Tailless Species Research Articles

Cymric, a Maternal and Zygotic HTK-16-Like SHARK Family Tyrosine Kinase Gene, Is Disrupted in Molgula occulta, a Tailless Ascidian.

AbstractWe describe the cloning and expression of a nonreceptor tyrosine kinase, cymric (Uro-1), a HTK-16-like (HydraTyrosineKinase-16) gene, identified in a subtractive screen for maternal ascidian cDNAs in Molgula oculata, an ascidian species with a tadpole larva. The cymric gene encodes a 4-kb mRNA expressed in gonads, eggs, and embryos in the tailed M. oculata but is not detected in eggs or embryos of the closely related tailless species Molgula occulta. There is a large insertion in cymric in the M. occulta genome, as shown by transcriptome and genome analyses, resulting in it becoming a pseudogene. The cymric amino acid sequence encodes a nonreceptor tyrosine kinase with an N-terminal region containing two SH2 domains and five ankyrin repeats, similar to the HTK-16-like gene found in other ascidians. Thus, the ascidian cymric genes are members of the SHARK (Src-homology ankyrin-repeat containing tyrosine kinase) family of nonreceptor tyrosine kinases, which are found throughout invertebrates and missing from vertebrates. We show that cymric is lacking the tyrosine kinase domain in the tailless M. occulta, although the truncated mRNA is still expressed in transcriptome data. This maternal and zygotic HTK-16-like tyrosine kinase is another described pseudogene from M. occulta and appears not to be necessary for adult development.

Loss of collagen gene expression in the notochord of the tailless tunicate Molgula occulta.

In tunicates, several species in the Molgulidae family have convergently lost the tailed, swimming larval body plan, including the morphogenesis of the notochord, a major chordate-defining trait. Through the comparison of tailless M. occulta and a close relative, the tailed species M. oculata, we show that notochord-specific expression of the Collagen Type I/II Alpha (Col1/2a) gene appears to have been lost specifically in the tailless species. Using CRISPR/Cas9-mediated mutagenesis in the tailed laboratory model tunicate Ciona robusta, we demonstrate that Col1/2a plays a crucial role in the convergent extension of notochord cells during tail elongation. Our results suggest that the expression of Col1/2a in the notochord, although necessary for its morphogenesis in tailed species, is dispensable for tailless species. This loss is likely a result of the accumulation of cis-regulatory mutations in the absence of purifying selective pressure. More importantly, the gene itself is not lost, likely due to its roles in other developmental processes, including during the adult stage. Our study further confirms the Molgulidae as an interesting family in which to study the evolutionary loss of tissue-specific expression of indispensable genes.

The Degenerate Tale of Ascidian Tails.

Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the "notoball." We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

Correction to 'Substrate use drives the macroevolution of mammalian tail length diversity'.

