Symmetry and the tentacular apparatus in Cnidaria

Comparative analysis provides evidence that bilateral symmetry is a primary character of Cnidaria. All anthozoan taxa are characterized by bilateral symmetry. The anthozoan pharyngeal plane is a plane of bilateral symmetry of mesenteries and, at the same time, it is a plane of bilateral symmetry of regulatory gene expression in anthozoan morphogenesis. In Medusozoa, the bilateral symmetry is replaced by radial symmetry, but some hydrozoans (for example, Corymorphidae) demonstrate bilateral symmetry. The bilateral symmetry of Cnidaria is thought to be inherited from the common ancestors of both cnidarians and triploblastic bilaterians. The secondary radial symmetry of Cnidaria evidently is a result of the adaptation to the sessile mode of life. The presence of both the marginal and labial rings of tentacles is supposed to be a plesiomorphic character of Cnidaria. In some groups of cnidarians, one of the tentacle rings may be reduced.     

Did internal transport,rather than directed locomotion,favor the evolution of bilateral symmetry in animals?

The standard explanation for the origin of bilateral symmetry is that it conferred an advantage over radial symmetry for directed locomotion. However, recent developmental and phylogenetic studies suggest that bilateral symmetry may have evolved in a sessile benthic animal, predating the origin of directed locomotion. An evolutionarily feasible alternative explanation is that bilateral symmetry evolved to improve the efficiency of internal circulation by affecting the compartmentalization of the gut and the location of major ciliary tracts. This functional design principle is illustrated best by the phylum Cnidaria where symmetry varies from radial to tetraradial, biradial and bilateral. In the Cnidaria, bilateral symmetry is manifest most strongly in the internal anatomy and the disposition of ciliary tracts. Furthermore, the bilaterally symmetrical Cnidaria are typically sessile and, in those bilaterally symmetrical cnidarians that undergo directed locomotion, the secondary body axis does not bear a consistent orientation to the direction of locomotion as it typically does in Bilateria. Within the Cnidaria, the hypothesized advantage of bilateral symmetry for internal circulation can be tested by experimental analysis and computer modeling of fluid mechanics. The developmental evolution of symmetry within the Cnidaria can be further explored through comparative gene expression studies among species whose symmetry varies.     

Early evolution of symmetry and polarity in metazoan body plans

The early diverging metazoan lineages have highly disparate adult body plan geometries, which can be characterised in terms of five major types of symmetry (asymmetrical, spherical, cylindrical, n-radial, bilateral). Patterns of evolutionary changes in symmetry types and the homology of body axes across lineages are discussed here by confronting evidence from comparative anatomy, phylogeny, genomics and evo-devo. The conventional scenario, postulating a graded complexification from asymmetry to radial and finally bilateral symmetry, is considered untenable. Cylindrical symmetry is likely to be the ancestral type from which derived all remaining types through multiple convergences. Recent proposals prompted by molecular data that the bilateral anatomies of many cnidarians and of the Bilateria are homologous are clearly not supported. The Hox-based patterning system operating along the antero-posterior axis of the Bilateria does not seem to predate their divergence with the Cnidaria, but intercellular signalling systems, notably the Wnt pathway, could have been involved in generating the main body axis in the last common ancestor of the Metazoa. To cite this article: M. Manuel, C. R. Biologies 332 (2009).     

Floral symmetry: pollinator-mediated stabilizing selection on flower size in bilateral species

Pollinator-mediated stabilizing selection (PMSS) has been proposed as the driver of the evolutionary shift from radial to bilateral symmetry of flowers. Studies have shown that variation in flower size is lower in bilateral than in radial species, but whether bilateral flowers experience more stabilizing selection pressures by employing fewer, more specialized pollinators than radial flowers remains unclear. To test the PMSS hypothesis, we investigate plant–pollinator interactions from a whole community in an alpine meadow in Hengduan Mountains, China, to examine: (i) variance in flower size and level of ecological generalization (pollinator diversity calculated using functional groups) in 14 bilateral and 13 radial species and (ii) the role pollinator diversity played in explaining the difference of variance in flower size between bilateral and radial species. Our data showed that bilateral species had less variance in flower size and were visited by fewer pollinator groups. Pollinator diversity accounted for up to 40 per cent of the difference in variance in flower size between bilateral and radial species. The mediator effect of pollinator diversity on the relationship between floral symmetry and variance in flower size in the community is consistent with the PMSS hypothesis.     

