Diversity of Organisms: How Much Do We Know?1 1075

AMER. ZOOL., 29:1075-1084 (1989)

Diversity of Organisms: How Much Do We Know?1

ROBERT D. BARNES

Department of Biology, Gettysburg College,Gettysburg, Pennsylvania 17325

SYNOPSIS. The history of Invertebrate Zoology over the past 40 years can be used toillustrate interest in organisms and some of the ways in which the symposium's questionmay be interpreted. The study of animal organisms from a holistic perspective has pro-gressed enormously as reflected in changes in described and estimated numbers of species,in the discovery of new higher taxa and in the growth of literature. Generalizations onthe biology of animal organisms, however, rest on relatively small samples, and many ofthe same organisms that have received the most attention in the past continue to receivethe most today. Symbiosis and colonial organization have been two important meanswhereby new organizational levels for organisms have evolved. Ultrastructural researchover the past 20 years has provided new evidence in support of the hypothesis promulgatedlong ago that multicellular animals (metazoans) may have evolved from colonial protistans.Some polymorphic, colonial metazoans have approached or crossed the threshold to astill more complex level of organism.

INTRODUCTION

There has been a great accummulationof information about the diversity of ani-mal organisms over the past 40 years. Thegrowing understanding of this diversityreflects not only interest in the study oforganisms but in some of the ways of inter-preting the symposium's question—Is theorganism necessary?

There are almost one and a half milliondescribed species of eukaryotic organismsliving on our planet. Most are animals,of which there are over a million. Twohundred thirty-five thousand are plants;80,000 are fungi; and 87,082 are protis-tans (algae and protozoans) (Fig. 1). Thediversity is enormous, especially amonganimals. Animal motility and heterotro-phic nutrition have led to many differentlife styles and body plans.

NUMBERS OF DESCRIBED SPECIES

How many species of animals have beendescribed to date? A reasonably good esti-mate is that there are about 1,035,250. Thefive largest groups in order of magnitudewould be insects, arachnids, mollusks, ver-tebrates and crustaceans (Fig. 2). The esti-mates have changed over the past 35 years,

1 From the Symposium on Is the Organism Necessary?presented at the annual meeting of the AmericanSociety of Zoologists, 27-30 December 1987, at NewOrleans, Louisiana.

but, surprisingly, the numbers were notalways smaller.

Compare Mayr et al. (1953) with the mostrecent edition of Barnes (1987) whosenumbers (Table 1) are derived from var-ious sources, but especially from theMcGraw-Hill Synopsis and Classification ofLiving Organisms (Parker, 1982). In 1953Mayr, Linsley and Usinger estimated1,090,235 species of animals having beendescribed as compared to 1,035,185 today,about 55,000 less. In this comparison,mollusks, myriapods and insects showdecreased numbers. Arnett's (1985) cur-rent figure of 751,000 species of insects isa very careful estimate. Some groups likecnidarians and rotifers show little change,and sponges, nematodes, annelids, bryo-zoans and echinoderms show moderateincreases. The really large increases are inflatworms, crustaceans and arachnids.

Yet even for groups that show littlechange in the two lists, a large number ofnew species have been described over thisperiod. For example, the number of newspecies of polychaetes described between1972 and 1983 averaged 92 a year. But thetotal increase in the estimates of describedspecies of annelids is only 1,700 and thatincludes oligochaetes and leeches, as well.

Several factors may contribute to theforegoing, apparent paradox. Earlier esti-mates vary in their accuracy and a consid-erable number of new species as well as oldones fall into synonymy. Herbert Levi at

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1076 ROBERT D. BARNES

80,000

6%

EMBRYOPHYTES16%

METAZOAN ANIMALS

1.035.250 7 2 %

FIG. 1. Estimated numbers of species comprising theeukaryotic kingdoms. Based on figures from Barnes(1987), Bold et al. (1987), and Corliss (1984).

the Museum of Comparative Zoology hasinformed me that in his revisions of neo-tropical orbweaving spiders, he has foundthat about 70% have been unnamed, butof the 30% that have names, they have beennamed three or four times! This is not trueof all groups. For example, relatively fewpolychaetes fall to synonomy (K. Fauchald,personal communication). Almost all of thesmall groups show increases in estimatednumbers. But this is not surprising; thesmaller the number of described species,the more accurately can new speciesdescriptions be evaluated and counted.