You have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Mincer S. T. and Russo G. A. 2020Correction to ‘Substrate use drives the macroevolution of mammalian tail length diversity’Proc. R. Soc. B.2872020247720202477http://doi.org/10.1098/rspb.2020.2477SectionYou have accessCorrectionsCorrection to ‘Substrate use drives the macroevolution of mammalian tail length diversity’ S. T. Mincer S. T. Mincer http://orcid.org/0000-0003-2275-7971 Google Scholar Find this author on PubMed Search for more papers by this author and G. A. Russo G. A. Russo Google Scholar Find this author on PubMed Search for more papers by this author S. T. Mincer S. T. Mincer http://orcid.org/0000-0003-2275-7971 Google Scholar Find this author on PubMed Search for more papers by this author and G. A. Russo G. A. Russo Google Scholar Find this author on PubMed Search for more papers by this author Published:28 October 2020https://doi.org/10.1098/rspb.2020.2477See original article.Substrate use drives the macroevolution of mammalian tail length diversityProc. R. Soc. B287, 20192885. (Published 5 February 2020). (doi:10.1098/rspb.2019.2885)There are two minor errors in figure 2 of the originally published manuscript. The scale colour bar accompanying the summary tree to signify slope value was flipped so that it was incorrectly labelled. In the corrected figure red represents slope of −4.4, and blue represents slope of 3.3. The labels for two branches in the summary tree representing significant shifts in slope and intercept for Caprinae and Bovinae were also switched. Correct slope and intercept values for all branches are included in the electronic supplementary material, and the errors described here have now been fixed in the figure submitted in this correction. These errors do not change the interpretation or accuracy of results. Figure 2. Summary tree of the evolutionary best-fit model for tail length ∼ body length. Black represents the ancestral condition and each colour represents a statistically significant evolutionary shift (regime) in slope and intercept (available in electronic supplementary material), coloured by the slope for each group. Tailless species (dashed lines) were not included in the evolutionary best-fit model but have been subsequently added back onto the tree to show where tail loss has occurred in phylogeny.Download figureOpen in new tabDownload PowerPoint Previous Article FiguresRelatedReferencesDetailsRelated articlesSubstrate use drives the macroevolution of mammalian tail length diversityFeb 5, 2020, 12:00:00 AMProceedings of the Royal Society B: Biological Sciences This Issue28 October 2020Volume 287Issue 1937 Article InformationDOI:https://doi.org/10.1098/rspb.2020.2477PubMed:33109016Published by:Royal SocietyPrint ISSN:0962-8452Online ISSN:1471-2954History: Published online28/10/2020Published in print28/10/2020 License:© 2020 The Author(s)Published by the Royal Society. All rights reserved. Citations and impact Subjectsevolution Large datasets are available through Proceedings B's partnership with Dryad

Primate tails: Ancestral state reconstruction and determinants of interspecific variation in primate tail length.

Living primates vary considerably in tail length-body size relation, ranging from tailless species to those where the tail is more than twice as long as the body. Because the general pattern and determinants of tail evolution remain incompletely known, we reconstructed evolutionary changes in relative tail length across all primates and sought to explain interspecific variation in this trait. We combined data on tail length, head-body length, intermembral index (IMI), habitat use, locomotion type, and range latitude for 340 species from published sources. We reconstructed the evolution of relative tail length to identify all independent cases of regime shifts on a primate phylogeny, using several methods based on Ornstein-Uhlenbeck (OU) models. Accounting for phylogeny, we also examined the effects of habitat, locomotion type, distance from the equator and IMI on interspecific variation in tail length-body size relation. Primate tail length is not sexually dimorphic. A phylogenetic reconstruction allowing multiple optima explains the observed regime shifts best. During the evolutionary history of primates, relative tail length changed 50 times under an OU model. Specifically, relative tail length increased 26 and decreased 24 times. Most of these changes occurred among Old World primates. Among the variables tested here, interspecific variation in IMI and the difference between leaping and non-leaping locomotion explained interspecific variation in relative tail length: Evolutionary decreases in relative tail length are generally associated with an increase in IMI and an absence of leaping behavior. Regime shifts for relative tail length in living primates occurred in concert with fundamental changes in IMI and a change from leaping to non-leaping locomotion, or vice versa. Exceptions from this general pattern are linked to the presence of a prehensile tail or specialized foraging strategies. Thus, the primate tail appears to have evolved in functional coordination with limb proportions, presumably to assist body balance.