The origins of axial patterning in the metazoa: how old is bilateral symmetry?

Bilateral symmetry is a hallmark of the Bilateria. It is achieved by the intersection of two orthogonal axes of polarity: the anterior-posterior (A-P) axis and the dorsal-ventral (D-V) axis. It is widely thought that bilateral symmetry evolved in the common ancestor of the Bilateria. However, it has long been known that members of the phylum Cnidaria, an outgroup to the Bilateria, also exhibit bilateral symmetry. Recent studies have examined the developmental expression of axial patterning genes in members of the phylum Cnidaria. Hox genes play a conserved role in patterning the A-P axis of bilaterians. Hox genes are expressed in staggered axial domains along the oral-aboral axis of cnidarians, suggesting that Hox patterning of the primary body axis was already present in the cnidarian-bilaterian ancestor. Dpp plays a conserved role patterning the D-V axis of bilaterians. Asymmetric expression of dpp about the directive axis of cnidarians implies that this patterning system is similarly ancient. Taken together, these result imply that bilateral symmetry had already evolved before the Cnidaria diverged from the Bilateria.     

Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances

A striking aspect of flowering plant (angiosperm) diversity is variation in flower symmetry. From an ancestral form of radial symmetry (polysymmetry, actinomorphy), multiple evolutionary transitions have contributed to instances of non-radial forms, including bilateral symmetry (monosymmetry, zygomorphy) and asymmetry. Advances in flowering plant molecular phylogenetic research and studies of character evolution as well as detailed flower developmental genetic studies in a few model species (e.g. Antirrhinum majus, snapdragon) have provided a foundation for deep insights into flower symmetry evolution. From phylogenetic studies, we have a better understanding of where during flowering plant diversification transitions from radial to bilateral flower symmetry (and back to radial symmetry) have occurred. From developmental studies, we know that a genetic programme largely dependent on the functional action of the CYCLOIDEA gene is necessary for differentiation along the snapdragon dorsoventral flower axis. Bringing these two lines of inquiry together has provided surprising insights into both the parallel recruitment of a CYC-dependent developmental programme during independent transitions to bilateral flower symmetry, and the modifications to this programme in transitions back to radial flower symmetry, during flowering plant evolution.     

Ancient origins of axial patterning genes: Hox genes and ParaHox genes in the Cnidaria

Among the bilaterally symmetrical, triploblastic animals (the Bilateria), a conserved set of developmental regulatory genes are known to function in patterning the anterior–posterior (AP) axis. This set includes the well-studied Hox cluster genes, and the recently described genes of the ParaHox cluster, which is believed to be the evolutionary sister of the Hox cluster ( Brooke et al. 1998 ). The conserved role of these axial patterning genes in animals as diverse as frogs and flies is believed to reflect an underlying homology (i.e., all bilaterians derive from a common ancestor which possessed an AP axis and the developmental mechanisms responsible for patterning the axis). However, the origin and early evolution of Hox genes and ParaHox genes remain obscure. Repeated attempts have been made to reconstruct the early evolution of Hox genes by analyzing data from the triphoblastic animals, the Bilateria ( Schubert et al. 1993 ; Zhang and Nei 1996 ). A more precise dating of Hox origins has been elusive due to a lack of sufficient information from outgroup taxa such as the phylum Cnidaria (corals, hydras, jellyfishes, and sea anemones). In combination with outgroup taxa, another potential source of information about Hox origins is outgroup genes (e.g., the genes of the ParaHox cluster). In this article, we present cDNA sequences of two Hox-like genes ( anthox2 and anthox6 ) from the sea anemone, Nematostella vectensis. Phylogenetic analysis indicates that anthox2 (=Cnox2) is homologous to the GSX class of ParaHox genes, and anthox6 is homologous to the anterior class of Hox genes. Therefore, the origin of Hox genes and ParaHox genes occurred prior to the evolutionary split between the Cnidaria and the Bilateria and predated the evolution of the anterior–posterior axis of bilaterian animals. Our analysis also suggests that the central Hox class was invented in the bilaterian lineage, subsequent to their split from the Cnidaria.     