NEW HIGHER TAXA

The increase in numbers of describedspecies over the past 45 years has beenimpressive. Just as impressive, however, isthe increase in descriptions of new speciesthat have been assigned to new higher taxa.Since 1950 eight new classes and phyla ofanimals have been discovered. The mostrecent is a class of tiny echinoderms (2.6-9.0 mm in diameter) which look a little likemedusae, and have been assigned to thenew taxon, the class Concentricycloidea.The class was first described in 1986 fromspecimens collected on wood in deep water

off the coast of New Zealand (Baker et al.,1986). Prior to that representatives of anew phylum, the phylum Loricifera, werediscovered by Kristensen (1983) in theinterstitial spaces of marine gravel off thecoast of France. Loriciferans belong to theaschelminth assemblage and look some-what like a cross between a rotifer and akinorhynch.

Over the past 45 years representativesof four new classes of crustaceans have beendiscovered. All are minute animals; mostare less than a millimeter in length. A num-ber of species of the class Remipedia havebeen collected from marine caves. Firstdescribed by Yager in 1981, these highlymetameric arthropods, which look some-what like polychaetes, are perhaps the mostprimitive known crustaceans. The Tantu-locarida is a class of marine ectoparasitesrelated to copepods, first described byBoxshall and Lincoln in 1983. Members ofthe class Cephalocarida are primitive crus-taceans first described in 1955 by Sandersfrom sediments in Long Island Sound andsince taken from many other locations.Finally, the class Mystacocarida, reportedby Pennak and Zinn in 1943, contains elon-gate, interstitial species related to cope-pods.

In 1980 Rieger described a strangeinterstitial worm, Lobatocerebrum, that is cil-iated and acoelomate like flatworms buthas metameric ventral ganglia and proto-nephridia like annelids. It has arbitrarilybeen placed with the annelidan oligo-chaetes but may eventually have a highertaxon of its own.

Among mollusks the first living mono-placophorans were discovered in 1952 indeep water off the coast of Chile. Theyhave since been taken from a number ofsites in the world's oceans and eleven speciesbelonging to three genera (Neopilina,Monoplacophorus and Vema) are now known(Wingstrand, 1985).

To these new groups we should add sev-eral others, which although discoveredearlier, have really only become known toany degree during the past 35 years.Included here are the marine Trichoplax,representing the monotypic phylum Pla-cazoa (Grell, 1982) and the pterobranchs,

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DIVERSITY OF ORGANISMS 1077

FIG. 2. Estimated numbers of species of the major groups of metazoan animals. From Barnes (1987).

whose biology has only received attentionduring the past 15 years and most recentlyby Lester (1985). Up until about 1950 thepogonophorans, a deep water gutless groupof worms related to annelids, were repre-sented by a relatively small number of spec-imens found in miscellaneous dredgingsamples. With improved collecting tech-niques and wider oceanographic samplingthe collection of pogonophorans increaseddramatically. Then, beginning in 1970s ourknowledge of the biology of pogonopho-rans began to move forward.

NUMBERS OF UNDESCRIBED SPECIES

How many species are yet to bedescribed? A report from the Office ofTechnological Assessment (1987) suggests5-10 million. If so, only 15 to 30% oforganisms have been described. The greatnumbers that have been postulated are

largely arthropods. Certainly the mostmind-boggling are the projections of Erwin(1983) for insects of the rainforest canopy,one of the few remaining frontiers ofunknown organisms.

Erwin conjectures that there may be asmany as 30 million species of insects, a stag-gering figure given the 750,000 describedspecies estimated by Arnett (1985). Erwin'sprojections are based on canopy collectionsmade in Brazil and other parts of theAmerican tropics. An insecticide foggingdevice was elevated to various levels in therainforest. The insects thus disturbed andkilled rain downward and are collected onplastic trays containing a collecting bottlein the center. The beetle fauna, with whichErwin was largely concerned, consists ofsmall species belonging to about six fami-lies. His projections are based on the enor-mous number of endemic species charac-

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TABLE 1. Numbers of described speciesma Is.*

PoriferaCnidariaCtenophoraPlatyhel-

minthesNemerteaMesozoaAcanthoceph-

alaRotiferaGastrotrichaKinorhynchaNematodaNematomor-

phaAnnelidaPogonophoraEchiuraSipunculaMolluscaTardigradaOnychophoraChelicerataCrustaceaInsectaMyriapodaBryozoaEntoproctaPhoronidaBrachiopodaEchinoder-

mataChaetognathaHemichordataUrochordataVertebra ta

Total

1953, Mayrrtal.