Morphology and fluorescence of the parapodial light glands in Tomopteris helgolandica and allies (Phyllodocida: Tomopteridae)

The histology of putative light organs in the parapodia of five species of Tomopteris (pelagic annelids) is examined and compared using light, epifluorescence and scanning electron microscopy. The structural homology of rosette glands in the parapodial pinnae of the tail-bearing species T. helgolandica and T. pacifica, and hyaline glands of the tail-less species T. carpenteri, T. planktonis and T. septentrionalis is highlighted. However, the rosette glands point towards the ramus of the coelomic cavity inside the parapodia, whereas the hyaline glands point towards the surrounding water and penetrate the pinnal surface on the posterior side of the parapodia. Further, in order to assess the photogenic properties of rosette glands from T. helgolandica, we analysed the distribution and the temporal dynamics of their endogenous fluorescence in isolated parapodia in response to light emission induced by KCl and carbachol. The gradual extinction of bioluminescence was combined to the centrifugal spread of fluorescence from the core of the gland. We suggest this fluorescence to be produced by a “breakdown” product of the chemiluminescent reaction. Finally, both gland types probably evolved from a common light-emitting structure and differentiated along a functional and migrational axis extending from endocrine secretion close to the coelomic ramus to exocrine secretion close to the lateral margin of the pinna.

Developmental mode influences diversification in ascidians

Ascidian species (Tunicata: Ascidiacea) usually have tailed, hatching tadpole larvae. In several lineages, species have evolved larvae that completely lack any tail tissues and are unable to disperse actively. Some tailless species hatch, but some do not hatch before going through metamorphosis. We show here that ascidian species with the highest speciation rates are those with the largest range sizes and tailed hatching larval development. We use methods for examining diversification in binary characters across a posterior distribution of trees, and show that mode of larval development predicts geographical range sizes. Conversely, we find that species with the least dispersive larval development (tailless, non-hatching) have the lowest speciation rates and smallest geographical ranges. Our speciation rate results are contrary to findings from sea urchins and snails examined in the fossil record, and further work is necessary to reconcile these disparate results.

Molgula pugetiensis is a Pacific Tailless Ascidian Within the Roscovita Clade of Molgulids

The molgulid ascidians are a monophyletic group of tunicates that share specific morphological features, includ ing a heart/kidney complex and hermaphroditic gonads found on both sides of the adult. The larval body plan, however, is quite plastic, with tailless species evolving at least four times independently within the molgulids. The Roscovita clade of molgulid ascidians includes three spe cies that are found sympatrically in Roscojf, France, and includes both tailed and tailless species. Another species in this clade, Molgula pacifica, is a direct developing tailless molgulid ascidian within the Roscovita clade found on the outer coast of Vancouver Island. M. pacifica never hatch, and they undergo metamorphosis within the chorion. Molgula pugetiensis is a molgulid ascidian found in the San Juan Islands, but its development has not been reported. Here, we show that this species, belonging to the Roscovita clade, is a tailless ascidian species with that hatch from the chorion before metamorphosis, similar to the well described tailless Molgula occulta from Roscojf, France. Immediately after hatching, the tailless larvae develop am pullae and proceed to metamorphose into juveniles. Tunicates in the Molgulidae are a monophyletic group of solitary ascidians (Ascidiacea: Stolidobranchia) that are sis ter group to the families Styelidae and Pyuridae (1-3). Evidence from 18S rDNA shows that the stolidobranch tunicates may also be closely related to the appendicular ians, which include the pelagic oikopleurid tunicates that build mucous houses (2-4). Molgulidae is an interesting group for research on the evolution of body plans because it contains species that have urodele, tailed, chordate larvae with notochord and muscle as well as closely related species