Symmetry of priapulids (Priapulida). 1. Symmetry of adults

Priapulids possess a radial symmetry that is remarkably reflected in both external morphology and internal anatomy. It results in the appearance of 25-radial (a number divisible by five) symmetry summarized as a combination of nonaradial, octaradial, and octaradial (9+8+8) symmetries of scalids. The radial symmetry is a secondary appearance considered as an evolutionary adaptation to a lifestyle within the three-dimensional environment of bottom sediment. The eight anteriormost, or primary, scalids retain their particular position because of their innervation directly from the circumpharyngeal brain. As a result of a combination of the octaradial symmetry of primary scalids, pentaradial symmetry of teeth, and the 25-radial symmetry of scalids, the initial bilateral symmetry remains characterized by the single sagittal plane.     

Evolutionary Trends in the Flowers of Asteridae: Is Polyandry an Alternative to Zygomorphy?

Background and Aims

Floral symmetry presents two main states in angiosperms, actinomorphy (polysymmetry or radial symmetry) and zygomorphy (monosymmetry or bilateral symmetry). Transitions from actinomorphy to zygomorphy have occurred repeatedly among flowering plants, possibly in coadaptation with specialized pollinators. In this paper, the rules controlling the evolution of floral symmetry were investigated to determine in which architectural context zygomorphy can evolve.

Methods

Floral traits potentially associated with perianth symmetry shifts in Asteridae, one of the major clades of the core eudicots, were selected: namely the perianth merism, the presence and number of spurs, and the androecium organ number. The evolution of these characters was optimized on a composite tree. Correlations between symmetry and the other morphological traits were then examined using a phylogenetic comparative method.

Key Results

The analyses reveal that the evolution of floral symmetry in Asteridae is conditioned by both androecium organ number and perianth merism and that zygomorphy is a prerequisite to the emergence of spurs.

Conclusions

The statistically significant correlation between perianth zygomorphy and oligandry suggests that the evolution of floral symmetry could be canalized by developmental or spatial constraint. Interestingly, the evolution of polyandry in an actinomorphic context appears as an alternative evolutionary pathway to zygomorphy in Asteridae. These results may be interpreted either in terms of plant–pollinator adaptation or in terms of developmental or physical constraints. The results are discussed in relation to current knowledge about the molecular bases underlying floral symmetry.Key words: Floral symmetry, architectural constraints, Asteridae, comparative analysis, composite tree, correlated evolution, evolutionary scenario     

Generation of bilateral symmetry in Anthozoa: a model

Polyps of Anthozoa usually display bilateral symmetry with respect to their mouth opening, to their pharynx, and in particular to the arrangement of their mesenteries. Mesenteries, which are endodermal folds running from the apical to the basal end of the body, subdivide the gastric cavity into pouches. They form in a bilateral symmetric sequence. In this article I propose that early in polyp development the endoderm subdivides successively into three different types of compartments. A mesentery forms at the border between compartments. Two of the compartments are homologous to those of Scyphozoa. They form by mutual activation of cell states that locally exclude each other. The third compartment leads to siphonoglyph formation and is an evolutionary innovation of the Anthozoa. The mechanism that controls the number and spatial arrangement of the third type of compartment changes the radial symmetry into a bilateral one and occasionally into a different one. The dynamics of its formation indicate an activator-inhibitor mechanism. Computer models are provided that reproduce decision steps in the generation of the mesenteries.     

Diverse radial symmetry among the Cambrian Fortunian fossil embryos from northern Sichuan and southern Shaanxi provinces,South China

The Cambrian Fortunian fossil embryos exhibit embryonic development of ancient animals and hence have important bearings on evolutionary developmental biology. They have radial symmetry, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Additionally, we recovered two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo II with octaradial symmetry, and they are tentatively suggested to represent new cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit stable tetraradial symmetry in medusozoans, biradial symmetry in anthozoans, and bilateral symmetry in siphonophores (Hydrozoa). The current study supports the view that the tetraradial symmetry of modern medusozoans is a surviving remnant of their Fortunian relatives.     