4,5009,000

90

6,000750

50

3001,500

175100

10,000

1007,000

160

25080,000

18065

35,00025,000

850,00013,0003,300

604

250

4,0003080

1,60037,790

1,090,235

1987, Barnes

5,0009,000

50

12,700900

50

1501,500

460100

12,000

2308,700

80140320

50,000400

7068,00042,000

751,01210,5004,000

15010

325

6,0007085

1,25049,933

1,035,185

of living ani-

Change

+ 500—

- 4 0

+6,700+ 150

-150

+ 285—

+ 2,000

+ 130+ 1,700

+ 79+ 80+ 70

-30,000+ 220

+ 5+ 33,000+ 17,000-99,998

-2,500+700

+ 90+ 6

+ 75

+ 2,000+40

+ 5-350

+ 12,143-55,050

* Changes in estimates between 1953 and 1987.

teristic of each of the four forest types hesurveyed. Fifty-eight to 78% of the speciesof beetles within each type were endemic.

Mites number about 30,000 species, andthis is believed to represent only about 20%of the actual fauna. Moreover, since thebulk of these species is thought to live intropical forests, many are predicted tobecome extinct before they are ever dis-covered. But insects and arachnids areprobably the only two groups for whichthere are still very large numbers of unde-scribed species. I expect the larger marinegroups will exhibit only modest increases

in the years ahead. Although even withregard to these marine groups, the projec-tions of various workers vary. For example,Kristian Fauchald (personal communica-tion) speculates that only about 60% of thepolychaetes are known. Echinoderms areabout 90% known, according to CynthiaAhern (personal communication). Inter-estingly, coral reefs are providing many ofthe new species of polychaetes, but coralreef echinoderms, such as brittle stars andfeather stars are fairly well known, and itis the deep sea that is the last frontier ofnew echinoderms.

LITERATURE GROWTH

How much do we know about the biol-ogy of this enormous diversity of animals?If the volume of literature is any indica-tion, we know a great deal. In 1985, forexample, out of about 220,000 publica-tions in Biology, approximately 62,000,one-fourth, were devoted to animals. Ofthese 62,000 articles concerned with ani-mal organisms, over a quarter were oninsects. Articles on birds, mammals, fish,mollusks and crustaceans, in that order,together had half.

The growth of literature on organismsin this century can be illustrated by con-sidering publications on a few groups ofanimal species. The graphs in Figures 3and 4 show the number of papers pub-lished each year for bryozoans, arachnidsand myriapods over the past 85 years. Thetabulations are derived from ZoologicalRecord, and these groups were selectedbecause of the ease of obtaining the fig-ures. Groupings of phyla have continuallychanged in Zoological Record. The annualoutput of papers on bryozoans (Fig. 3) hasfluctuated but within a relatively modestrange of about 25 to 75 papers during thefirst part of this century. Following WorldWar II, the number increased several fold,reaching a peak of 282 papers in 1970.More recently it has dropped to less than200. Myriapods had a similar history up toWorld War II, but subsequently rose mark-edly, exceeding 300 papers a year since1979. The growth of papers on arachnidsis much more spectacular. In 1900 there

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2 8 0

2 4 0

2 0 0

160

1 2 0

8 0

4 0

1900 1908 1916 1924 1932 1940 1948 1956 1964 1972 1980

FIG. 3. Annual publications on bryozoans from 1900 to 1985. Based on figures from Zoological Record.

were 131 papers published; after 1978there has been an output of about 2,000papers a year.

