Hydra, for example, has four types of nematocytes: stenoteles, desmonemes, atrichous isorhizas and holotrichous isorhizas. All cnidarians have two membrane layers in the body: the epidermis and the gastrodermis; between both layers they have the mesoglea, which is a connective layer. How does it move planula, it swims 5). Images courtesy of Gnter Plickert (A,B), Thomas C. Bosch (C,D), Catherine Dana and R.E.S. Comparisons between these morphologically diverse species might provide insights into the constraints of the underlying developmental programs. II. Additional cnidarian species used for developmental studies are included in Table 1. Gastrulation and germ layer formation in the sea anemone Numerous variations on these classical transplantations have been carried out (MacWilliams, 1983a, MacWilliams, 1983b; Broun and Bode, 2002; Gee et al., 2010), including recombination of tissue layers (Schmid and Tardent, 1984; Takano and Sugiyama, 1984), removal and transplantation of interstitial cells (Campbell, 1976; Heimfeld and Bode, 1984) and the dissociation and reaggregation of Hydra cells (Gierer et al., 1972; Technau et al., 2000). Analysis of chimaeric hydra produced from a normal and a slow-budding strain (L4), Morphology and morphodynamics of the stenotele nematocyst of Hydra attenuata Pall. Components of the BMP pathway are expressed during embryogenesis in the anthozoans Nematostella and Acropora (Technau et al., 2005). The cookie is used to store the user consent for the cookies in the category "Analytics". Scale bar: 100 m. Gain-of-function studies by injection of synthetic mRNAs and loss-of-function studies by injection of mRNAs encoding dominant-negative versions of proteins have been performed successfully in embryos from Nematostella and Clytia (Lee et al., 2007; Momose and Houliston, 2007; Wikramanayake et al., 2003). Nematostella has a typical anthozoan life cycle (Fig. As the stolon grows, new polyps arise from it in a spaced manner. (D,E) During the planula stage, expression of chordin remains lateral to the blastopore (D), whereas dpp is largely expressed in an endodermal stripe and in a spot at the border of the blastopore (E), on the side of chordin expression. Whereas many hydrozoan medusa have simple eyes at the rim of the bell, the most sophisticated eyes are found in the cubozoan jellyfish. Cnidarian morphology: Cnidarians have two distinct body plans the medusa (a) and the polyp (b). diploblastic All jellyfish have mesoglea which is what? How many germ layers do cnidarians have and name them 2 What are the two body forms of cnidarians and make a sketch of each below? For many years, studies of cnidarian development, particularly of pattern formation, stem cells and regeneration, were dominated by research using the adult Hydra polyp, allowing only indirect comparisons with bilaterian embryonic development. (A) A Hydra polyp is essentially a two-layered tube, with a ring of tentacles around the mouth opening at the tip of the hypostome. 1. Cnidarians are divided into two major groups (Fig. Wnt signaling also appears to play an important role in axial patterning in the embryo and polyp of Hydractinia echinata (Duffy et al., 2010; Plickert et al., 2006). Sensory neurons are located between epithelial cells and connect to ganglion neurons (purple), which are at the base of the epithelium on top of the myofibers and sometimes cross the mesogloea. In fact, Hydra can be cut into ~20 fragments, each of which will regenerate a complete polyp (Bode and Bode, 1980). Read More, In case of sale of your personal information, you may opt out by using the link Do Not Sell My Personal Information. In ectodermal epithelial cells (green), the fibers are oriented longitudinally, and in endodermal epithelial cells (pink) they are oriented circumferentially (ring muscle). Find out more and apply to Developments 2023 Journal Meeting here. We also discuss the challenges that researchers need to overcome to expand developmental studies using cnidarians. The two epithelial cell lineages (ectodermal and endodermal) are maintained by division of cells in the body column. I. Image in F reproduced with permission (Renfer et al., 2010). two 6) (Finnerty et al., 2004; Hayward et al., 2002; Matus et al., 2006a; Matus et al., 2006b; Rentzsch et al., 2006). 3I-K). 