forced through a fine gauze sieve so that it is broken down into separate cells, these will eventually reorganise themselves into a new sponge, each kind of cell finding its appropriate place within the body. Most remarkably of all, if you take two sponges of the same species and treat them both in this extreme way and then mix cells from the two, they will reconstitute themselves into a single mixed-parentage entity.

      Massive barrel sponge (Xestospongia testudinaria) and diver. Tubbataha Reef National Marine Park, Palawan, Philippines.

      Some sponges produce a soft, flexible substance around their cells which supports the whole organism. This, when the cells themselves have been killed by boiling and washed away, is what we use in our baths. Other sponges secrete tiny needles, called spicules, either of calcium carbonate or silica, which mesh together to form a scaffold in which the cells are set. How one cell orientates itself and produces its spicule so that it fits perfectly into the overall design is totally unknown. When you look at a complex sponge skeleton such as that made of silica spicules which is known as Venus’ flower basket, the imagination is baffled. How could quasi-independent microscopic cells collaborate to secrete a million glassy splinters and construct such an intricate and beautiful lattice? We do not know. But even though sponges can produce such miraculous complexities as this, they are not like other animals. They have no nervous system, no muscle fibres. The simplest creatures to possess these physical characteristics are the jellyfish and their relatives.

      A typical jellyfish is a saucer fringed with stinging tentacles. This form is called a medusa after the unfortunate woman in a Greek myth who was loved by the god of the sea and as a result had her hair changed by a jealous goddess into snakes. Jellyfish are constructed from two layers of cells. The jelly which separates them gives the organism a degree of rigidity needed to withstand the buffeting of the sea. They are quite complex creatures. Their cells, unlike those of the sponge, are incapable of independent survival. Some are modified to transmit electric impulses and are linked into a network which amounts to a primitive nervous system; others are able to contract in length and so can be considered as simple muscles. There are also stinging cells with coiled threads inside them, the unique possessions of the jellyfish tribe. When food or an enemy comes near, the cell discharges the thread, which is armed with spines like a miniature harpoon and often loaded with poison. It is these cells in the tentacles that will sting you if you unluckily brush against a jellyfish when swimming.

      Jellyfish reproduce by releasing eggs and sperm into the sea. The fertilised egg does not develop into another jellyfish directly but becomes a free-swimming creature quite different from its parents. It eventually settles down on the bottom of the sea and grows into a tiny flower-like organism called a polyp. In some species, this sprouts, through branching twigs, into other polyps. They filter-feed with the aid of tiny beating cilia. Eventually, the polyps bud in a different way and produce miniature medusae which detach themselves and wriggle away to take up the swimming life once more.

      Portugese man o’war (Physalia physalis) split level showing float and tentacles, Indo-pacific.

      This alternation of form between generations has allowed all kinds of variations within the group. The true jellyfish spend most of their time as free-floating medusae with only the minimum period fixed to the rocks. Others, like the sea anemones, do the reverse. For all their adult lives they are solitary polyps, glued to the rock, their tentacles waving in the water ready to trap prey that may touch them. Yet a third kind are colonies of polyps but ones that have, confusingly, given up their attachment to the sea bottom and sail free like medusae. The Portuguese man o’war is one of these. Chains of polyps dangle from a float filled with gas. Each chain has a specialised function. One kind produces reproductive cells; another absorbs sustenance from captured prey; another, heavily armed with particularly virulent stinging cells, trails behind the colony for up to fifty metres, paralysing any fish that blunder into it.

      It seems an obvious assumption that these relatively simple organisms appeared very early in the history of animal life, but for a long time there was no proof that they actually did so. Hard evidence could only come from the rocks. Even if microorganisms can be preserved in chert, it is difficult to believe that a creature as large but as fragile and insubstantial as a jellyfish could retain its shape long enough to be fossilised. But in the 1940s some geologists noticed very odd shapes in the ancient Ediacara Sandstones of the Flinders Ranges in southern Australia. These rocks, now thought to be about 650 million years old, were believed to be completely unfossiliferous. Judging from the size of the sand grains of which they

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