The number of books and reviews hasalso increased dramatically. In 1960 whenI was working on the first edition of myInvertebrate Zoology, the books in Englishcovering the general biology of free-livinginvertebrates other than insects werelargely limited to five volumes of LibbieHyman's series (1940-1959; she had notyet completed the volume on mollusks), twovolumes on the physiology of crustaceansand a few British volumes on the arachnids,some of which were already quite old atthat point. In the intervening 37 years theoutput of books on invertebrates has beenenormous. Almost every phylum of any sizehas been covered, even relatively small oneslike nemerteans and brachiopods. Thereare volumes on pycnogonids, leeches andtunicates. Still missing are a general biol-ogy of the cnidarians and one on rotifers.However, they will soon be covered in themultivolume work on the MicroscopicAnatomy of Invertebrates now in processunder the editorship of Frederick Harri-son.

So, how much do we know? I believe that

we know a great deal. Of course, there hasbeen a great accumulation of informationabout individual species. But as I look backover the past thirty years, I am impressedwith major areas of knowledge about ani-mal organisms for which we had little orno information thirty years ago. Let mepoint out just four. A whole new world hasbeen revealed in the interstitial fauna, theanimals that live between sand grains. Ourknowledge of animal symbiosis, especiallymutualistic symbiosis with unicellularorganisms, has greatly increased. There hasbeen the recognition of the fact that shellsand skeletons secreted by animals providea record of their age and environmentalconditions. For example, there are inter-tidal bivalves which record by fine lines ontheir shells, not only the daily occurrenceof low tides, but of spring and neap tides(Evans, 1972). The series of lines can there-fore be read like a tide chart with thesequence of spring and neap tides quitevisible.

A most important contribution to theadvancement of our general knowledge ofanimals, especially to our better under-standing of their evolutionary relation-ships, has come from ultrastructural

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400

300

100

Combined with

arachnids

1900 1908 1916

2400

2000

1600.

800

1972 1980

Combined with

myriapods

1900 1908 1916 1924 1972 1980

FIG. 4. Annual publications on myriapods (uppergraph) and arachnids (lower graph) from 1900 to 1985.Based on figures from Zoological Record. The figuresfor myriapods also include a small number of publi-cations on some very small groups, such as tardigradesand pycnogonids, which are sometimes contained inthis section.

research. Through electron microscopygreat advances have been made in ourunderstanding of animal ciliation, proto-nephridia, podocytes, vascular and coelo-mic linings or their lack, to name but a few.

KNOWLEDGE SAMPLE SIZE

In all of these advances in our knowledgeof the biology of metazoans, one might askhow broadly based is the advance. I haveoften wondered how far our knowledge ofthe biology of species extends beyond thefamiliar names that appear over and overagain in the literature. This symposiumprompted me to investigate. Using poly-chaete annelids again, the lists in Table 2show the number of papers published perspecies in 1983 and 1984 and the names

of the species with the record number ofpapers. The reader may note that manyare familiar. And if you compare them withthe polychaetes that won the publicationrace in the first quarter of the twentiethcentury, you will find that many are thesame. This is not surprising. The animalsthat we know best are those that are athand, those easiest to obtain and maintainin a laboratory. Among marine animalsthey are the common, easily collectedspecies around marine laboratories or aca-demic institutions located near the coast.Of course there has been some increase inthe variety of species studied over the pastthirty years, but much attention continuesto be focused on a group of favorites.

How broadly based are the generaliza-tions we make about animals? The infor-mation to answer that question is not easilyobtained, but some data are available fora few areas. For example, textbooks makethe generalization that the ophiuroids, orbrittle stars, possess pluteus larvae, a larvaltype similar to that of sea urchins, and thatsome brood their eggs. There are about2,000 species of brittlestars. On what isthat generalization based? In 1975 Hen-dler surveyed the literature and found thatof the 2,000 species of brittlestars 71 areknown to have larvae and 55 to brood.That means we know something of thedevelopmental pattern of about 6% of theophiuroids. The figure is probably a littlehigher today.

The study of protonephridia providesanother example. Protonephridia are blind-ending excretory tubules found in someten phyla. The terminal cells bear one ormore cilia. The general structure of theseorgans was delineated by the end of thelast century, but the function of proto-nephridia has been little understood sinceprotonephrida commonly occur in animalsthat are too small to require organs for theremoval of metabolic wastes. In 1958 thefirst EM work on protonephridia wasundertaken by Kummel on the sheep liverfluke. Subsequent investigators examinedthe protonephridia and found fenestra-tions around the barrel of the cell. Showingsome resemblance to podocytes, the fenes-trations appear to be the sites of passage

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TABLE 2. Numbers of papers published per species of living polychaetes in 1983 and 1984.

