Soaking up the limelightSystema Porifera: A Guide to theClassification of Spongesedited by John N. A. Hooper & Rob W. M. van SoestKluwer Academic/Plenum: 2002. 1,810 pp.$595, £398, E625

Lorraine Berry

Sponges are “among the most successful lifeforms that have ever existed, with an estimated15,000 species alive today”, according to theeditors of this new work on their classifi-cation. The study of sponges has providedinformation about fundamental biologicalissues, including the biosynthesis of chemi-cals, totipotency (the ability of a cell to differ-entiate into other cell types), the evolution ofeukaryotic immunology, and cellular theory.Sponges are also a source of therapeuticdrugs, and can serve as biomarkers of pollu-tion and proxies for palaeoclimatic change.

Nomenclature problems have plaguedsponge taxonomists for over a century, how-ever. The classification of sponges is compli-cated by their plasticity of form. They lackrecognizable organs, cell types often migrate,and detritus (including skeletal parts calledspicules from other sponges) can be incor-porated, transforming the sponges beyondrecognition. As a result there are many syn-onyms, which are difficult to reconcile withspecies descriptions (from different authorsat different localities at different times) andwith type material that is dispersed amongmuseums around the world.

Systema Porifera attempts to resolve thehigher systematics of the phylum Porifera,incorporating spongiomorphs such as theArchaeocyatha, ‘Stromatoporoidea’ and‘Sphinctozoa’, which previously resided inother phyla. The book represents the work

of 45 researchers, who took 7 years to re-evaluate and define the 2,100 nominal genera.It also provides a review of the taxonomic literature, creating an invaluable database ofsponge biodiversity and a platform for thefuture development of sponge systematics.

Interest in sponges has been growingsince the 1960s, when the discovery of bioactive metabolites (used to synthesize compounds with antibiotic and other bio-medical properties) caught the attention ofdrug-development agencies. Since then, thenumber of recognized species of sponge has doubled, and today they are studied by several hundred researchers in various institutes around the world. This increasehas led to an explosion in the discovery and documentation of species, which until nowwas dispersed in journals and monographs.Systema Porifera provides an important service by drawing information from these disparate sources together and updating it.

The editors are well qualified to do thisbecause of their work on the rich sponge faunas of the Indo-Malay Archipelago. JohnHooper has also revised the systematics ofAustralasian sponges, and Rob van Soest hasconcentrated on sponges from the north-eastern Atlantic and the Caribbean. Togetherthey have done an excellent job, making Systema Porifera interesting and accessible to a wider scientific audience than puresponge taxonomists.

Descriptions of each class, order andfamily of sponge are organized under aseries of uniform subheadings, makinginformation easy to locate. Bibliographiesare comprehensive and include many recentreviews. The authors discuss the history ofnomenclature and classification, and recog-nize the possibility of future taxonomicchanges, highlighting the fact that spongescience is still evolving.

Most sections contain an identificationkey for families and genera — somethingnever previously attempted for Porifera —

although I would recommend, as the editorssuggest, referring to the Thesaurus of SpongeMorphology, edited by Nicole Boury-Esnaultand Klaus Rützler (Smithsonian Contri-butions to Zoology 596, 1–55; 1997), to usethem. Some diagrams and electron micro-graphs in Systema Porifera are a bit small,although enlarging them would have furtherincreased the size of this already enormous— but invaluable — book. ■

Lorraine Berry is in the Department of InvertebrateZoology, Royal Belgian Institute of NaturalSciences, Rue Vautier, Brussels B-1000, Belgium.

Universal building blocksThe Road to Galaxy Formationby William C. Keel Springer: 2002. 230 pp. E99.95, $109, £60

Carlton Baugh

Galaxies are the building blocks of the Universe. Understanding the origin of these vast agglomerations of stars, gas and dust isone of the main challenges facing cosmolo-gists today. Our ideas about the physicalprocesses responsible for shaping galaxieshave matured rapidly over the past decade, in response to the staggering advances inastronomical technology over this period.

The advent of 10-metre ground-basedtelescopes, the success of satellite-borneinstruments such as NASA’s Hubble SpaceTelescope, and the opening up of the electro-magnetic spectrum far beyond the opticalregion have revolutionized our view of theUniverse. We can now observe galaxies that are so far away that the time it takes light totravel from them is a significant fraction ofthe age of the Universe. This means that wesee these objects as they appeared when the Universe was just a billion or so years old.The task confronting theoreticians is daunt-ing: to explain the formation and evolutionof galaxies over more than ten billion years of cosmic history. It is certainly time for abook devoted to setting out the availableclues as to how galaxies are made.

William Keel’s book outlines the twocompeting theories of galaxy formation: the‘monolithic collapse’ and the hierarchicalscenario. The former is a throwback to the1960s, when it was first suggested that galax-ies were born in some violent episode in theearly Universe. The discovery of galaxieswith seemingly elderly stellar populations athigh redshifts (which correspond to a timewhen the Universe was relatively young)gives credence to the monolithic picture,with the result that this simple model hasstubbornly refused to die.

The hierarchical scenario, on the otherhand, attacks the problem from the opposite

book reviews

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Hanging on: some 15,000 species of sponge, including Leucetta chagosensis shown here, are alive today.

© 2003 Nature Publishing Group

direction. In this model, galaxies are built upby repeated mergers of smaller fragments. Inthis case, there is no single epoch of galaxyformation, but rather a steady build-up of stars. This model is appealing because it is set in the context of a model for the formation of structure in the dark-matter component of the Universe. Models in which the mass of the Universe is dominatedby weakly interacting particles known ascold dark matter, and which also have a significant cosmological constant or dark-energy component, are receiving impressivesupport from measurements of the cosmicmicrowave background radiation and theHubble diagram of distant supernovae.However, this evidence is circumstantial:dark matter has not been detected in the laboratory, and there is no convincing theoretical explanation of dark energy.