5). Unlike Hydra, it has a typical hydrozoan life cycle that includes planula larva and medusa stages (Fig. These findings demonstrate the existence of a molecularly defined second body axis, perpendicular to the oral-aboral body axis, called the directive axis. Research in the R.E.S. 1 / 13 Flashcards Learn Test Match Created by SARAH_RIHA Terms in this set (13) What level of organization do Cnidarians exhibit? Images were taken by Jens Fritzenwanker and U.T. (C) A somatic patch of transgenic ectodermal epithelial cells expressing eGFP under the control of an actin promoter, demonstrating normal axial tissue displacement with growth. Endoderm -> Inside skin. Even more extraordinarily, Hydra can be dissociated into a suspension of cells which, when reaggregated, regenerates polyps de novo, consistent with a reaction-diffusion-based mechanism (see Glossary, Box 1) of pattern formation (Gierer et al., 1972; Technau et al., 2000; Technau and Holstein, 1992). The endoderm is the inner layer ('endo' for 'within'), and the ectoderm is the outer layer ('ecto' for 'outer'). Germ layer, any of three primary cell layers, formed in the earliest stages of embryonic development, consisting of the endoderm (inner layer), the ectoderm (outer layer), and the mesoderm (middle layer). In this respect, the hydrozoan Clytia, which has a generation time of 3-4 weeks, perhaps holds more promise. In Hydra, which is the best-studied case, new epithelial cells arise by the division of existing epithelial cells, whereas neurons, nematocytes, gland cells and gametes arise from multipotent stem cells called interstitial cells (David and Murphy, 1977) (reviewed by Bode, 1996; Bosch, 2009). A coelenterate with specialized stinging features on its tentacles that surround its mouth is known as cnidaria. How Many Germ Layers Do Cnidaria Have? Thus, in cnidarians, Wnt signaling appears to play a decisive role in establishing the animal/oral pole that subsequently develops into the hypostomal organizer of the polyp. Note the differences in size between different cnidarians. Surprisingly, in Nematostella bmp2/4 and chordin do not form opposing gradients of expression as they do in vertebrates or flies but instead are expressed on the same side after initially being expressed in a radial pattern around the blastopore (Rentzsch et al., 2006). Early-branching animal lineages (excluding the Bilateria), i.e. Scale bars: 2 mm in A; 500 m in C-E; 200 m in F; 250 m in G; 100 m in H. In summary, BMPs and chordin (or other BMP-binding molecules) are components of an ancient molecular system used to generate axial asymmetries. Regeneration in Hydra is a morphallactic process, i.e. of the formation of inner germ layers, endoderm and mesoderm. How Many Germ Layers Do Cnidarians Have And Name Them - Realonomics Use of small molecules to study cnidarian development. The registration deadline is Friday 21 July. We'll assume you're ok with this, but you can opt-out if you wish. Explore More at BYJU'S NEET How Many Germ Layers Do Cnidaria Have? Cnidarians also have another thing in common - they have two germ layers. Ultimately, transient expression of GFP under the control of a Hydra actin promoter was achieved by bombardment of adult Hydra polyps with gold particles coated with plasmid DNA (Bottger et al., 2002). Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. (F) Transgenic F1 primary polyp expressing mCherry (red) under the control of a muscle-specific promoter (MyHC::mCherry). About This Quiz & Worksheet. Learn all about the three germ layers and how they shape the d. J. Comp. We thank David Miller, Zoe Richards and Eldon Ball for photographs of Acropora; Thomas Holstein for a picture of a Hydra polyp; Hanna Kraus for photographs of Clytia hemisphaerica; Hanna Kraus for the drawings of the Nematostella, Clytia and Acropora life cycles and the Hydra polyp in Fig. How many germ layers do cnidarians have and name them 2 What are the two body forms of cnidarians and make a sketch of each? What is the germ layer of Cnidaria? Cnidarians are generally regarded as diploblastic animals, possessing endoderm and ectoderm, but lacking mesoderm. However, these are unlikely to be separate species; H. magnipapillata should probably be renamed H. vulgaris (Martinez et al., 2010). Cnidarians: an evolutionarily conserved model system for regeneration? Porifera (sponges), Ctenophora (comb jellies), Cnidaria (jellyfish) and Placozoa (Trichoplax). The phylum-defining trait of Cnidaria is the stinging cell, the nematocyte (see Glossary, Box 1), an extrusive organelle used for predation, adhesion and defence (Holstein, 1981; Lengfeld et al., 2009; Tardent and Holstein, 1982). Body has two cell layers (ectoderm and endoderm) separated by a layer of jelly-like " mesoglea ". What germ layers do cnidarians have and name them? The outer layer, the ectoderm, has cells that aid in capturing food and cells that secrete mucus. How many germ layers do cnidarians have? Cnidarians have an ectoderm and endoderm, both of which are germ layers. Explore the fascinating world of cnidarians and their unique biology in this must-watch