The Uncertainty Principle The position and momentum of a particle cannot be simultaneously measured with arbitrarily high precision. There is a minimum for the product of the uncertainties of these two measurements. There is likewise a minimum for the product of the uncertainties of the energy and time. This is not a statement about the inaccuracy of measurement instruments, nor a reflection on the quality of experimental methods; it arises from the wave properties inherent in the quantum mechanical description of nature. Even with perfect instruments and technique, the uncertainty is inherent in the nature of things. Graphical interpretation of uncertainty principle

Index Uncertainty principle concepts

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Uncertainty Principle Important steps on the way to understanding the uncertainty principle are wave-particle duality and the DeBroglie hypothesis. As you proceed downward in size to atomic dimensions, it is no longer valid to consider a particle like a hard sphere, because the smaller the dimension, the more wave-like it becomes. It no longer makes sense to say that you have precisely determined both the position and momentum of such a particle. When you say that the electron acts as a wave, then the wave is the quantum mechanical wavefunction and it is therefore related to the probability of finding the electron at any point in space. A perfect sinewave for the electron wave spreads that probability throughout all of space, and the "position" of the electron is completely uncertain. DeBroglie wavelength Forms of uncertainty principle Application example: required energy to confine particles

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Particle Confinement The uncertainty principle contains implications about the energy that would be required to contain a particle within a given volume. The energy required to contain particles comes from the fundamental forces, and in particular the electromagnetic force provides the attraction necessary to contain electrons within the atom, and the strong nuclear force provides the attraction necessary to contain particles within the nucleus. But Planck's constant, appearing in the uncertainty principle, determines the size of the confinement that can be produced by these forces. Another way of saying it is that the strengths of the nuclear and electromagnetic forces along with the constraint embodied in the value of Planck's constant determine the scales of the atom and the nucleus. The following very approximate calculation serves to give an order of magnitude for the energies required to contain particles. Show calculation Uncertainty principle Compare to particle in box. More detailed treatment of 3-D box Energies in electron volts (eV) Application example: required energy to confine particles

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Confinement Calculation If you examine this calculation in detail, you will note that a gross approximation was made in the relationship Δp = h/Δx. This was done to get a qualitative relationship that shows the role of Planck's constant in the relationship between Δx and Δp and thus the role of h in determining the energy of confinement. The other reason for doing it was to get an electron confinement energy close to what is observed in nature for comparison with the energy for confining an electron in the nucleus. If you actually use the limiting case allowed by the uncertainty principle, Δp = hbar/2Δx, the confinement energy you get for the electron in the atom is only 0.06 eV. This is because this approach only confines the electron in one dimension, leaving it unconfined in the other directions. For a more realistic atom you would need to confine it in the other directions as well. A better approximation can be obtained from the three-dimensional particle-in-a-box approach, but to precisely calculate the confinement energy requires the Shrodinger equation (see hydrogen atom calculation). Confinement discussion Uncertainty principle Calculation for 3-D box

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Wavefunction Contexts

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Arthropods Made Easy
The Phylum Arthropoda

Phylum Arthopoda Review

1. What are some examples of arthropods?

Ants, flies, cockroaches, shrimps, crabs, spiders and scorpions are examples of arthropods.

Phylum Arthropoda - Image Diversity: arthropods

2. What are the classes into which the phylum Arthropoda is divided? What are the three main ones and some of their representative species?
The three main classes of arthropods are: insects (cockroachs, ants, flies, bees, beetles, butterflies), crustaceans (crabs, lobsters, shrimps, barnacles) and arachnids (scorpions, spiders, mites). Other classes are onychophorans (velvet worms), diplopods (millipedes) and chilopods (centipedes).

3. What are the main morphological features of arthropods?
Arthropods present three distinguishing features: they are metameric beings (segmented body), they have an exoskeleton made of chitin and they present articulated limbs.

Phylum Arthropoda - Image Diversity: arthropod structure

4. Concerning germ layers and the presence of coelom how are arthropods characterized?
Arthropods are triploblastic (they have three germ layers) and coelomate beings.

5. Considering the presence of segmentation (metameres) in their body to which other already studied phylum are arthropods proximal?
Considering their metameric feature arthropods are proximal to annelids that also have segmented bodies. In the embryonic development of some arthropods there are fusions of metameres forming structures like, for example, the cephalothorax of arachnids.