The origin of tetraradial symmetry in cnidarians

Serially arranged sets of eight septa‐like structures occur in the basal part of phosphatic tubes of Sphenothallus from the early Ordovician (early Floian) Fenxiang Formation in Hubei Province of China. They are similar in shape, location and number, to cusps in chitinous tubes of extant coronate scyphozoan polyps, which supports the widely accepted cnidarian affinity of this problematic fossil. However, unlike the recent Medusozoa, the tubes of Sphenothallus are flattened at later stages of development, showing biradial symmetry. Moreover, the septa (cusps) in Sphenothallus are obliquely arranged, which introduces a bilateral component to the tube symmetry. This makes Sphenothallus similar to the Early Cambrian Paiutitubulites, having similar septa but with even more apparent bilateral disposition. Biradial symmetry also characterizes the Early Cambrian tubular fossil Hexaconularia, showing a similarity to the conulariids. However, instead of being strictly tetraradial like conulariids, Hexaconularia shows hexaradial symmetry superimposed on the biradial one. A conulariid with a smooth test showing signs of the ‘origami’ plicated closure of the aperture found in the Fenxiang Formation supports the idea that tetraradial symmetry of conulariids resulted from geometrical constrains connected with this kind of closure. Its minute basal attachment surface makes it likely that the holdfasts characterizing Sphenothallus and advanced conulariids are secondary features. This concurs with the lack of any such holdfast in the earliest Cambrian Torellella, as well as in the possibly related Olivooides and Quadrapyrgites. Bilaterally arranged internal structures in polyps representing probably the oldest medusozoans support the suggestions based on developmental evidence that the ancestor of cnidarians also was a bilaterally symmetrical animal. This is one more example of fossil data that strictly fit the molecular phylogenetic evidence but not necessarily morphology‐based zoological interpretations.     

Beyond bilateral symmetry: geometric morphometric methods for any type of symmetry

Background

Studies of symmetric structures have made important contributions to evolutionary biology, for example, by using fluctuating asymmetry as a measure of developmental instability or for investigating the mechanisms of morphological integration. Most analyses of symmetry and asymmetry have focused on organisms or parts with bilateral symmetry. This is not the only type of symmetry in biological shapes, however, because a multitude of other types of symmetry exists in plants and animals. For instance, some organisms have two axes of reflection symmetry (biradial symmetry; e.g. many algae, corals and flowers) or rotational symmetry (e.g. sea urchins and many flowers). So far, there is no general method for the shape analysis of these types of symmetry.

Results

We generalize the morphometric methods currently used for the shape analysis of bilaterally symmetric objects so that they can be used for analyzing any type of symmetry. Our framework uses a mathematical definition of symmetry based on the theory of symmetry groups. This approach can be used to divide shape variation into a component of symmetric variation among individuals and one or more components of asymmetry. We illustrate this approach with data from a colonial coral that has ambiguous symmetry and thus can be analyzed in multiple ways. Our results demonstrate that asymmetric variation predominates in this dataset and that its amount depends on the type of symmetry considered in the analysis.

Conclusions

The framework for analyzing symmetry and asymmetry is suitable for studying structures with any type of symmetry in two or three dimensions. Studies of complex symmetries are promising for many contexts in evolutionary biology, such as fluctuating asymmetry, because these structures can potentially provide more information than structures with bilateral symmetry.     

Phylogenetic analysis of the Cnidaria

The recent members of the phylum Cnidaria were analyzed with phylogenetic methodology and the help of the PAUP Computer program. The Cnidaria are established as a monophylum by their cnidocysts, planula larva, and a polyp stage. The Ctenophora were seen as the most probable sister group of the Cnidaria. Arguments for the monophyly of the cnidarian classes Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa were providea. For the ground plan of the Cnidaria the following characters were postulated: triphasic life cycle consisting of a planula larva, a benthic polyp stage, and a sexually propagating medusa like stage. For the polyp a radial symmetry, lack of septae, and hollow tentacles were assumed. The original medusa probably was tetraradial and developed from the polyp stage by a total metamorphosis. Twelve polarized characters were used to generate cladograms. The most parsimonious one showed the Anthozoa as the first offshoot of the tree with the united Scyphozoa, Cubozoa and Hydrozoa forming its sister group. Within this sister group the Scyphozoa and Cubozoa were seen as sistergroups to each other. Both groups united are then the sistergroup of the Hydrozoa. A bootstrap analysis yielded the same tree with high probabilities for the internal nodes. Despite assuming a planktonic origin of the Cnidaria in this investigation, the resulting cladogram is also compatible with an evolution of the medusa stage within the Cnidaria after the splitting-off of the Anthozoa. The possible loss of the medusa stage in the Anthozoa is discussed.     