video! For example, how development of the cnidarian nervous system is controlled and how its development relates to nervous system development in bilaterians are of obvious interest. Cnidarians nominally develop from two germ layers, the ectoderm and endoderm, whereas the presence of a third germ layer, the mesoderm, traditionally characterizes higher animals. While it is conceivable that cnidarians as the sister group to Bilateria could provide insights into the ancestral gastrulation mechanisms, it turns out that among the diversity of different cnidarian species, virtually all known mechanisms of gastrulation can be found (Kraus and Markov, 2017; Tardent, 1978) (Fig. 2. Considerable effort is now being made to explain regeneration in Hydra at the molecular level, by examining the involvement of conserved signaling pathways and transcription factors, such as the Wnt pathway and Brachyury (Bielen et al., 2007; Broun and Bode, 2002; Broun et al., 2005; Chera et al., 2009; Galliot, 2000; Galliot et al., 2008; Gee et al., 2010; Hobmayer et al., 2000; Holstein et al., 1991; Holstein et al., 2003; Lengfeld et al., 2009; Technau and Bode, 1999). What are the 3 germ layers in embryonic development? 1. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. I. Head-body proportion regulation, GFP expression in Hydra: lessons from the particle gun, Genetic screen for signal peptides in Hydra reveals novel secreted proteins and evidence for non-classical protein secretion, Characterization of the head organizer in hydra, Formation of the head organizer in hydra involves the canonical Wnt pathway, The production of new hydrants by the insertion of small grafts, Conserved and novel gene expression between regeneration and asexual fission in Nematostella vectensis, Elimination by Hydra interstitial and nerve cells by means of colchicine, Cell cycle kinetics and development of Hydra attenuata. sea anemones) and for studying the formation of the calcified skeleton that defines the coral reef. Although the recent development of methods for genetically manipulating cnidarians has facilitated studies of gene function, many of the tools available for more mature model organisms are still lacking for cnidarians. Registered Charity 277992 | Registered in England and Wales | Company Limited by Guarantee No 514735. Is phylum Cnidaria is diploblastic or triploblastic? The phylum Cnidaria (sea anemones, corals, hydras and jellyfish) is the likely sister group of the triploblastic Bilateria. 1)Radially Symmetrical. This site uses different types of cookies. EST analysis of the cnidarian Acropora millepora reveals extensive gene loss and rapid sequence divergence in the model invertebrates, Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions, Assembly of the cnidarian camera-type eye from vertebrate-like components, The blastoporal organiser of a sea anemone, Strabismus-mediated primary archenteron invagination is uncoupled from Wnt/SZ-catenin-dependent endoderm cell fate specification in Nematostella vectensis (Anthozoa, Cnidaria): implications for the evolution of gastrulation, Migration and differentiation potential of stem cells in the cnidarian Hydractinia analysed in eGFP-transgenic animals and chimeras, Unexpected complexity of the Wnt gene family in a sea anemone, A WNT of things to come: evolution of Wnt signaling and polarity in cnidarians, Asymmetric developmental potential along the animal-vegetal axis in the anthozoan cnidarian, Nematostella vectensis, is mediated by Dishevelled, Multiple Wnts are involved in Hydra organizer formation and regeneration, Silencing of developmental genes in Hydra, Hydra transplantation phenomena and the mechanism of hydra head regeneration. (H) Double negative-feedback loop between Dpp and Chordin as suggested by morpholino-mediated gene knockdown experiments (Saina et al., 2009). (D) The hydrozoan Clytia hemisphaerica forms a colony with feeding polyps (autozooids) and medusae-bearing gonozooids. 3D,E). Cnidarians in both groups have an external radial symmetry, yet internal asymmetries and bilaterality are displayed in many groups. Scale bars: 1 cm in A; 70 m in B,K; 80 m in C; 5 cm in D; 150 m in E; 500 m in F,J; 250 m in G,H; 100 m in I. Morpholino-mediated knockdown of endogenous transcripts has been successfully performed in Nematostella and Clytia (Momose et al., 2008; Momose and Houliston, 2007; Rentzsch et al., 2008; Saina et al., 2009). A phylogenetic tree [based on the results of Collins (Collins, 2002) and Collins et al. The inner tissue layer of a cnidarian polyp, larva or medusa. (Hydrozoa, Cnidaria), Cell sorting during the regeneration of Hydra from reaggregated cells, Phenotypic maturation of neurons and continuous