Phylum Arthropoda - Image Diversity: athropod cephalothorax

6. What is the external rigid carapace of arthropods called? Of which substance is it made? Which type of organic molecule is that substance?
The external carapace of arthropods is called exoskeleton. The arthropod exoskeleton is made of chitin, a nitrogen-containing polysaccharide.

Phylum Arthropoda - Image Diversity: exoskeleton

7. How do arthropods grow?
Due to the presence of exoskeleton the growth of an arthropod is periodical. During the growth period the animal loses the exoskeleton, grows and develops a new exoskeleton. This process is named ecdysis, or molting.

Phylum Arthropoda - Image Diversity: ecdysis

8. How does the presence of exoskeleton explain the general small size of arthropods?
Since they have exoskeleton and periodic ecdysis, the growth of arthropods is limited to avoid the animal becoming vulnerable to environmental harm. There are however some arthropod species with relatively large-sized individuals, like “giant” cockroaches, crabs and spiders.

9. How can the features of the arthropod exoskeleton explain the terrestrial adaptation of some species of the phylum?
In the arthropod exoskeleton there is a layer of wax which is impermeable. This feature was fundamental for primitive arthropods from the sea to survive on dry land without losing excessive water to the environment.

10. What is the type of digestive system present in beings of the phylum Arthropoda? Are these animals protostomes or deuterostomes?
The digestive tube of arthropods is complete, containing mouth and anus. Arthropods are protostome animals, i.e., in their embryonic development the blastopore originates the mouth.

11. How is the extracorporeal digestion associated to predation in arachnids?
Arachnids can inoculate poison to paralyze or kill their preys using structures called chelicerae. The prey is partially digested outside the body of the arachnid by digestive enzymes inoculated together with the venom or injected posteriorly. After this extracorporeal digestion the food is ingested and gains the digestive tube of the predator where the extracellular digestion continues.

12. Which organs or respiratory adaptations do aquatic and terrestrial arthropods respectively present?
In crustaceans, typical aquatic beings, there are richly vascularized gills that make contact with water and permit gas exchange. In terrestrial insects the respiration is tracheal and gases flow inside small tubes that connect the animal external surface and ramify to tissues and cells without the participation of blood. In arachnids, besides the tracheal respiration, book lungs (thin folds resembling leaves in a book) may also exist.

Phylum Arthropoda - Image Diversity: crustacean respiratory system insect respiratory system arachnid respiratory system



13. In arthropods why isn't gas exchange done through cutaneous diffusion?
In arthropods the impermeability of the exoskeleton makes the passage of gases difficult. In addition the new methods of respiration present in arthropods were preserved by evolution because they were more efficient for those animals.

14. What is the type of circulatory system present in arthropods? Do these animals have heart and respiratory pigments?
In arthropods the respiratory system is open (lacunar). Blood, also known as hemolymph, is pumped by a heart and falls into cavities (lacunas) irrigating and draining tissues.

All arthropods have a heart. Crustaceans and arachnids have respiratory pigments. Most insects do not have respiratory pigments since their blood does not carry gases (in them gases reach tissues and cells through tracheal structures). However, some few insects do have respiratory pigments, hemoglobin (contribution from Jacob Campbell).

15. What are respiratory pigments? What is the respiratory pigment present in some arthropods? Which is the analogous molecule in humans?
Respiratory pigments are molecules able to carry oxygen and other respiratory gases present in circulatory fluids.

In crustaceans and in arachnids hemocyanin is the respiratory pigment. In humans the analogous pigment is hemoglobin.

16. How is the respiratory system of insects (with its independence between circulation and respiration) related to the motor agility of some species of this arthropod class?
Even having low speed and low pressure circulatory system, since it is a lacunar (open) circulatory system, insects perform extremely fast and exhaustive movements with their muscle fibers, like wing beating. This is possible because in these animals the respiration is independent from the open circulation. Gas exchange is done with great speed and efficiency by the tracheal system that puts cells in direct contact with air. Muscles can then work fast and hard.

17. How are the excretory systems of the three main arthropod classes constituted?
In crustaceans a pair of excretory organs called green glands exists. The green glands collect residuals from the blood and other parts of the body. They are connected by ducts to excretory pores located under the base of the antennae and these pores release the excretions outside.

In insects small structures called malpighian tubules gather wastes from the blood and throw them into excretory ducts that open in the intestine. In these animals excretions are eliminated together with feces.