One paper each forTwo papers each forThree papers each forFour papers each forFive papers each

Six papers each

Eight papers

Ten papersEleven papersTwelve papersFourteen papersNineteen papersTwenty papers

1983 1984

220 species36 species13 species4 species

Glycera dibranchiataNeanthes succineaNeanthes arenaceodentaPomatoceras lamarckii

Capitella capitataNeanthes diversicolor

Neanthes virensArenicola marinaNeanthes diversicolor

466 species27 species

8 species3 species

Sabellaria alveolata

Chaetopterus variopedatus

Capitella capitataPerinereis cultrifera

Arenicola marinaNeanthes virens

of fluid into the interior of the tubule, thecilia providing the filtration pressure. Howbroadly are these generalizations based? Isour sample size adequate? The ten phylaof animals with protonephridia includemore than 17,000 species excluding poly-chaetes. Since Kummel's first paper in 1958there have been at least 44 papers pub-lished on protonephridia from about 40species that belong to 9 of the 10 phyla inwhich protonephridia occur (Fig. 5).Although the sample size is still small, Ithink this is a remarkable record.

A final example relates to the feedingmechanism of crinoids. The crinoidsinclude about 80 species of sea lilies and450 species of feather stars. In 1960 DavidNichols published one of the first detailedpapers on the feeding mechanism of a cri-noid, Antedon bifida, the European featherstar. Since that time there have been obser-vations on the feeding posture of a rela-tively large number of species, probablyaround thirty, largely from the work of D.L. Meyer and D. B. Macurda. However,published work on the actual feedingmechanism has been limited to some sevenstudies that have been made on only twospecies, the European Antedon bifida andthe North Pacific Florometra serratissima.This investigational base needs broaden-ing.

I do not feel uneasy about the empiricaldata base of our generalizations about ani-mal organisms. As I have watched infor-

mation accumulate in animal biology, gen-eralizations have usually become refined,not discarded. Structures and processes thatdepart markedly from what has been pre-viously known are usually described andthose that follow the pattern are usuallynot. Therefore our knowledge is probablygreater than the numbers of publisheddescriptions might lead us to believe. Weare never going to have a complete record;generalizations are always going to be basedon just a sample. Nevertheless, we must becareful not to invest our generalizationswith more status than they merit.

ORGANISM BOUNDARIES

Finally, I would like to look at the wayour growing knowledge of animal diversityhas contributed to concepts of organismboundaries. Of the many contributions EMstudies have made to animal biology, oneof the most interesting, from an evolution-ary point of view, has been studies of cil-iation. The work of Rieger (1976) hasprovided convincing evidence that mono-ciliated epidermal cells, i.e., one cilium percell, is the primitive condition for animals.Monociliated cells are characteristic of pla-cozoans, sponges, cnidarians, gnathosto-mulids, and some gastrotrichs. They arealso found among deuterostomes, such asechinoderms and pterobranchs. They donot occur in the flatworm, mollusk, annelidand arthropod assemblage (protostomes).

Recent work on choanoflagellates has

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CEPHALOCHOROATES45

POLYCHAETES

PRIAPULIDS9

ACANTHOCEPHALANS1,150

KINORYNCHS100

ROTIFERS1,500

GASTROTRICHS460

GNATHOSTOMUL1DS80

NEMERTEANS900

CESTODES4,000

FLUKES I8,000

TURBELLARIANS (

3,000 1958

I I

I III I II

J I

J I

J-M ,—L-, 1 I , ' , r-1962 1966 1970 1974 1978 1982 1986

FIG. 5. Publications on the ultrastructure of protonephridia. Publications are indicated by bars. Figure undername of group indicates approximate number of described species. The number of species of polychaetespossessing protonephridia as opposed to metanephridia was not known to author.

demonstrated that their mitochondria andciliation are strikingly similar to those ofmetazoans (Nielsen, 1985). Indeed, thesimilarities indicate that the choanoflagel-lates may be the protozoan flagellate ances-torsofthe Animal Kingdom. Nielsen (1985)even assigns the choanoflagellates to theAnimal Kingdom, but I think this is goingtoo far. Such a decision requires a funda-mental change in the way the Animal King-dom is currently defined. |

I have called the reader's attentiori/tcrcurrent research findings indicating a phy-logenetic relationship of choanoflagellatesto metazoans because it has provided a reaf-firmation of a position long held by manybiologists—that the multicellular condi-tion of metazoans is derived from a pro-tistan colony. Following the origin of thefirst cells, evolution to a new level of organ-ism may have occurred in at least four ways:(1) by symbiosis, (2) by intracellular differ-entiation, (3) by unicellular, colonial orga-nization with intercellular differentiation,and (4) by multicellular, colonial organi-zation with polymorphism.

