Flagellar basal apparatus and its utility in phylogenetic analyses of the porifera

Ciliary transition zone evolution and the root of the eukaryote tree: implications for opisthokont origin and classification of kingdoms Protozoa, Plantae, and Fungi

I thoroughly discuss ciliary transition zone (TZ) evolution, highlighting many overlooked evolutionarily significant ultrastructural details. I establish fundamental principles of TZ ultrastructure and evolution throughout eukaryotes, inferring unrecognised ancestral TZ patterns for Fungi, opisthokonts, and Corticata (i.e., kingdoms Plantae and Chromista). Typical TZs have a dense transitional plate (TP), with a previously overlooked complex lattice as skeleton. I show most eukaryotes have centriole/TZ junction acorn-V filaments (whose ancestral function was arguably supporting central pair microtubule-nucleating sites; I discuss their role in centriole growth). Uniquely simple malawimonad TZs (without TP, simpler acorn) pinpoint the eukaryote tree's root between them and TP-bearers, highlighting novel superclades. I integrate TZ/ciliary evolution with the best multiprotein trees, naming newly recognised major eukaryote clades and revise megaclassification of basal kingdom Protozoa. Recent discovery of non-photosynthetic phagotrophic flagellates with genome-free plastids (Rhodelphis), the sister group to phylum Rhodophyta (red algae), illuminates plant and chromist early evolution. I show previously overlooked marked similarities in cell ultrastructure between Rhodelphis and Picomonas, formerly considered an early diverging chromist. In both a nonagonal tube lies between their TP and an annular septum surrounding their 9+2 ciliary axoneme. Mitochondrial dense condensations and mitochondrion-linked smooth endomembrane cytoplasmic partitioning cisternae further support grouping Picomonadea and Rhodelphea as new plant phylum Pararhoda. As Pararhoda/Rhodophyta form a robust clade on site-heterogeneous multiprotein trees, I group Pararhoda and Rhodophyta as new infrakingdom Rhodaria of Plantae within subkingdom Biliphyta, which also includes Glaucophyta with fundamentally similar TZ, uniquely in eukaryotes. I explain how biliphyte TZs generated viridiplant stellate-structures.

The architecture of cell differentiation in choanoflagellates and sponge choanocytes

Although collar cells are conserved across animals and their closest relatives, the choanoflagellates, little is known about their ancestry, their subcellular architecture, or how they differentiate. The choanoflagellate Salpingoeca rosetta expresses genes necessary for animal development and can alternate between unicellular and multicellular states, making it a powerful model for investigating the origin of animal multicellularity and mechanisms underlying cell differentiation. To compare the subcellular architecture of solitary collar cells in S. rosetta with that of multicellular ‘rosette’ colonies and collar cells in sponges, we reconstructed entire cells in 3D through transmission electron microscopy on serial ultrathin sections. Structural analysis of our 3D reconstructions revealed important differences between single and colonial choanoflagellate cells, with colonial cells exhibiting a more amoeboid morphology consistent with higher levels of macropinocytotic activity. Comparison of multiple reconstructed rosette colonies highlighted the variable nature of cell sizes, cell–cell contact networks, and colony arrangement. Importantly, we uncovered the presence of elongated cells in some rosette colonies that likely represent a distinct and differentiated cell type, pointing toward spatial cell differentiation. Intercellular bridges within choanoflagellate colonies displayed a variety of morphologies and connected some but not all neighbouring cells. Reconstruction of sponge choanocytes revealed ultrastructural commonalities but also differences in major organelle composition in comparison to choanoflagellates. Together, our comparative reconstructions uncover the architecture of cell differentiation in choanoflagellates and sponge choanocytes and constitute an important step in reconstructing the cell biology of the last common ancestor of animals.

3D electron microscopy of choanoflagellates and sponge choanocytes reveals a remarkable variety of cell architecture and suggests that cell type differentiation may have been present in the stem lineage leading to the animals.

Choanoflagellates are microscopic aquatic organisms that can alternate between single-celled and multicellular states, and sequencing of their genomes has revealed that choanoflagellates are the closest single-celled relatives of animals. Moreover, choanoflagellates are a form of ‘collar cell’—a cell type crowned by an array of finger-like microvilli and a single, whip-like flagellum. This cell type is also found throughout the animal kingdom; therefore, studying the structure of the choanoflagellate collar cell can shed light on how this cell type and animal multicellularity might have evolved. We used electron microscopy to reconstruct in 3D the total subcellular composition of single-celled and multicellular choanoflagellates as well as the collar cells from a marine sponge, which represents an early-branching animal lineage. We found differences between single-celled and multicellular choanoflagellates in structures associated with cellular energetics, membrane trafficking, and cell morphology. Likewise, we describe a complex system of cell–cell connections associated with multicellular choanoflagellates. Finally, comparison of choanoflagellates and sponge collar cells revealed subcellular differences associated with feeding and cellular energetics. Taken together, this study is an important step forward in reconstructing the biology of the last common ancestor of the animals.