precursor migration in the formation of the peduncle nerve net in Hydra, HyBra1, a Brachyury homologue, acts during head formation in Hydra, Origin and evolution of endoderm and mesoderm, Parameters of self-organization in Hydra aggregates, Arrested apoptosis of nurse cells during Hydra oogenesis and embryogenesis, Maintenance of ancestral complexity and non-metazoan genes in two basal cnidarians, A non-tree-based comprehensive study of metazoan Hox and ParaHox genes prompts new insights into their origin and evolution, Mmoires pour servir l'histoire d'un genre de polypes d'eau douce, Move it or lose it: axis specification in Xenopus, An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation, Transgenic Hydra allow in vivo tracking of individual stem cells during morphogenesis, This site uses cookies. Ectoderm and endoderm are separated by an acellular matrix called the mesogloea (gray). Main cnidarian model systems used in developmental biology. Tentacles radiate outward from the rim of the mouth. lab is supported by the NIH National Institute of General Medical Sciences. The Abstract deadline for our 2023 Journal Meeting Unconventional and Emerging Experimental Organisms in Cell and Developmental Biology has been reached. To trace the evolutionary origin of the mesoderm, researchers have searched for cnidarian homologs of genes involved in bilaterian mesoderm formation. 1). Early attempts at this in Hydra were hampered by the low numbers of embryos that can be obtained and by the lengthy embryonic dormancy that most Hydra strains undergo. This suggests that mesoderm might have arisen from endomesoderm (see Glossary, Box 1) in the common ancestor of bilaterians by an altered combination of interactions between these developmental regulators; in fact, Hydra Brachyury can induce mesoderm in Xenopus (Marcellini et al., 2003), suggesting that it is not the gene but rather the regulatory context that has evolved. Cnidaria is an early-branching metazoan lineage, and four of the five current cnidarian classes (Anthozoa, Hydrozoa, Cubozoa, Scyphozoa and Staurozoa) have been identified in the Cambrian fossil record (Cartwright et al., 2007), indicating that considerable cnidarian diversification had occurred by ~500 million years ago. In summary, with the availability of genome sequences and the advent of gene knockdown techniques and transgenics it is now possible to carry out experimental studies of developmental processes in cnidarians that would have been impossible only a few years ago. All cnidarians have two membrane layers with a jelly-like mesoglea between them. (B) The bilayered cellular organization of a Hydra polyp. 1 What germ layers do cnidarians have and name them? (A-C) Nematostella vectensis, showing adult polyp (A), planula larva (B) and primary polyp (C). Cnidarians are a little more complex than sponges. What are germ layers name them? Thecate hydrozoans form an exoskeleton (theca) around the polyps and stolon mat; by contrast, athecate hydrozoans lack this structure. Are cnidarians motile or sessile? A. millepora also contains intracellular symbiotic dinoflagellates (see Glossary, Box 1), and so is an important system for understanding how this symbiosis is established and maintained, which has a bearing on the current widespread phenomenon of coral bleaching (see Glossary, Box 1). Porifera is a diploblastic . Future efforts will certainly expand on recent studies using cnidarians to understand eye evolution, as cnidarians are the only animals among the four basal metazoan phyla that have evolved sophisticated eyes. Subsequently, it was found that bmp2/4, its co-factor bmp5-8, the BMP-like ligand gdf5, the antagonists chordin and gremlin1, as well as several other genes, such as most of the Hox genes, are expressed asymmetrically with respect to the oral-aboral axis in Nematostella (Fig. Ectoderm -> Outer skin. A study published today (September 11) in Nature Ecology & Evolution suggests that instead of two distinct germ layers, cnidarians may indeed have three. Cnidarians nominally develop from two germ layers, the ectoderm and endoderm, whereas the presence of a third germ layer, the mesoderm, traditionally characterizes higher animals. 2A, Fig. Animals with only two germ layers are called diploblastic animals. 