The notochord

What is the notochord? The notochord is the defining structure of the phylum Chordata, which includes all vertebrates. In its ‘mature’ state the notochord is a rod of large cells constrained by a thick extracellular sheath and positioned between the developing spinal cord and gut. Turgor pressure generated by notochord cells exerted against the sheath gives the notochord stiffness, imparting the mechanical properties required for one of its main functions as the central axial skeletal element of the developing embryo. What does it do? The notochord exists transiently during the life of most vertebrates. During its short life, however, the notochord plays another critical role for developing embryos. It produces a variety of secreted growth factors, such as Sonic hedgehog, which instruct surrounding tissues to acquire particular differentiated fates. A number of embryological experiments, for example, tell us that, without notochord-derived signals, motorneurons in the spinal cord would not form, the pancreas insulin-producing cell would not differentiate and the body muscles would not be produced properly. Without the axial skeletal support of the notochord, many lower vertebrate larvae, such as fish fry and tadpoles, become severely shortened and cannot swim or feed properly. Thus many of the mutations affecting zebrafish notochord development are named after the seven dwarves. Where does it come from? One of the truly classic experiments of embryology is the ‘organiser’ experiment of Spemann and Mangold. In the original experiment, a piece of the dorsal side of a newt embryo was transferred from one species (with dark pigment) to the ventral side of an equivalently staged embryo of another, lightly pigmented newt species. Amazingly, the resulting embryos were conjoined twins, with one original body axis and a second body axis induced by the transplanted dorsal tissue. For the notochord enthusiast, the most astounding thing about this is that the majority of tissue derived from the graft is notochord, while the induced second axis is derived mostly from the host. The same activity has been identified for other vertebrate phyla, with an equivalent piece of dorsal notochord progenitor tissue able to induce the formation of a second axis. Figure 1 shows an example of such an experiment with zebrafish. Thus, well before acquiring the morphological characteristics of a notochord the progenitor tissue possesses the remarkable ability to organise embryogenesis. From an evolutionary point of view recent studies of a group of lower chordates, called ascidians, are beginning to give us insight into some of the genes important for notochord formation. Ascidians, also known as tunicates or sea-squirts, are most familiar in their sessile adult stages, but as free-swimming larvae ascidians possess a notochord that serves as the main skeletal element. In one study a gene called Manx, which is important for notochord formation, was identified by comparing gene expression in two related species of ascidian, one possessing a normally differentiated notochord with another tail-less species. Where does it go? By the time the notochord is mature it expresses a constellation of genes that is essentially identical to that of developing cartilage. This is consistent with the mechanical role of the notochord. Indeed, in birds, most fish and mammals the notochord becomes segmented and in regions of vertebrae expresses type X collagen and is replaced by bone while in the intervertebral regions notochord cells contribute to the centre of the discs. Where can I find out more?

Evolution of ciliary patterns in the Oligotrichida (Ciliophora, Spirotricha) and its taxonomic implications

Although the somatic ciliature of the Oligotrichida typically comprises only a girdle and ventral kinety, a considerable diversity of ciliary patterns occurs. The four main girdle kinety patterns are identically found in tailed and tail-less species. The contractile tail has a complicated and unique ultrastructure and is potentially useful for the cell's movement and/or stabilization during feeding. Accordingly, I assume that this structure has evolved only once, namely, in the Tontoniidae nov. fam., and that the different girdle kinety patterns developed convergently in the tailed and tail-less taxa. Further distinct features suggest the establishment of the families Cyrtostrombidiidae nov. fam. (with cyrtos-like pharyngeal fibres and lack of ventral membranelles and endoral) and Pelagostrombidiidae nov. fam. (with neoformation organelle). An attempt is made to reconstruct the evolution of the kinety patterns based on morphologic, ontogenetic, and ultrastructural data. Some genera of tail-less Oligotrichida base on differences in the ciliary pattern; Omegastrombidium nov. gen. is erected for a further girdle kinety pattern. Likewise, the tailed genus Tontonia is split, resulting in two new genera, viz., Pseudotontonia nov. gen. and Spirotontonia nov. gen. Furthermore, the genus Spirostrombidium is split due to the different origin of the parallel course of girdle and ventral kinety, and Parallelostrombidium nov. gen. is established. However, the genus Thigmostrombidium is rejected because its enlarged thigmotactic membranelles are interpreted as an adaptation to the benthic lifestyle, which occurred several times within different girdle kinety patterns.

Brachyury expression in tailless Molgulid ascidian embryos.