The Origin of Animal Multicellularity and Cell Differentiation

Over 600 million years ago, animals evolved from a unicellular or colonial organism whose cell(s) captured bacteria with a collar complex, a flagellum surrounded by a microvillar collar. Using principles from evolutionary cell biology, we reason that the transition to multicellularity required modification of pre-existing mechanisms for extracellular matrix synthesis and cytokinesis. We discuss two hypotheses for the origin of animal cell types: division of labor from ancient plurifunctional cells and conversion of temporally alternating phenotypes into spatially juxtaposed cell types. Mechanistic studies in diverse animals and their relatives promise to deepen our understanding of animal origins and cell biology.

The ontogeny of choanocyte chambers during metamorphosis in the demosponge Amphimedon queenslandica

BACKGROUND

The aquiferous body plan of poriferans revolves around internal chambers comprised of choanocytes, a cell type structurally similar to choanoflagellates. These choanocyte chambers perform a range of physiological and developmental functions, including the capture of food and the generation of stem cells. Despite the increasing interest for choanocytes as sponge stem cells, there is limited knowledge on the development of choanocyte chambers. Using a combination of cell lineage tracing, antibody staining and EdU labeling, here we examine the development of choanocytes and the chambers they comprise during metamorphosis in the marine demosponge Amphimedon queenslandica.

RESULTS

Lineage-tracing experiments show that larval epithelial cells transform into mesenchymal pluripotent stem cells, resembling archeocytes, within 24 h of initiating metamorphosis. By 36 h, some of these labeled archeocyte-like cells have differentiated into choanocytes that will form the first postlarval choanocyte chambers. Non-labeled cells also contribute to these primary choanocyte chambers, consistent with these chambers being a chimera of multiple transdifferentiated larval cell types and not the proliferation of a single choanocyte precursor. Moreover, cell proliferation assays demonstrate that, following the initial formation of choanocyte chambers, chambers grow at least partially by the proliferation of choanocytes within the chamber, although recruitment of individual cells into established chambers also appears to occur. EdU labeling of postlarvae and juveniles reveals that choanocyte chambers are the primary location of cell proliferation during metamorphosis.

CONCLUSION

Our results show that multiple larval cell lineages typically contribute to formation of individual choanocyte chambers at metamorphosis, contrary to previous reports in other species that show sponge choanocyte chambers form clonally. Choanocytes in postlarval and juvenile A. queenslandica chambers can also divide, with choanocyte chambers being the primary location of cell proliferation. Interestingly, the level of cell proliferation varies greatly between chambers and appears to be contingent on the size, location and developmental state of the chamber. Small chambers on the periphery of the body tend to possess more dividing cells. As choanocytes can also dedifferentiate into archeocyte-like cells, cell proliferation in chambers may not only contribute to chamber growth and self-renewal but also increase the number of pluripotent archeocytes.

Choanoflagellate and choanocyte collar-flagellar systems and the assumption of homology

The similarities between the choanoflagellates and the choanocytes of sponges have been discussed for more than a century yet few studies allow a direct comparison of the two. We reviewed current knowledge of the collar and flagellum and compared their structure and function in the choanoflagellate Monosiga brevicollis and the sponge Spongilla lacustris. Collar microvilli were of similar length and number, but the shape of the collar differed between the two cells. In Monosiga, collars were flared and microvilli were joined by a single band of glycocalyx mid-way along their length; in Spongilla, collars formed a tube and microvilli were joined by a mesh of glycocalyx. Monosiga flagella beat at least four times faster than those in Spongilla. Flagellar vanes were found in both cell types. In both cells, the flagella and so probably also the vanes maintained moving points of contact with the microvilli, which suggested that collars and flagella were integrated systems rather than independent units. There were fundamental differences in how the collar and flagella interacted, however. In Spongilla, the flagellum bent upon contact with the collar; the flagellar amplitude was fitted to the collar diameter. In Monosiga, the flagellar amplitude was unaffected by the collar; instead the collar diameter appeared fitted to the flagellum. These differences suggest that though choanocytes and choanoflagellates are similar, homology cannot be taken for granted. Similarities in collar-flagellum systems separated by 600 million years of evolution, whether maintained or convergent, suggest that these form important adaptations for optimizing fluid flow through micro-scale filters.