Influence of auxin on the establishment of bilateral symmetry in monocots

To study the influence of auxin on the shift from radial to bilateral symmetry during monocot embryogenesis, the fate of young wheat (Triticum aestivum L.) zygotic embryos has been manipulated in vitro by adding auxins, an auxin transport inhibitor and an auxin antagonist to the culture medium. The two synthetic auxins used, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), induced identical phenotypes. In the most severe cases, the shift from radial to bilateral symmetry was blocked resulting in continuous uniform radial growth. The natural auxin indole-3-acetic acid (IAA) induced the same phenotype. The effect of 2,4,5-T and 2,4D depended on their concentrations and on the developmental stage of the isolated embryos. In the presence of 2,3,5-triiodobenzoic acid (TIBA), an auxin transport inhibitor, the overall embryo symmetry was abnormal. The relative position of the shoot apical meristem in comparison with the scutellum was anomalous. The quality of shoot apical meristem and the scutellum differentiation was altered compared with normal developed embryos. No root meristem was differentiated. The effect of TIBA depends on its concentration and on the developmental stage of the isolated embryos. By contrast, 2-(pchlorophenoxy)-2-methylpropionic acid (PCIB) which is described as an auxin antagonist, has no visible direct effect on the embryonic symmetry. These observations indicate that auxin influences the change from radial symmetry to embryonic polarity during monocot embryogenesis. A model of auxin action during early wheat embryo development is proposed.     

A Diploblastic Radiate Animal at the Dawn of Cambrian Diversification with a Simple Body Plan: Distinct from Cnidaria?

Background

Microfossils of the genus Punctatus include developmental stages such as blastula, gastrula, and hatchlings, and represent the most complete developmental sequence of animals available from the earliest Cambrian. Despite the extremely well-preserved specimens, the evolutionary position of Punctatus has relied only on their conical remains and they have been tentatively assigned to cnidarians. We present a new interpretation of the Punctatus body plan based on the developmental reconstruction aided by recent advances in developmental biology.

Results

Punctatus developed from a rather large egg, gastrulated in a mode of invagination from a coeloblastura, and then formed a mouth directly from the blastopore. Spiny benthic hatchlings were distinguishable from swimming or crawling ciliate larvae found in cnidarians and sponges. A mouth appeared at the perihatching embryonic stage and was renewed periodically during growth, and old mouths transformed into the body wall, thus elongating the body. Growing animals retained a small blind gut in a large body cavity without partitioning by septa and did not form tentacles, pedal discs or holdfasts externally. A growth center at the oral pole was sufficient for body patterning throughout life, and the body patterning did not show any bias from radial symmetry.

Conclusions

Contrary to proposed cnidarian affinity, the Punctatus body plan has basic differences from that of cnidarians, especially concerning a spacious body cavity separating ectoderm from endoderm. The lack of many basic cnidarian characters in the body patterning of Punctatus leads us to consider its own taxonomic group, potentially outside of Cnidaria.     

Preferences for Symmetry in Conspecific Facial Shape Among Macaca mulatta

In human males and females, bilateral symmetry of facial shape influences assessments of attractiveness. It is possible, however, that other primate species also possess preferences for conspecific facial symmetry. To assess this experimentally, we presented 13 adult rhesus macaques (8 females, 5 males) with computer-manipulated images of symmetrical and asymmetrical versions of opposite-sexed conspecific faces. We utilized looking behavior to assess visual preferences for these factors. We found significant preferences for symmetry, raising the possibility that human preferences for facial symmetry are more deeply rooted in our evolutionary history than previously realized. Our results also have implications for the use of facial shape as a mechanism for attractiveness appraisals across the Primates.     

Developmental regulatory genes and echinoderm evolution

Modified interactions among developmental regulatory genes and changes in their expression domains are likely to be an important part of the developmental basis for evolutionary changes in morphology. Although developmental regulatory genes are now being studied in an increasing number of taxa, there has been little attempt to analyze the resulting data within an explicit phylogenetic context. Here we present comparative analyses of expression data from regulatory genes in the phylum Echinodermata, considering the implications for understanding both echinoderm evolution as well as the evolution of regulatory genes in general. Reconstructing the independent evolutionary histories of regulatory genes, their expression domains, their developmental roles, and the structures in which they are expressed reveals a number of distinct evolutionary patterns. A few of these patterns correspond to interpretations common in the literature, whereas others have received little prior mention. Together, the analyses indicate that the evolution of echinoderms involved: (1) the appearance of many apomorphic developmental roles and expression domains, some of which have plesiomorphic bilateral symmetry and others of which have apomorphic radial symmetry or left-right asymmetry; (2) the loss of some developmental roles and expression domains thought to be plesiomorphic for Bilateria; and (3) the retention of some developmental roles thought to be plesiomorphic for Bilateria, although with modification in expression domains. Some of the modifications within the Echinodermata concern adult structures; others, transient larval structures. Some changes apparently appeared early in echinoderm evolution (> 450 Ma), whereas others probably happened more recently (< 50 Ma). Cases of likely convergence in expression domains suggest caution when using developmental regulatory genes to make inferences about homology among morphological structures of distantly related taxa.     