In arachnids, besides malpighian tubules, there are coxal glands located in the cephalothorax near the limbs that also participate in excretion.

Phylum Arthropoda - Image Diversity: arthropod excretory system

18. What are the noteworthy features of the nervous system of arthropods?
In arthropods the nervous system has more sophisticated sensory receptors with well-advanced cephalization. In the anterior region of the body there is a fusion of ganglia forming a brain connected to two ventral ganglial chains having motor and sensory nerves.

The boosted development of the sensory system of arthropods provides more adaptive possibilities for these animals to explore many different environments.

Phylum Arthropoda - Image Diversity: arthropod nervous system

19. What are compound eyes?
Arthropods have compound eyes made of several visual units called ommatidia. Each ommatidium transmits visual information through the optic nerve to the brain, which interprets the image. Because they are round and numerous, these ommatidia, whose external surfaces point in different directions creating independent images, cause arthropod eyes have a large visual field, larger than the visual field of vertebrates. Some insects have one or more simple eye besides their pair of compound eyes.

Phylum Arthropoda - Image Diversity: compound eyes

20. How is arthropod reproduction characterized?
Reproduction in beings of the phylum Arthropoda is sexual, with larval stage in some insects and crustaceans (arachnids present only direct development).

21. What are the types of fecundation that occur in arthropods? What is the predominant type?
In arthropods there are species having external fecundation and other species having internal fecundation. Internal fecundation is predominant.

22. How is fecundation done in insects (external or internal)? Is there copulation between insects?
Fecundation in insects is internal, with copulation.

23. How are the main classes of arthropods classified according to the presence of larval stage in their embryonic development?
In crustaceans there are species with direct and others with indirect development. In insects there are species without larval stage (ametabolic insects), others undergoing indirect development beginning with an egg stage followed by a nymph stage (hemimetabolic insects) and others with indirect development beginning with the larval stage (holometabolic insects).

The transformation of a larva into an adult individual is called metamorphosis. Hemimetabolic insects undergo incomplete metamorphosis while holometabolic insects undergo complete metamorphosis.

Phylum Arthropoda - Image Diversity: metamorphosis

24. What are nymph and imago?
Nymphs are larvae of hemimetabolic insects (like grasshoppers). They are very similar to the adult insect although smaller. In holometabolic insects (like butterflies) the larva makes a cocoon (chrysalis, pupa) where it lives until emerging into the adult form. Imago is the name given to the adult form of insects with indirect development.

25. Is the stage when an insect larva is within a cocoon a stage of total biological inactivity?
The period when the larva is within its cocoon is a time of intense biological activity since the larva is being transformed into an adult animal.

26. How are the three main arthropod classes characterized according to the presence of wings?
Crustaceans and arachnids do not have wings. Most insects have wings.

27. Most insects have wings. Which is the other animal phylum that contains creatures with analogous organs?
Besides the phylum Arthropoda another animal phylum with flying creatures is the chordate phylum, birds and chiropterans mammals (bats) have wings. In the past some reptiles that possibly originated the aves had wings too. There are also amphibians and fishes that jump high exploring the aerial environment.

28. How are the three main arthropod classes characterized according to the presence of antennae?
Crustaceans have two pairs of antennae; insects have one pair; arachnids do not have antennae.

29. How are the three main arthropod classes characterized according to the body division?
In crustaceans and arachnids the head is fused with the thorax forming the cephalothorax. Their body thus is divided into cephalothorax and abdomen.

In insects there are head, thorax and abdomen.

30. How are the three main arthropod classes characterized according to the number of limbs?
Most crustaceans have five pairs of limbs. Insects have three pairs and arachnids present four pairs of limbs.

31. Which arthropod class is the most diversified animal group of the planet? How can this evolutionary success be explained?
The insects are the animal group with most diversity of species. Almost 750000 insect species are known, about 55% of the total already cataloged species of living beings (compare with mammals, with no more than 4000 known species). It is calculated however that the number of unknown species of insects may be over 2 million. The insect population on the planet is estimated to be more than 10 quintillion (1000000000000000000) individuals.

The great evolutionary success of insects is due to factors such as: small size and alimentary diversity, making possible the exploration of numerous different ecological niches; wings that provided more geographic spread; the tracheal respiration that gave them motor agility; high reproductive rates with production of great numbers of descendants.