Ultrastructure of the ciliated cells of the free-swimming larva, and sessile stages, of the marine sponge Haliclona indistincta (Demospongiae: Haplosclerida)

We provide a detailed, comparative study of the ciliated cells of the marine haplosclerid sponge Haliclona indistincta, in order to make data available for future phylogenetic comparisons at the ultrastructural level. Our study focuses on the description and analysis of the larval epithelial cells, and choanocytes of the metamorphosed juvenile sponge. The ultrastructure of the two cell types is sufficiently different to prevent our ability to conclusively determine the origin of the choanocytes from the larval ciliated cells. However, ciliated, epithelial cells were observed in a migratory position within the inner cell mass of the larval stages. Some cilia were observed within the cell's cytoplasm, which is indicative of the ciliated epithelial cell undergoing transdifferentiation into a choanocyte; while traces of other ciliated epithelial cells were contained within phagosomes, suggesting they are phagocytosed. We compared our data with other species described in the literature. However, any phylogenetic inference must wait until further detailed comparisons can be made with species whose phylogenetic position has been determined by other means, such as phylogenomics, in order to more closely link genomic, and morphological information.

Deep phylogeny and evolution of sponges (phylum Porifera)

Sponges (phylum Porifera) are a diverse taxon of benthic aquatic animals of great ecological, commercial, and biopharmaceutical importance. They are arguably the earliest-branching metazoan taxon, and therefore, they have great significance in the reconstruction of early metazoan evolution. Yet, the phylogeny and systematics of sponges are to some extent still unresolved, and there is an on-going debate about the exact branching pattern of their main clades and their relationships to the other non-bilaterian animals. Here, we review the current state of the deep phylogeny of sponges. Several studies have suggested that sponges are paraphyletic. However, based on recent phylogenomic analyses, we suggest that the phylum Porifera could well be monophyletic, in accordance with cladistic analyses based on morphology. This finding has many implications for the evolutionary interpretation of early animal traits and sponge development. We further review the contribution that mitochondrial genes and genomes have made to sponge phylogenetics and explore the current state of the molecular phylogenies of the four main sponge lineages (Classes), that is, Demospongiae, Hexactinellida, Calcarea, and Homoscleromorpha, in detail. While classical systematic systems are largely congruent with molecular phylogenies in the class Hexactinellida and in certain parts of Demospongiae and Homoscleromorpha, the high degree of incongruence in the class Calcarea still represents a challenge. We highlight future areas of research to fill existing gaps in our knowledge. By reviewing sponge development in an evolutionary and phylogenetic context, we support previous suggestions that sponge larvae share traits and complexity with eumetazoans and that the simple sedentary adult lifestyle of sponges probably reflects some degree of secondary simplification. In summary, while deep sponge phylogenetics has made many advances in the past years, considerable efforts are still required to achieve a comprehensive understanding of the relationships among and within the main sponge lineages to fully appreciate the evolution of this extraordinary metazoan phylum.

Choanocyte ultrastructure in Halisarca dujardini (Demospongiae, Halisarcida)

Understanding poriferan choanocyte ultrastructure is crucial if we are to unravel the steps of a putative evolutionary transition between choanoflagellate protists and early metazoans. Surprisingly, some aspects of choanocyte cytology still remain little investigated. This study of choanocyte ultrastructure in the halisarcid demosponge Halisarca dujardini revealed a combination of minor and major distinctive traits, some of them unknown in Porifera so far. Most significant features were 1) an asymmetrical periflagellar sleeve, 2) a battery of specialized intercellular junctions at the lateral cell surface complemented with an array of lateral interdigitations between adjacent choanocytes that provides a particular sealing system of the choanoderm, and 3) a unique, unexpectedly complex, basal apparatus. The basal apparatus consists of a basal body provided with a small basal foot and an intricate transverse skeleton of microtubules. An accessory centriole, which is not perpendicular to the basal body, is about 45 degrees . In addition, a system of short striated rootlets (periodicity = 50-60 nm) arises from the proximal edge of the basal body and runs longitudinally to contact the nuclear apex. This is the first flagellar rootlet system ever found in a choanocyte. The accessory centriole, the rootlet system, and the nuclear apex are all encircled by a large Golgi apparatus, adding another distinctive feature to the choanocyte cytology. The set of distinct features discovered in the choanocyte of H. dujardini indicates that the ultrastructure of the poriferan choanocyte may vary substantially between sponge groups. It is necessary to improve understanding of such variation, as the cytological features of choanocytes are often coded as characters both for formulation of hypotheses on the origin of animals and inference of phylogenetic relationships at the base of the metazoan tree.