The T-box transcription factor gene Brachyury is important for the differentiation of notochord in all chordates, including the ascidians Halocynthia roretzi and Ciona intestinalis. We isolated Brachyury from molgulid ascidians, which have evolved tailless larvae multiple times independently, and found the genes appear functional by cDNA sequence analyses. We then compared the expression of Mocu-Bra in tailed Molgula oculata embryos to two tailless species, Molgula occulta (Mocc-Bra) and Molgula tectiformis (Mt-Bra). Here we show that both tailless species express Brachyury in the notochord lineage during embryogenesis. Initial expression of Mocu-Bra is normal in tailed M. oculata embryos; 10 precursor notochord cells divide twice to result in 40 notochord cells that converge and extend to make a notochord down the center of the tail. In contrast, in tailless Molgula occulta, Mocc-Bra expression disappears prematurely, and there is only one round of division, resulting in 20 cells in the final notochord lineage that never converge or extend. In M. occulta x M. oculata hybrid embryos, expression of Mocu-Bra is prolonged, and the embryos form a tail with 20 notochord cells that converge and extend normally. However, in Molgula tectiformis, a different tailless ascidian, Mt-Bra was expressed only in the 10 notochord precursor cells, which never divide, converge, or extend. In summary, neither Brachyury function nor the early establishment of the notochord lineage appears to be impaired in tailless embryos. In light of these results, we are continuing to investigate how and why notochord development is lost in tailless molgulid ascidian embryos.

The forkhead gene FH1 is involved in evolutionary modification of the ascidian tadpole larva

The forkhead gene FH1 encodes a HNF-3β protein required for gastrulation and development of chordate features in the ascidian tadpole larva. Although most ascidian species develop via a tadpole larva, the conventional larva has regressed into an anural (tailless) larva in some species. Molgula oculata (the tailed species) exhibits a tadpole larva with chordate features (a dorsal neural sensory organ or otolith, a notochord, striated muscle cells, and a tail), whereas its sister species Molgula occulta (the tailless species) has evolved an anural larva, which has lost these features. Here we examine the role of FH1 in modifying the larval body plan in the tailless species. We also examine FH1 function in tailless species×tailed species hybrids, in which the otolith, notochord, and tail are restored. The FH1 gene is expressed primarily in the presumptive endoderm and notochord cells during gastrulation, neurulation, and larval axis formation in both species and hybrids. In the tailless species, FH1 expression is down-regulated after neurulation in concert with arrested otolith, notochord, and tail development. The FH1 expression pattern characteristic of the tailed species is restored in hybrid embryos prior to the development of chordate larval features. Antisense oligodeoxynucleotides (ODNs) shown previously to disrupt FH1 function were used to compare the developmental roles of this gene in both species and hybrids. As described previously, antisense FH1 ODNs inhibited endoderm invagination during gastrulation, notochord extension, and larval tail formation in the tailed species. Antisense FH1 ODNs also affected gastrulation in the tailless species, although the effects were less severe than in the tailed species, and an anural larva was formed. In hybrid embryos, antisense FH1 ODNs blocked restoration of the otolith, notochord, and tail, reverting the larva back to the anural state. The results suggest that changes in FH1 expression are involved in re-organizing the tadpole larva during the evolution of anural development.

A multigene locus containing the Manx and bobcat genes is required for development of chordate features in the ascidian tadpole larva.

The Manx gene is required for the development of the tail and other chordate features in the ascidian tadpole larva. To determine the structure of the Manx gene, we isolated and sequenced genomic clones from the tailed ascidian Molgula oculata. The Manx gene contains 9 exons and encodes both major and minor Manx mRNAs, which differ in the length of their 5' untranslated regions. The coding region of the single-copy bobcat gene, which encodes a DEAD-box RNA helicase, is embedded within the first Manx intron. The organization of the bobcat and Manx transcription units was determined by comparing genomic and cDNA clones. The Manx-bobcat gene locus has an unusual organization in which a non-coding first exon is alternatively spliced at the 5' end of two different mRNAs. The bobcat and Manx genes are expressed coordinately during oogenesis and embryogenesis, but not during spermatogenesis, in which bobcat mRNA accumulates independently of Manx mRNA. Similar to Manx, zygotic bobcat transcripts accumulate in the embryonic primordia responsible for generating chordate features, including the dorsal neural tube and notochord, are downregulated during embryogenesis in the tailless species Molgula occulta and are upregulated in M. occulta X M. oculata hybrids, which restore these chordate features. Antisense experiments indicate that zygotic bobcat expression is required for development of the same suite of chordate features as Manx. The results show that the Manx-bobcat gene complex has a role in the development of chordate features in ascidian tadpole larvae.