Cytological aspects of similarity and difference of Myxozoa and Cnidaria

A comparative cytomorphological analysis of Myxozoa and parasitic Cnidaria Polypodium hydriforme has been carried out in view of the Weill (1938) hypothesis, which regards Myxozoa as a reduced Cnidaria. The question on the relation of Myxozoa and Cnidaria was arising several times with the application of some new methods during the Myxozoa studies. At present the idea on their phylogenetic relationships has appeared again in connection with an absolutely new understanding of the myxozoan life cycle (Wolf, Markiw, 1984), as well as with the application of molecular-biological methods for their phylogenetic studies. The latter, however, provided some diverse results. So far no comparative cytomorphological analysis of Myxozoa and Polypodium has been carried out. The present paper is to fill the gap on the basis of accumulated facts. According to Weill (1938), the features of similarity of parasitic Cnidaria and Myxozoa are the following: 1) the presence in both of extrusomes (nematocysts and polar capsules) whose structure and development are surprizingly similar; 2) the nuclear dimorphism and somato-generative segregation; 3) the presence of a somatic nutritional cell, surrounding the multiplying generative cells; at present it is known that polyploidy of somatic nuclei and the absence of parasitophorous vacuole are characteristic of trophamnion of Polypodium and trophozoite of Myxozoa; 4) the presence of radial symmetry in both groups; 5) the construction of a diblastic organism made of a cluster of endodermal cells and a few ectodermal cells; 6) the similarity of their cell contacts (Grassé, 1970). At present it is possible to add to Weill's (1938) list of features common for parasitic Cnidaria and Myxozoa the number of important similarities between Polypodium and Myxozoa, some of which being not characteristic of Cnidaria: 1) the "cell in cell" organization of all Polypodium parasitic stages and all Myxozoa life cycle stages; 2) the presence of gametophore supplied with extrusomes; 3) both organisms have haplophase in their life cycles preceded by two-step meiosis; 4) there are mitochondria with tubular cristae in both organisms; 5) the absence of spermatozoa and eggs in both organisms; 6) the similarity of Polypodium cnidocile structure and the cone-like formation situated at the anterior end of polar capsule of actinospore (Lom. Dykova, 1997); 7) the participation of MTOC in the formation of extrusomes in both animals. In spite of the obvious similarity between Myxozoa and parasitic Cnidaria (including Polypodium) it is, however, necessary to take into account differences between them, the main being as follows: the absence in Myxozoa of flagellated stages, centrioles, tissues and organs, true blastophylla, planula-like larvae, gastrulation; the presence of low cell integrations in Myxozoa; Cnidaria and Myxozoa have different types of mitosis, their life cycles and the discharge mechanism of their stinging apparatus being also different. We consider as quite valid a suggestion by Siddall et al. (1995) that parasitic Cnidaria could present an early separated branch of the cnidarian evolution. Further studies of Myxozoa life cycle may show their more definite relation to parasitic Cnidaria. The problem has not yet been solved completely since the available molecular-biological data are rather contradictory and moreover there is no distinct idea as to the Eumetazoa ancestor so far. A further thorough investigation is badly needed in the feelds of developmental cycle, cytomorphology and molecular biology of the variety of narcomedusae and representatives of Myxozoa. This may help to find some transitional forms and stages of the animals and to understand whether we deal with a regressive evolution of parasitic Cnidaria or with a parallel evolution of taxa originated from the common ancestor.     

Culture studies on some Florida species of Caulerpa: Morphological responses to reduced illumination

The morphological development of three species of Caulerpa, C. sertularioides, C. paspaloides and C. racemosa has been studied in low light culture. The resulting morphologies are described and contrasted with the typical morphologies. Culture forms in all cases were unlike the field forms but each had affinities with other taxa described in the literature. The most important generalized response was a change in the symmetry of the assimilators from radial to bilateral. This response supports an earlier morphologically based theory on the evolutionary relationships among the species of Caulerpa.