Six major steps in animal evolution: are we derived sponge larvae?

A review of the old and new literature on animal morphology/embryology and molecular studies has led me to the following scenario for the early evolution of the metazoans. The metazoan ancestor, "choanoblastaea," was a pelagic sphere consisting of choanocytes. The evolution of multicellularity enabled division of labor between cells, and an "advanced choanoblastaea" consisted of choanocytes and nonfeeding cells. Polarity became established, and an adult, sessile stage developed. Choanocytes of the upper side became arranged in a groove with the cilia pumping water along the groove. Cells overarched the groove so that a choanocyte chamber was formed, establishing the body plan of an adult sponge; the pelagic larval stage was retained but became lecithotrophic. The sponges radiated into monophyletic Silicea, Calcarea, and Homoscleromorpha. Homoscleromorph larvae show cell layers resembling true, sealed epithelia. A homoscleromorph-like larva developed an archenteron, and the sealed epithelium made extracellular digestion possible in this isolated space. This larva became sexually mature, and the adult sponge-stage was abandoned in an extreme progenesis. This eumetazoan ancestor, "gastraea," corresponds to Haeckel's gastraea. Trichoplax represents this stage, but with the blastopore spread out so that the endoderm has become the underside of the creeping animal. Another lineage developed a nervous system; this "neurogastraea" is the ancestor of the Neuralia. Cnidarians have retained this organization, whereas the Triploblastica (Ctenophora+Bilateria), have developed the mesoderm. The bilaterians developed bilaterality in a primitive form in the Acoelomorpha and in an advanced form with tubular gut and long Hox cluster in the Eubilateria (Protostomia+Deuterostomia). It is indicated that the major evolutionary steps are the result of suites of existing genes becoming co-opted into new networks that specify new structures. The evolution of the eumetazoan ancestor from a progenetic homoscleromorph larva implies that we, as well as all the other eumetazoans, are derived sponge larvae.

Reproduction in a carnivorous sponge: the significance of the absence of an aquiferous system to the sponge body plan

Sponges usually produce, release, and capture gametes via the aquiferous system, and so the absence of both choanocytes and an aquiferous system in the carnivorous sponge Asbestopluma occidentalis has led to unusual characteristics of development for this Phylum. Sperm are highly specialized elongate cells tightly packed into spermatic cysts in the peripheral tissue of the sponge. Mature spermatozoa have proacrosomal vesicles at the anterior end and a ciliary pit surrounding the flagellum. Clusters of four to five oocytes are in synchronous stages of cleavage, suggesting that fertilization is synchronous. All stages of embryos occur in the same individual. Early cleavage was holoblastic and equal; blastomeres in two-, four- and eight-cell embryos were compact and 16-cell stage embryos were bi-layered. Late-stage embryos show three cellular regions along the anterior-posterior axis: the anterior hemisphere with heterogeneous cells, a mid-region with cells lying perpendicular to the A-P axis in a collagenous matrix, and small cells at the posterior pole. Unusually for Porifera, multiciliated cells cover all but the posterior pole. It is inferred that fertilization occurs by capture of intact spermatic cysts whose surrounding forceps spicules become trapped in the anisochelae of neighboring sponges. The elongate shape of sperm may be designed to penetrate the loose collagenous mesohyl, such that the arrival of a packet of sperm would lead to simultaneous fertilization of oocytes in a cluster. Loss of the water canal system in carnivorous sponges has allowed the evolution of features that are highly specialized for the habitat of this animal, but such modifications were not necessarily a prerequisite for the subsequent evolution of metazoans. Given the extremely versatile mechanisms of gametogenesis, embryogenesis, and tissue/body structure in sponges, generalizations regarding basal metazoan reproduction, development, and structure must be approached with caution.