1).Regardless of the drastically different mechanisms used, the resulting post . Most of the quiz questions . These cookies ensure basic functionalities and security features of the website, anonymously. After fertilization, the zygote undergoes cleavage to form the blastula. Despite their relatively simple anatomies, cnidarians have a surprisingly complex genetic toolkit. Do cnidarians have germ layers? Cnidarians are a particular group of aquatic animals, and this quiz will appraise your level of expertise concerning Cnidarian germ layers. Members of the phylum Chordata have three main germ cell layers, each with its own unique role in development. where it could regulate the differentiation of the retractor muscles (Finnerty et al., 2004; Saina and Technau, 2009). Ciliated rim on one or both ends of the slit-like pharynx, defining the symmetry of the directive axis. (I-K) Clytia hemisphaerica showing autozooid and gonozooid polyps (I), young medusa (J) and planula larva (K). Neurons are found in both the endoderm and ectoderm. Thus, the differentiated epithelial cells of the body column also serve as stem cells. Other recent studies have revealed the evolution and expansion of taxon-restricted genes in cnidarians, i.e. Comparison of the Nematostella and Hydra genome sequences (browsers are available for both genomes: Hydra, http://hydrazome.metazome.net/cgi-bin/gbrowse/hydra; Nematostella, http://www.metazome.net/cgi-bin/gbrowse/Nvectensis) shows that Hydra has undergone considerable gene loss compared with Nematostella (Chapman et al., 2010). Strikingly, in Acropora the bmp2/4 homolog is asymmetrically expressed (Hayward et al., 2002). Peter Rugg-Gunn, Naomi Moris and Patrick Tam highlight several technical challenges to studying early human development and propose ways to overcome some of these constraints. As first reported in 1909, organizer activity is also present at the oral end of the Hydra polyp, at the hypostome, which directly develops from the embryonic blastopore (Browne, 1909). Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). The Forest of Biologists is a biodiversity initiative created by The Company of Biologists, with support from the Woodland Trust. The cnidarians, or the jellyfish and their kin, are the simplest animal group that displays true tissues, although they possess only two tissue layers. Genetic mapping and cloning of the histocompatibility complex has been carried out in the hydrozoan Hydractinia (Nicotra et al., 2009), indicating that genetic approaches are feasible in cnidarians. Cnidarians are basically radially symmetrical and have tentacles. Polyps Cnidaria are diploblastic animals; in other words, they have two main cell layers, while more complex animals are triploblasts having three main layers. Distinguishing features Cnidarians form a phylum of animals that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. Most interstitial cells and nematoblast clusters are located between ectodermal epithelial cells. How many cell germ layers do cnidarians have? What are the names of Acropora millepora). Cnidarians (corals, sea anemones, jellyfish and hydroids) form a diverse phylum that contains ~9000 species, which live in aquatic (predominantly marine) environments. The typical ciliated postgastrula larval stage in cnidarians. Cnidaria A phylum that comprises the sea anemones, jellyfish, and corals, and which is known from the late Precambrian . This Wikipedia site . The process of fertilization is tightly controlled to ensure that only one sperm fuses with one egg. They belong to the phylum Cnidaria. The resulting embryos develop into ciliated planula larvae that undergo either a gradual (Nematostella) or more dramatic (Acropora) metamorphosis into a sessile primary polyp, which involves calcification and formation of the skeleton in corals. Three primary germ layers Gastrulation is a key phase in embryonic development when pluripotent stem cells differentiate into the three primordial germ layers: ectoderm, mesoderm and endoderm. 28.2A: Phylum Cnidaria - Biology LibreTexts In the oral part, the mesenteries are connected to the endodermal part of the pharynx. Since the morphological consequences of the deployment of this system in cnidarians are drastically different from those in bilaterians, it is premature to homologize the cnidarian directive axis with the D-V axis of bilaterians. 2C, Fig. Two distinct body plans are found in Cnidarians: the polyp or tuliplike "stalk" form and the medusa or "bell" form (Figure 2). Initial attempts to determine the evolutionary relationship between cnidarian and vertebrate stem cells involved searching sequenced cnidarian genomes and EST datasets for homologs of the four pluripotency genes that are known to be expressed in vertebrate stem cells (Klf4, Oct4, Sox2 and Nanog). PDF Animal Diversity I: Porifera, Cnidaria, Ctenophora, Platyhelminthes Some cnidarians are dimorphic, that is, they exhibit both body plans during their life cycle. See answer Advertisement MichalFalkner Cnidarians have three germ layers .Names of germ layers are ectoderm (outer layer), mesoderm (middle layer), and endoderm (inside layer). Clytia has been used to study the role of Wnt signaling in egg polarization and in nematocyte differentiation in the medusa (Denker et al., 2008; Momose et al., 2008; Momose and Houliston, 2007). For example, morphologically indistinguishable nerve cells can have very different patterns of gene expression (Bode, 1992; Galliot et al., 2009; Technau and Holstein, 1996). Development have published a recent Editorial showcasing some of our most popular features, as well as introducing some new initiatives. Medusa (adult) 2. 