Keel’s own research has covered a widerange of topics, which is reflected in the rich-ness and variety of subjects covered in thisbook. It is refreshing, in a market dominatedby theorists, to come across a book on galaxyformation written from an observationalperspective. After all, any model of galaxyformation, no matter how appealing from a theoretical point of view, is eventuallyjudged by how well it describes what is actually out there.

In a book of this length it is impossible to cover in detail all of the most excitingadvances of the past few years. Consequently,the discussion of Lyman break galaxies —the first significant population of galaxies tobe identified at high redshift — is rather briefin view of the tremendous impact that theseobjects have had upon the subject. Also,galaxies that are detected by their emission at submillimetre wavelengths (which occurswhen the dust that they contain is heated bystarlight or by material falling into a centralblack hole) receive little attention until thefinal chapter. The book provides a flavour ofthe physics of galaxy formation, rather than a rigorous review of the theory. Readers whorequire a comprehensive theoretical briefingcould turn instead to Cosmological Physics byJohn Peacock (Cambridge University Press,1999) or Cosmology, 2nd edn by Peter Coles& Francesco Lucchin (Wiley, 2002).

Nevertheless, The Road to Galaxy Forma-tion should prove to be a handy primer onobservations of galaxies for graduate stu-dents, advanced undergraduates and theor-eticians who feel too shy to visit a telescope. A particularly useful feature is the biblio-graphy at the end of each chapter, which contains a brief résumé of selectedresearch papers and will no doubt beinvaluable to newcomers to the field whoneed guidance in selecting further readingfrom a burgeoning literature. ■

Carlton Baugh is in the Department of Physics,University of Durham, South Road, Durham DH1 3LE, UK.

book reviews

Leonardo lifts offA wing designed by Leonardo da Vinci proves to be aerodynamic. Martin KempLeonardo da Vinci is famous, among otherthings, for inventions that did not work, or ratherthat could not have worked in his own era. Hewas, as the cliché has it, a man ahead of his time. But like many clichés, this obscures ratherthan clarifies. Although he was undoubtedlyproductive as a ‘jobbing’ engineer, his grand,visionary projects, such as the flying machine andthe tank, blended genuine aspirations with boutsof visual boasting directed at prospective patrons.

The brilliance and practical limitations ofLeonardo’s more extravagant visions have recentlybeen vividly demonstrated in the project to build a flying machine undertaken by ITN Factual forChannel 4 Television in Britain. I was called in as their consultant, to work with SkysportEngineering of Bedfordshire in England, whichspecializes in the resurrection of early aircraft.

The brief was to construct something thatwould actually fly, but with the minimum ofdeviation from Leonardo’s own designs, in detailand in spirit. We were determined to use nothingthat he did not or could not have envisaged. Thegreat uccello (bird) that he envisaged in the 1490s,with its flapping wings, had no chance of flying,so we favoured the gliding mechanisms to whichhe later resorted.

Our construction was based on a series ofLeonardo’s bat-like designs, which combinedlarge spans with skeletal lightness (at least interms of the materials available to him).Leonardo’s concept was based on his convictionthat nature’s own inventions operated with noinsufficiency and no redundancy in the context of natural law. Deciding to remake nature on itsown terms, he worked on the principle that theaspiring aviator should not literally imitate a birdbut should create a mechanical body that worksanalogously, in conformity with the causesbehind natural effects. It is this approach thatplaces the flying machine in much the samecategory as the Mona Lisa , that most syntheticallycontrived of portraits (Nature 389, 799; 1997).

The problem that we faced, like everyone elsewho has attempted to fly Leonardo’s machines, wasthat of achieving sufficient lift to raise into the air

the relatively heavy materials available to him. Ourbreakthrough came with the observation of adetail on a sheet in the Biblioteca Ambrosiana inMilan that had appeared insignificant to my eyes.The Skysport engineers saw it differently. Near thelateral rib adjacent to the pole at the inner marginof the wing, they noticed a looped line that passesover the front edge (circled in red, below). The rearof the loop is labelled “panno” (cloth). What thisdetail said to the engineers was ‘leading edge’ and‘aerofoil’ — the necessary features for lift-off.

However, Leonardo had no sense of thedynamic laws that underlie modern aerofoildesign, which involve compression beneath thewing and rarefaction above. During his manyhours watching birds wheeling on currents of air,fish swimming in streams, and boats tackingagainst the wind, he never doubted that water and air (both of which are fluids) behave in thesame way. He did not pay attention to the fact that air, unlike water, is compressible, and had notconsidered such a possibility. But he had observedthe robustness of the leading edge of a bird’s wingand the relatively broad and blunt heads of somefish, which told him that air or water needed topushed strongly out of the way.

The Milanese wing design achievedtriumphant lift-off high on the Sussex Downs ona breezy autumn morning about 500 years after itwas drawn. In the version that flew, Leonardo’swing design was doubled up, and we omitted thewindlass, which was intended to adjust the angleof the wings but would have served no usefulpurpose. Also added was a leonardesque tail for stability and a somewhat less leonardesque A-frame for the intrepid pilot.

A new formula is needed to replace the cliché.Leonardo, with extraordinary visual inventiveness,envisaged more of the potentiality of his period’sscience and technology than any of hiscontemporaries. And, like many highly originalinventors, he needed his slice of luck to come upwith the right answer for the wrong reasons.Martin Kemp is in the Department of the History of Art, University of Oxford, Oxford OX1 2BE, UK.

Science in culture

Winging it: a flying machine based on plans byLeonardo da Vinci has been flown. The red circleshows the loop that indicates the leading edge .

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