32. What are some examples of beings of the phylum Arthropoda that present a high level of behavioral sophistication?
Insects like some species of bees, wasps, ants and termites form societies that include hierarchy and job division among members. Spiders build sophisticated external structures, webs, mainly to serve as a trap for capturing prey. Another example is the communication mechanism in some bees known as the bee dance by which an individual signal to others information about the spatial position of flower fields and other nectar sources.

33. Arthropod identity card. How are arthropods characterized according to examples of representing beings, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?
Examples of representing beings: cockroaches, flies, crabs, lobsters, shrimps, spiders, scorpions, mites. Basic morphology: segmented body (metameric), articulated limbs, chitinous exoskeleton, periodic ecdysis. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete. Respiratory system: tracheal in insects, branchial in crustaceans, tracheal and book lungs in arachnids. Circulatory system: open, hemocyanin in crustaceans and arachnids. Excretory system: malpighian tubules in insects, green glands in crustaceans, malpighian tubules and coxal glands in arachnids. Nervous system: ganglial. Types of reproduction: sexual, with or without larval stage in insects and crustaceans, metamorphosis in some insects, no larval stage in arachnids.

Annelida

Segmented worms: bristleworms, ragworms, earthworms, leeches and their allies

Greg W. Rouse, Fredrik Pleijel, and Damhnait McHugh

Figure 1: Summary of phylogenetic hypothesis in Rouse and Pleijel (2001). Alternative arrangements are discussed in the Discussion of Phylogenetic Relationships. The position of Clitellata/Oligochaeta (includes earthworms and leeches) and Echiura is yet to be resolved.

Containing group: Bilateria

Introduction

Annelida is a group commonly referred to as segmented worms, and they are found worldwide from the deepest marine sediments to the soils in our city parks and yards. Through most of the 20th century Annelida was split into three major groups; Polychaeta, Oligochaeta (earthworms etc.) and Hirudinea (leeches). Earthworms and leeches are the familiar annelids for most people, but polychaetes comprise the bulk of the diversity of Annelida and are found in nearly every marine habitat, from intertidal algal mats downwards. There are even pelagic polychaetes that swim or drift, preying on other plankton, and a few groups occurring in fresh water and moist terrestrial surroundings. Around 9000 species of polychaetes are currently recognized with several thousand more names in synonymy, and the overall systematics of the group remains unstable (Rouse and Pleijel, 2001).
It is now recognized that Oligochaeta and Hirudinea, comprised of several thousand species, form a clade and should be referred to either as Oligochaeta (Siddall et al., 2001) or Clitellata (Martin, 2001). Moreover, it is possible that this group may well belong inside Polychaeta, thus making Polychaeta synonymous with Annelida (McHugh, 1997; Westheide, 1997; Westheide et al., 1999). Echiura (spoon worms), at one time regarded as an annelid group (Sedgwick, 1898), has been excluded from Annelida for many years (Newby, 1940). Evidence now suggests they are in fact annelids (Hessling and Westheide, 2002; McHugh, 1997), though their placement within the group is unresolved. The former phyla Pogonophora and Vestimentifera have also recently become regarded as a single, clearly annelid, group (Bartolomaeus, 1995; Nielsen, 1995; Rouse and Fauchald, 1995), and are now known by the original name, Siboglinidae (see Rouse and Fauchald , 1997 and McHugh 1997). Undoubted annelid fossils, such as Canadia, are known from the Burgess Shale deposits.
Until relatively recently the most commonly used system to divide polychaetes was as 'Errantia' and 'Sedentaria'. This was essentially a system of convenience with no real intention of depicting evolutionary relationships. This classification was supplanted in the 1960s and 1970s by ones which split polychaetes into as many as 22 orders with no explicit linkage between them (Fauchald and Rouse, 1997). A recent cladistic analysis of Annelida and other groups has resulted in a new classification of polychaetes (Rouse and Fauchald, 1997), with the group split into two main clades Scolecida and Palpata. Scolecida is a small group of less than 1000 named species, and these worms are all burrowers of one form or another, with bodies reminiscent of earthworms. Palpata comprises the vast majority of polychaetes and is divided into Aciculata and Canalipalpata. Aciculata contains about half of the polychaete species and largely encompasses the old taxonomic group Errantia. Representatives of this lineage are characterized by having internal supporting chaetae, or aciculae, in the parapodia. It includes major groups such as Phyllodocida and Eunicida, which tend to be mobile forms with well developed eyes and parapodia for rapid locomotion. Canalipalpata, a group with more than 5000 named species, is distinguished by having long grooved palp structures that are used for feeding. Canalipalpata is divided into Sabellida, Spionida and Terebellida. Most of these groups’ members live in tubes and use their palps to feed in various ways.