Coordinated contractions effectively expel water from the aquiferous system of a freshwater sponge

In response to mechanical stimuli the freshwater sponge Ephydatia muelleri (Demospongiae, Haplosclerida, Spongillidae) carries out a series of peristaltic-like contractions that is effective in expelling clumps of waste material from the aquiferous system. Rates of contraction depend on the region of tissue they are propagating through: 0.3–1 μm s–1 in the peripheral canals, 1–4 μm s–1 in central canals, and 6–122 μm s–1 in the osculum. Faster events include twitches of the entire sponge choanosome and contraction of the sheet-like apical pinacoderm that forms the outer surface of the animal. Contraction events are temporally and spatially coordinated. Constriction of the tip of the osculum leads to dilation of excurrent canals; fields of ostia in the apical pinacoderm close in unison just prior to contraction of the choanosome, apical pinacoderm and osculum. Relaxation returns the osculum, canals and the apical pinacoderm to their normal state, and three such coordinated `inflation–contraction'responses typically follow a single stimulus. Cells in the mesohyl arrest crawling as a wave of contraction passes, suggesting an extracellular signal may pass between cells. Bundles of actin filaments traverse endopinacocytes of the apical pinacoderm. Actin-dense plaques join actin bundles in adjacent pinacocytes to form continuous tracts spanning the whole sponge. The orchestrated and highly repeatable series of contractions illustrates that cellular sponges are capable of coordinated behavioural responses even in the absence of neurons and true muscle. Propagation of the events through the pinacocytes also illustrates the presence of a functional epithelium in cellular sponges. These results suggest that control over a hydrostatic skeleton evolved prior to the origin of nerves and true muscle.

Poriferan paraphyly and its implications for Precambrian palaeobiology

Well-supported molecular phylogenies, combined with knowledge of modern biology, can lead to new inferences about the sequence of character acquisition in early animal evolution, the taxonomic affinity of enigmatic Precambrian and Cambrian fossils, and the Proterozoic Earth system in general. In this paper we demonstrate, in accordance with previous molecular studies, that sponges are paraphyletic, and that calcisponges are more closely related to eumetazoans than they are to demosponges. In addition, our Bayesian analysis finds the Homoscleromorpha, previously grouped with the demosponges, to be even more closely related to eumetazoans than are the calcisponges. Hence there may be at least three separate extant ‘poriferan’ lineages, each with their own unique skeleton. Because spiculation is convergent within ‘Porifera’, differences between skeletonization processes in enigmatic Cambrian taxa such as Chancelloria and modern sponges does not mean that these Problematica are not organized around a poriferan body plan, namely a benthic, sessile microsuspension feeding organism. The shift from the anoxic and sulphidic deep ocean that characterized the mid-Proterozoic to the well-ventilated Phanerozoic ocean occurs before the evolution of macrozooplanton and nekton, and thus cannot have been caused by the advent of faecal pellets. However, the evolution and ecological dominance of sponges during this time interval provides both a mechanism for the long-term generation of isotopically-light CO2 that would be recorded in carbon isotopic excusions such as the ‘Shuram’ event, and an alternative mechanism for the drawdown and sequestration of dissolved organic carbon within the sediment.

Embryogenesis and larval differentiation in sponges

Having descended from the first multicellular animals on earth, sponges are a key group in which to seek innovations that form the basis of the metazoan body plan, but sponges themselves have a body plan that is extremely difficult to reconcile with that of other animals. Adult sponges lack overt anterior–posterior polarity and sensory organs, and whether they possess true tissues is even debated. Nevertheless, sexual reproduction occurs as in other metazoans, with the development of embryos through a structured series of cellular divisions and organized rearrangements of cellular material, using both mesenchymal and epithelial movements to form a multicellular embryo. In most cases, the embryo undergoes morphogenesis into a spatially organized larva that has several cell layers, anterior–posterior polarity, and sensory capabilities. Here we review original data on the mode of cleavage, timing of cellular differentiation, and the mechanisms involved in the organization of differentiated cells to form the highly structured sponge larva. Our ultimate goal is to develop interpretations of the phylogenetic importance of these data within the Porifera and among basal Metazoa.

The unicellular ancestry of animal development

The transition to multicellularity that launched the evolution of animals from protozoa marks one of the most pivotal, and poorly understood, events in life's history. Advances in phylogenetics and comparative genomics, and particularly the study of choanoflagellates, are yielding new insights into the biology of the unicellular progenitors of animals. Signaling and adhesion gene families critical for animal development (including receptor tyrosine kinases and cadherins) evolved in protozoa before the origin of animals. Innovations in transcriptional regulation and expansions of certain gene families may have allowed the integration of cell behavior during the earliest experiments with multicellularity. The protozoan perspective on animal origins promises to provide a valuable window into the distant past and into the cellular bases of animal development.