Thus the first metazoan animals—motile,multicellular heterotrophs—evolved byincreasing differentiation and interdepen-dence of cells within a flagellate colony,probably a choanoflagellate colony. Atsome point the high degree of cellularinterdependence resulted in the colonybecoming a multicellular organism.

Within the Metazoa, various groups haveestablished mutualistic symbiotic relation-ships with bacteria, cyanobacteria, green^lgae, diatoms and dinoflagellates. The hostgroups are fairly restricted: sponges, cni-darians, flatworms, pogonophorans andmollusks. In some sponges and corals theunicellular symbionts contribute a largepart of the total biomass. Have any of thesesymbiotic relationships approached thethreshold of a new level of organism?

Polymorphism, whereby the members ofa colony become structurally specialized fordifferent functions, has evolved in a num-ber of colonial groups: hydroid cnidarians,siphonophoran cnidarians, hydrocorals,pennatulacean cnidarians, bryozoans, andsocial insects. In all but the social insects,

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the individuals of these polymorphic col-onies are attached together. When a feed-ing individual is predominant and hasretained the primitive ground plan of thephylum, such as the gastrozooid of hydroidsand the autozooids of bryozoans, colonialorganization remains distinct. Where spe-cialization has blurred that original groundplan, then the colony could cross thethreshold to a new level of organizationand organism. I believe there is one groupof animals which approaches that thresh-old. These are the siphonophores. Thesemarine colonies have developed extremepolymorphism. Their composite individu-als have commonly lost the distinctive,radial, tentaculate form and the colony withits swimming bells, floats, fishing polyps andreproductive individuals functions as anintegrated whole, one organism. I wouldwager that if all cnidarians were extinct butsiphonophores, we would be hard pressedto recognize the individuals even if weguessed it was a colony.

There is still another, less familiar,example of how our increasing knowledgeof diverse kinds of animals can disturb ourconventional ideas about the boundaries ofthe organism. The old debate as to whethera leuconoid sponge with many oscula is oneindividual or a colony, is rarely heard today.The notion that multiple oscula, likemouths, must indicate more than one indi-vidual, has disappeared with the recogni-tion that multiple oscula are simply a fea-ture of leuconoid architecture. However,it is now known that where larval settle-ment is dense in some species, adjacentdeveloping individuals will fuse together(Fell and Jacob, 1979). The resultingsponge is indistinguishable from one whicharises from a single egg, but it is a geneticmosaic. Is it one organism or two?

ACKNOWLEDGMENTS

1 am grateful to Dr. Ralph A. Sorensenfor reviewing the manuscript.

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Pennak, R. W. and D.J. Zinn. 1943. Mystacocarida,a new order of Crustacea from the intertidalbeaches in Massachusetts and Connecticut.Smithsonian Misc. Coll. 103:1-11.

Rieger, R. M. 1976. Monociliated epidermal cells inGastrotricha: Significance for concepts of earlymetazoan evolution. Z. Zool. Syst. Evolutions-forsch. 14(3):198-226.

Rieger.R. M. 1980. A new group of interstitial worms,

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Lobatocerebridae, nov. fam., and its significance Wingstrand, K. G. 1985. On the anatomy and rela-for metazoan phylogeny. Zoomorphologie 95:41- tionships of recent Monoplacophora. Galathea84. Rep. 16:7-94.

Sanders, H. L. 1955. Cephalocarida, a new subclass Yager,). 1981. Remipedia, a new class of Crustaceaof Crustacea. Proc. Nat. Acad. Sci. U.S.A. 41: from a marine cave in the Bahamas. J. Crust. Biol.61-66. l(3):328-333.

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Diversity of Organisms: How Much Do We Know?1 1075