Characteristics

Synapomorphies of Annelida

The monophyly of Annelida is not well supported and only two morphological features are worthy of discussion; segmentation and chaetae. Nuchal organs represent another possible apomorphy and are discussed in the section on sensory structures (see plesiomorphies and other features).

1. Metamerism (segmentation)

Annelids have three body regions (Fig. 2). The majority of the body is comprised of repeated units called segments. The original French use of the name Annélides (Lamarck, 1802) comes from the Latin word ‘anellus’, meaning a little ring, in reference to the presence of the ring-like segments. Each segment is, in principle, limited by septa dividing it from neighbouring segments, and has a fluid-filled cavity within referred to as a coelom. Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment. Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior. The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium. The head is comprised of two units, the prostomium and the peristomium. The postsegmental pygidium includes the zone from which new segments are proliferated during growth. The proposed homology of segmentation seen in annelids with that seen in Arthropoda has been used to unite the two as Articulata, a grouping that dates back to Cuvier (1817). The homology of this segmentation has been questioned recently, with arthropods now viewed by many as closer to taxa such as Nematoda (Aguinaldo et al., 1997). This suggests that the form of segmentation seen in annelids may in fact represent an apomorphy. With regards to the supposedly unsegmented Echiura, their reinstatement within Annelida (see McHugh 1997) suggests that their apparently unsegmented body in fact represents a series of fused segments (see Hessling and Westheide 2002).

Figure 2. Ophryotrocha (Dorvilleidae). Sandgerdi Iceland. Copyright © 20001 Greg Rouse.

2. Chaetae

A distinctive feature of annelids are structures called chaetae (Fig. 3). Chaetae (also called setae) are bundles of chitinous, thin-walled cylinders held together by sclerotinized protein. They are produced by a microvillar border of certain invaginated epidermal cells and so can be defined as cuticular structures that develop within epidermal follicles. Chaetae show a huge amount of variation, from long thin filaments (capillary chaetae) to stout multi-pronged hooks (Fig. 3). Apart from annelids, chaetae are found in Echiura and Brachiopoda. There is now good evidence (Hessling and Westheide, 2002; McHugh, 1997) that the former group falls within Annelida. The position of Brachiopoda is controversial (Lüter, 2000b; Lüter and Bartolomaeus, 1997; Stechmann and Schlegel, 1999) and the homology of their chaetae with those of annelids is unresolved (Lüter, 2000a). There is a distinct possibility therefore that chaetae represent an apomorphy for Annelida.

Figure 3. Proscoloplos (Orbiniidae). Bondi, Australia. SEM and Light micrographs. Copyright © 2001 Greg Rouse.

Discussion of Phylogenetic Relationships

The most recent comprehensive systematization of polychaetes, that proposed by Rouse and Fauchald (1997) from their cladistic parsimony analyses, has been used here (Fig. 1) with some alterations (see Rouse and Pleijel, 2001). Allowing for the likely errors in the placement of many taxa, and the fact that there were conflicting results included in the original analyses, the most fundamental problem inherent in the systematization used here may be that of the placement of the root for any tree of Annelida.
Basal annelids, according to Rouse and Fauchald (1997), are taxa such as Clitellata/Oligochaeta and simple-bodied forms within Scolecida. Their trees also excludes Echiura from Annelida. This result was based on outgroup choices such as Mollusca and Sipuncula, and may well be misleading. Alternative hypotheses are therefore worth outlining, though they do not follow normal cladistic practice. Storch (1968), following a detailed study on the musculature of Annelida, proposed that scale-worms, a diverse clade within Phyllodocida, are representative of the plesiomorphic condition for Annelida. He suggested that there was a radiation from this group, but that Chrysopetalidae were most closely related to scale-worms. The implication of his hypothesis is that Phyllodocida represents a paraphyletic group, from which all other polychaete taxa arise. Westheide (1997) (and see Westheide et al., 1999) suggests that the basic (i.e., plesiomorphic) 'body plan' of Annelida comprises features meaning that the root of the Annelida tree would be placed with taxa from Aciculata. This would either result in a paraphyletic Phyllodocida or Amphinomida, depending on which taxon is used as the root (Fig. 4).