Cytoskeletal actin genes function downstream of HNF-3beta in ascidian notochord development.

We have examined the expression and regulation of cytoskeletal actin genes in ascidians with tailed (Molgula oculata) and tailless larvae (Molgula occulta). Four cDNA clones were isolated representing two pairs of orthologous cytoskeletal actin genes (CA1 and CA2), which encode proteins differing by five amino acids in the tailed and tailless species. The CA1 and CA2 genes are present in one or two copies, although several related genes may also be present in both species. Maternal CA1 and CA2 mRNA is present in small oocytes but transcript levels later decline, suggesting a role in early oogenesis. In the tailed species, embryonic CA1 and CA2 mRNAs first appear in the presumptive mesenchyme and muscle cells during gastrulation, subsequently accumulate in the presumptive notochord cells, and can be detected in these tissues through the tadpole stage. CA1 mRNAs accumulate initially in the same tissues in the tailless species but subsequently disappear, in concert with the arrest of notochord and tail development. In contrast, CA2 mRNAs were not detected in embryos of the tailless species. Fertilization of eggs of the tailless species with sperm of the tailed species, which restores the notochord and the tail, also results in the upregulation of CA1 and CA2 gene expression in hybrid embryos. Antisense oligodeoxynucleotide experiments suggest that CA1 and CA2 expression in the notochord, but not in the muscle cells, is dependent on prior expression of Mocc FHI, an ascidian HNF-3beta-like gene. The expression of the CA1 and CA2 genes in the notochord in the tailed species, downregulation in the tailless species, upregulation in interspecific hybrids, and dependence on HNF-3beta activity is consistent with a role of these genes in development of the ascidian notochord.

Requirement of the Manx gene for expression of chordate features in a tailless ascidian larva.

An evolutionary change in development was studied in two closely related ascidian species, one exhibiting a conventional tadpole larva and the other a modified tailless larva. Interspecific hybridization restores chordate features to the tailless larva. The zinc finger gene Manx is expressed in cells that generate chordate features in the tailed species but is down-regulated in the tailless species. Manx expression is restored in hybrid embryos. Antisense oligodeoxynucleotide treatment inhibited Manx expression and chordate features in hybrid embryos, which suggests that Manx is required for development of the chordate larval phenotype in ascidians.

Early stages of development in the caudal neural tube of the golden Syrian hamster (Mesocricetus auratus).

Secondary neurulation is the morphogenetic process whereby the caudal segments of the neural tube are derived from cells in the embryonic tail bud. Comparative studies have demonstrated similar characteristics in the mechanism of secondary neurulation among tailless species, which are thought to be due to the evolutionary reduction in tail length (Hughes and Freeman, 1974). In order to explore this hypothesis further, light and scanning electron microscopy was used to study early stages of neurulation in the tail buds of hamster embryos. The golden Syrian hamster is a relatively common laboratory rodent with a reduced tail. In this species, secondary neurulation first became apparent in embryos with approximately 17 pairs of somites. This was well before closure of the posterior neuropore which occurred at the 21-somite stage. The lumen of the neural tube appeared to extend into the tail bud in an even and progressive fashion accompanied by reorientation and rearrangement of tail-bud cells. The mechanism appeared to be similar to that reported in long-tailed rodents.