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Metamorphosis of coeloblastula performed by multipotential larval flagellated cells in the calcareous sponge Leucosolenia laxa

The calcareous sponge Leucosolenia laxa releases free-swimming hollow larvae called coeloblastulae that are the characteristic larvae of the subclass Calcinea. Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of cells: flagellated cells, bottle cells, vesicular cells, and free cells in a central cavity. The flagellated cells, the principal cell type of the larva, are arranged in a pseudostratified layer around a large central cavity. The larval flagellated cells characteristically have glutinous granules that are used as internal markers during metamorphosis. After a free-swimming period the larva settles on the substratum, and settlement apparently triggers the initiation of metamorphosis. The larval flagellated cells soon lose their flagellum and begin the process of dedifferentiation. Then the larva becomes a mass of dedifferentiated cells in which many autophagosomes are found. Within 18 h after settlement, the cells at the surface of the cell mass differentiate to pinacocytes. The cells beneath the pinacoderm differentiate to scleroblasts that form triradiate spicules. Finally, the cells of the inner cell mass differentiate to choanocytes and are arranged in a choanoderm that surrounds a newly formed large gastral cavity. We found glutinous granules in these three principal cell types of juvenile sponges, thus indicating the multipotency of the flagellated cells of the coeloblastula.

Colonial origin for Emetazoa: major morphological transitions and the origin of bilaterian complexity

A new hypothesis for the evolution of Bilateria is presented. It is based on a reinterpretation of the morphological characters shared by protostomes and deuterostomes, which, when taken together with developmental processes shared by the two lineages, lead to the inescapable conclusion that the last common ancestor of Bilateria was complex. It possessed a head, a segmented trunk, and a tail. The segmented trunk was further divided into two sections. A dorsal brain innervated one or more sensory cells, which included photoreceptors. "Appendages" or outgrowths were present. The bilaterian ancestor also possessed serially repeated "segments" that were expressed ontogenetically as blocks of mesoderm or somites with adjoining fields of ectoderm or neuroectoderm. It displayed serially repeated gonads (gonocoels), each with a gonoduct and gonopore to the exterior, and serially repeated "coeloms" with connections to both the gut and the exterior (gill slits and pores). Podocytes, some of which were serially repeated in the trunk, formed sites of ultrafiltration. In addition, the bilaterian ancestor had unsegmented coeloms and a contractile blood vessel or "heart" formed by coelomic myoepithelial cells. These cells and their underlying basement membrane confine the hemocoelic fluid, or blood, in the connective tissue compartment. A possible scenario to account for this particular suite of characters is one in which a colony of organisms with a cnidarian grade of organization became individuated into a new entity with a bilaterian grade of organization. The transformation postulated encompassed three major transitions in the evolution of animals. These transitions included the origins of Metazoa, Eumetazoa, and Bilateria and involved the successive development of poriferan, cnidarian, and bilaterian grades of organization. Two models are presented for the sponge-to-cnidarian transition. In both models the loss of a flow-through pattern of water circulation in poriferans and the establishment of a single opening and epithelia sensu stricto in cnidarians are considered crucial events. In the model offered for the cnidarian-to-bilaterian transition, the last common ancestor of Eumetazoa is considered to have had a colonial, cnidarian-grade of organization. The ancestral cnidarian body plan would have been similar to that exhibited by pennatulacean anthozoans. It is postulated that a colonial organization could have provided a preadaptive framework for the evolution of the complex and modularized body plan of the triploblastic ancestor of Bilateria. Thus, one can explore the possibility that problematica such as ctenophores, the Ediacaran biota, archaeocyaths, and Yunnanozoon reflect the fact that complexity originated early and involved the evolution of a macroscopic compartmented ancestor. Bilaterian complexity can be understood in terms of Beklemishev "cycles" of duplication and colony individuation. Two such cycles appear to have transpired in the early evolution of Metazoa. The first gave rise to a multicellular organism with a sponge grade of organization and the second to the modularized ancestor of Bilateria. The latter episode may have been favored by the ecological conditions in the late Proterozoic. Whatever its cause, the individuation of a cnidarian-grade colony furnishes a possible explanation for the rapid diversification of bilaterians in the late Vendian and Cambrian. The creation of a complex yet versatile prototype, which could be rapidly modified by selection into a profusion of body plans, is postulated to have affected the timing, mode, and extent of the "Cambrian explosion." During the radiations, selective loss or simplification may have been as creative a force as innovation. Finally, colony individuation may have been a unique historical event that imprinted the development of bilaterians as the zootype and phylotypic stage. (ABSTRACT TRUNCATED)