Figure 4.

From a molecular perspective, McHugh (1997) found Clitellata, Pogonophora (= Siboglinidae) and Echiura nested among various polychaetes using sequence data from elongation factor 1 alpha in a parsimony analysis (Fig. 4). The placement by McHugh (1997) of Pogonophora as a polychaete group was congruent with Rouse and Fauchald’s (1997) results, but those of Clitellata and Echiura were markedly different. The conflict between this molecular sequence data and the morphological results could be caused by several factors. One possibility is that Clitellata/Oligochaeta and Echiura have lost a number of morphological features that would help identify their sister group among polychaetes. Further morphological study, combined with sequence data, may uncover these 'losses' (see Hessling and Westheide, 2002). However, the molecular sequence data sets assembled to date have been marked by both a limited number of taxa and characters. An exception is Brown et al. (1999), where DNA sequence data from three nuclear genes across a wide taxonomic diversity of annelids was subject to parsimony analysis. They recovered some morphological groupings such as Cirratulidae, Terebellidae and Eunicida, but did not show a monophyletic Phyllodocida or Aciculata, nor did they find any parts of these taxa to be basal groups of Annelida. However, some expected groupings were not recovered. Martin (2001) found that the placement of Clitellata/Oligochaeta among polychaetes could not be resolved, and he could not recover a monophyletic Annelida owing to the placement of taxa such as Mollusca and Sipuncula. None of the major taxa used here, such as Palpata, Aciculata, Phyllodocida, Canalipalpata, Sabellida or Terebellida were recovered in Martin's (2001) analysis. Also less diverse taxa such as Nereididae, Spionidae and Aphroditiformia were not recovered.

Does the fossil record help?

In a review of the fossil record of annelids Rouse and Pleijel (2001) suggested that the oldest unequivocal fossil polychaetes, such as Canadia from the Cambrian, belong within Phyllodocida. Subsequent fossil polychaetes that can be confidently placed outside Phyllodocida do not appear until the Carboniferous. No other fossil polychaetes from the Cambrian can be unequivocally assigned to extant polychaete taxa. There are several likely appearances from the Ordovician, including Serpulidae, Spionidae and the radiation of Eunicida. Ensuing appearances suggest that by the end of the Carboniferous most major polychaetes lineages had appeared. The exception appears to be Scolecida, with the earliest known fossils being the dubious Archarenicola (Arenicolidae) from the Triassic, and one assignable to Paraonidae from the Cretaceous. With the rooting option employed in Figure 1, it appears that some of the earliest appearing fossil polychaetes belong to derived clades (e.g., Eunicida and Phyllodocida). This could be interpreted in two ways: (1) the root placement in Figure 1 is wrong, and so Aciculata, comprised of Amphinomida, Eunicida and Phyllodocida, may in fact represent a paraphyletic 'stem' group for the rest of polychaetes; (2) a number of major polychaete clades had already evolved in, or before, the 'Cambrian explosion', but fossils have not yet been found. The third possibility is that the overall topology used in Figure 1 may be profoundly incorrect. If we accept that the basic topology shown in Figure 1 is correct, but do not root the tree, then a diagram as shown in Figure 5 is the result. This may represent the most conservative representation of our understanding of annelid relationships.

Figure 5.

Other Names for Annelida

• Segmented worms

References

Aguinaldo, A. M. A., J. M. Turbeville, L. S. Linford, M. C. Rivera, J. R. Garey, R. A. Raff, and J. A. Lake. 1997. Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387:489-493.

Bartolomaeus, T. 1995. Structure and formation of the uncini in Pectinaria koreni, Pectinaria auricoma (Terebellida) and Spirorbis spirorbis (Sabellida): implications for annelid phylogeny and the position of the Pogonophora. Zoomorph. 115:161-177.

Brown, S., G. Rouse, P. Hutchings, and D. Colgan. 1999. Assessing the usefulness of histone H3, U2 snRNA and 28S rDNA in analyses of polychaete relationships. Aust. J. Zool. 47:499-516.

Cuvier, G. 1817. Le régne animal distribué d'après son organisation, pour servir de base à l'histoire naturelle des animaux et d'introduction à l'anatomie comparée. Deterville, Paris.