The mauthnerian apparatus in the Ichthyopsida; its nature and function and correlated problems of neurohistogenesis.

A FTER the investigations, made in 1859 by Mauthner, which led to the discovery of the two giant fibres in the spinal cord of Esox lucius, we can consider the two giant cells, from which these fibres originate, as a true functional system of nervous activity. This functional system, which is peculiar to the majority of fishes and amphibians, is formed by a single pair of symmetrical neurons of extremely large size, the cell bodies of which are located in the tegmentum of the medulla at the level of the VIIIth nerve root, and axons of which reach the tip of the neural cord, each running along the medial longitudinal fasciculus of opposite sides. Since the first investigations, and especially since the wider comparative analysis made by Tagliani (1905), it has been clear that Mauthner's fibres are not present in all the species of Ichthyopsida. As we shall see later, this condition is very important for the functional interpretation of this bineuronal apparatus. The absence of a typical mauthlnerian apparatus was noted in the Cyclostomata and, among fishes, in all the selachians and in some teleosteans, e.g., the Anguilloidea and Murenoidea (Anguilla, Conger, Muraena, Congromuraena, Gymnotus, Ophyctlis), in many bottom-living fishes, e.g., Lophius, Uranoscopus, and Blennius, and in tailless species, e.g., Orthagoriscus. Many exceptions were also found among urodele Amphibians, e.g., Geotriton, Cryptobranchus, Siren, Proteus, Diemictilus, and Salamandra maculosa (according to observations by Mason, 1879, 1888; Osborn, 1888; Koelliker, 1896; Klaussner, 1883; Gage, 1893; Tagliani, 1905; Beccari, 1907), while the M. (Mauthner) neurons were considered to be absent in all the anurans (Reissner 1864; Stieda, 1870; Sala, 1892; Koelliker, 1893, 1896; Athias, 1897; Van Geuchten, 1898; Gaupp, 1899; Tagliani, 1905). Yet, in more recent years, with the help of better techniques, many of these statements had to be rectified. Thus Schmidt (1885) described the M. cells in Proteus, Tuerckheim (1903) and Kingsbury (1895) in Cryptobranchus and Diemyctilus (Triturus) and Szepsenwol (1935) in Geotriton. Also the belief in the absence of these neurons in the anurans was found to be erroneous, for Larsell (1935) found them in tadpoles of Rana fusca and Bombinator pachypus (their presence in Rana fusca having been recently confirmed by Willis, 1947) and Stefanelli described them in Rana esculenta, Rana agilis, and Hyla arborea at the larval stage (Stefaneli and Osti, 1942) and in tadpoles of Discoglossus pictus, Hyla savignii, and Xenopus laevis (1949). In tadpoles of Rana esculenta Stefanelli was also able to show a pair of premauthnerian giant cells, at the level of the Vth nerve root. On the other hand, the absence of a true mauthnerian apparatus in the Bufonida (Bufo vulgaris and Bufo viridis) was confirmed by Stefanelli (1949) and by Zacchei (1949). A revised knowledge of the condition prevailing in the cyclostomes and teleosts brought a modification of many statements. In petromyzonts there are ten pairs of reticular giant cells, only one of which is homologous, as to location and connections, with the M. cells. Observations by De Angelis (in press) show the presence of the M. cells also in Blennius and in other species which, though typical bottom-living fishes, show an active movement of the tail in swimming (Clinus, Gobius, Trachinus, Solea). The morphological and functional significance of the M. giant neurons was not easily understood, and for a long time, as we shall see later, many wrong ideas dominated the subject. Many anatomical, comparative, morpho-ecological and physiological researches were necessary to elucidate the enigmatic significance of these neurons, and only after the most recent results can we consider this question at last resolved. Today the interest in the M. neurons goes beyond their morphological and functional significance and reaches the wider fields of the general problems of neurohistogenesis, because of the size of these elements, their easy localization, and their