1). Grafting, dissociation and separation of blastomeres and lineage tracing have also been successfully performed in cnidarian embryos (Freeman, 1990; Fritzenwanker et al., 2007; Kraus et al., 2007; Lee et al., 2007; Momose and Schmid, 2006). Endodermal folds in anthozoan polyps that extend from the body wall into the gastric cavity. Which sports bike is best for short riders? Whitening, caused by loss of symbiont dinoflagellates, that leads to the death of corals. 2 How many germ layers do cnidarians have and name them 2 What are the two body forms of cnidarians and make a sketch of each below? The ectoderm gives rise to the skin and the nervous system. In Hydra, from which genes encoding most of the components of the canonical Wnt pathway have been cloned, seven of the ten Wnt genes identified in Hydra are expressed in the hypostome (the oral dome of the polyp) (Hobmayer et al., 2000; Lengfeld et al., 2009). with inducible promoters or landing sites for recombination) are needed. Whereas studies of Hox genes have not been as illuminating as originally hoped with regard to the evolution of axes in metazoans, studies of the Wnt signaling pathway have been. The two main divisions of Cnidaria (Anthozoa and Medusozoa) are indicated in red. But opting out of some of these cookies may affect your browsing experience. Cnidarias have an internal sac for digestion which is called the gastrovascular cavity. Thus, RNAi in Hydra is not yet a robust technology. Do cnidarians have all three germ layers? - YouTube Sponges are diploblasts meaning that they develop from two basic germ layers: an ectoderm, or outer layer, and an endoderm, or inner layer. 5)Reproduction is asexual or sexual. (G) Schematic of the planula stage illustrating the asymmetric expression of chordin and dpp on one side, and of gdf5-like, a member of the BMP family, and of gremlin, a BMP antagonist, on the opposite side. However, because most studies have concentrated on vertebrate models we still have a lot to learn regarding the evolution of stem cells. How do Cnidaria reproduce? How many germ layers do cnidarians have? Well-fed polyps produce a new bud in 3-4 days. Note that a number of other genes (not shown for clarity) are also expressed asymmetrically, indicative of a directive axis. The Wnt pathway has also been manipulated by diacylglycerol and LiCl (Hassel and Bieller, 1996; Muller, 1990). Extracellular matrix between ectoderm and endoderm in cnidarian polyps and medusae; it is composed of laminins, collagens and proteoglycans. By clicking Accept All, you consent to the use of ALL the cookies. However, the application of RNAi to Hydra has its limitations. The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". Clear homologs of these genes have not been identified in the Nematostella or Hydra genome (Chapman et al., 2010), suggesting that either the role of these key genes is performed by related genes or, alternatively, that the circuitry for producing stem cells evolved independently in cnidarians and vertebrates. Necessary cookies are absolutely essential for the website to function properly. Gastrovascular cavity What does the mouth of the cnidarian open into? No, it has 3 tissue layers, no nematocysts, and shows bilateral symmetry: Go to 3 (F-H) Transgenic Nematostella. Cnidarians are generally considered to have only two germ layers (endoderm and ectoderm; see Glossary, Box 1) and are radially symmetric, although anthozoan polyps display an internal bilaterality in the asymmetric organization of the pharynx, the siphonoglyph (see Glossary, Box 1), and the retractor muscles in the mesenteries (see Glossary, Box 1) (e.g. The embryo remains attached to the parent polyp from fertilization through gastrulation. (A-C), David Miller (D), Eldon Ball (E), Tim Nchter and Thomas Holstein (F), U.T. Evolutionary crossroads in developmental biology: Cnidaria Most cnidarians have two tissue layers. The bacteria feeding approach requires treatment for up to 18 days and ultimately leads to starvation and cell death. two germ layers All epithelial cells in Hydra are myoepithelial, with myofibers on the basal side (red). Do Cnidaria Have Germ Layers? | Study.com