Eibye-Jacobsen, D., and R. M. Kristensen. 1994. A new genus and species of Dorvilleidae (Annelida, Polychaeta) from Bermuda, with a phylogenetic analysis of Dorvilleidae, Iphitimidae and Dinophilidae. Zool. Scri. 23:107-131.

Fauchald, K. 1977. The polychaete worms. Definitions and keys to the orders, families and genera. Nat. Hist. Mus. Los Angeles County. Sci. Ser. 28:1-188.

Fauchald, K., and G. W. Rouse. 1997. Polychaete systematics: Past and present. Zool. Scr. 26:71-138.

Hessling, R., and W. Westheide. 2002. Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis. J. Morph. 252:100-113.

Jamieson, B. G. M. 1981. The ultrastructure of the Oligochaeta. Academic Press, London.

Lamarck, J.-B. d. 1802. La nouvelle classes des Annélides. Bulletin du Muséum d'Histoire Naturelle, Paris An X:Disc. d'ouverture, 27 Floréal (Reprinted in 1907 in Bulletin biologique de la France et de la Belgique 60:56).

Lüter, C. 2000a. Ultrastructure of larval and adult setae of Brachiopoda. Zoologischer Anzeiger 239:75-90.

Lüter, C. 2000b. The origin of the coelom in Brachiopoda and its phylogenetic significance. Zoomorph. 120:15-28.

Lüter, C., and T. Bartolomaeus. 1997. The phylogenetic position of Brachiopoda- a comparison of morphological and molecular data. Zool. Scri. 26:245-253.

Martin, P. 2001. On the origin of the Hirudinea and the demise of the Oligochaeta. Proc. Royal Soc. London Ser. B: Biol. Sci. 268:1089-1098.

McHugh, D. 1997. Molecular evidence that echiurans and pogonophorans are derived annelids. Proc. natn. Acad. Sci. U.S.A. 94:8006-8009.

McHugh, D. 2000. Molecular phylogeny of the Annelida. Can. J. Zool. 78:1873-1884.

Newby, W. W. 1940. The embryology of the echiuroid worm Urechis caupo. Mem. Am. phil. Soc. 16:1-219.

Nielsen, C. 1995. Animal evolution. Oxford University Press, Oxford.

Purschke, G., R. Hessling, and W. Westheide. 2000. The phylogenetic position of the Clitellata and the Echiura - on the problematic assessment of absent characters. J. Zool. Syst. Evol. Res. 38:165-173.

Rouse, G. W. 2001. A cladistic analysis of Siboglinidae Caullery, 1914 (Polychaeta, Annelida): formerly the phyla Pogonophora and Vestimentifera. Zool. J. Linn. Soc. 132:55-80.

Rouse, G. W., and K. Fauchald. 1995. The articulation of annelids. Zool. Scr. 24:269-301.

Rouse, G. W., and K. Fauchald. 1997. Cladistics and polychaetes. Zool. Scr. 26:139-204.

Rouse, G. W., and F. Pleijel. 2001. Polychaetes. Oxford University Press, London.

Sawyer, R. T. 1986. Leech biology and behaviour. Clarendon Press, Oxford.

Sedgwick, A. 1898. A student's textbook of zoology. Swan Sonnenschein & Co. Ltd., London.

Siddall, M. E., K. Apakupakul, E. M. Burreson, K. A. Coates, C. Erseus, S. R. Gelder, M. Kallersjo, and H. Trapido-Rosenthal. 2001. Validating Livanow: Molecular data agree that leeches, branchiobdellidans, and Acanthobdella peledina form a monophyletic group of oligochaetes. Mol. Phyl. Evol. 21:346-351.

Stechmann, A., and M. Schlegel. 1999. Analysis of the complete mitochondrial DNA sequence of the brachiopod Terebratulina retusa places Brachiopoda within the protostomes. Proc. Royal Soc. London Ser. B: Biol. Sci. 266:2043-2052.

Storch, V. 1968. Zur vergleichenden Anatomie der segmentalen Muskelsysteme und zur Verwandtschaft der Polychaeten-Familien. Z. Morph. Ökol. Tiere 63:251-342.

Westheide, W. 1997. The direction of evolution within the Polychaeta. J. Nat. Hist. 31:1-15.

Westheide, W., D. McHugh, G. Purschke, and G. W. Rouse. 1999. Systematization of the Annelida: different approaches. Hydrobiol. 402:291-307.