New crinoids from the Baltic region (Estonia): fossil tip‐dating phylogenetics constrains the origin and Ordovician–Silurian diversification of the Flexibilia (Echinodermata)

This study documents previously unknown taxonomic and morphological diversity among early Palaeozoic crinoids. Based on highly complete, well preserved crown material, we describe two new genera from the Ordovician and Silurian of the Baltic region (Estonia) that provide insight into two major features of the geological history of crinoids: the early evolution of the flexible clade during the Great Ordovician Biodiversification Event (GOBE), and their diversification history surrounding the end‐Ordovician mass extinction. The unexpected occurrence of a highly derived sagenocrinid, Tintinnabulicrinus estoniensis gen. et. sp. nov., from Upper Ordovician (lower Katian) rocks of the Baltic palaeocontinent provides high‐resolution temporal, taxonomic and palaeobiogeographical constraints on the origin and early evolution of the Flexibilia. The Silurian (lower Rhuddanian, Llandovery) Paerticrinus arvosus gen. et sp. nov. is the oldest known Silurian crinoid from Baltica and thus provides the earliest Baltic record of crinoids following the aftermath of the end‐Ordovician mass extinction. A Bayesian ‘fossil tip‐dating’ analysis implementing the fossilized birth–death process and a relaxed morphological clock model suggests that flexibles evolved c. 3 million years prior to their oldest fossil record, potentially involving an ancestor–descendant relationship (via ‘budding’ cladogenesis or anagenesis) with the paraphyletic cladid Cupulocrinus. The sagenocrinid subclade rapidly diverged from ‘taxocrinid’ grade crinoids during the final stages of the GOBE, culminating in maximal diversity among Ordovician crinoid faunas on a global scale. Remarkably, diversification patterns indicate little taxonomic turnover among flexibles across the Late Ordovician mass extinction. However, the elimination of closely related clades may have helped pave the way for their subsequent Silurian diversification and increased ecological role in post‐Ordovician Palaeozoic marine communities. This study highlights the significance of studies reporting faunas from undersampled palaeogeographical regions for clade‐based phylogenetic studies and improving estimates of global biodiversity through geological time.     

When an epithelium ceases to exist – an ultrastructural study on the fate of the embryonic coelom in Epiperipatus biolleyi (Onychophora, Peripatidae)

It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda.     

Presumed postlarval pentacrinoids from the Lower Devonian Hunsrück Slate,Germany

Several tiny crinoids with crowns as small as 1 mm, or less, in width are newly recognized from the Hunsrück Slate of southwestern Germany. The presence of erect arms above an amorphous calyx in some specimens can be inferred. Based on comparison with the size and gross morphology of developmental stages in living crinoids, these tiny Hunsrück crinoids are judged to be at an early postlarval stage that is analogous to the pentacrinoid stage just after development from the stalked, but armless, smaller cystidean larval stage found in both living comatulids and isocrinids. Some of these tiny crinoids have a stalk up to 4 mm long attached to a now pyritized former substrate. Their clustered occurrence suggests gregarious settlement of larvae. Taxonomic identification of these presumed pentacrinoids is not possible, even to the sub‐class level, although they are preserved with larger juveniles of the cladids Propoteriocrinus and Lasiocrinus. These larger juveniles exhibit 3‐D pyritized calcite plates, whereas the probable pentacrinoids appear to be preserved as flattened, micro‐crystalline pyritized dermal tissues that enclosed lightly calcified, porous ossicles. The pentacrinoids were likely buried within weeks or months of hatching, based on developmental stages in similar‐sized living crinoids. These tiny crinoids, presumably pentacrinoids, are a further example of the extraordinarily detailed preservation of delicate tissues in pyrite from the Hunsrück Slate. They are most likely the pentacrinoid stage from one or more of the crinoid taxa (30 genera) present in the Hunsrück Slate. Assuming these are not microcrinoids, they are the first report of pentacrinoids from the fossil record and document that a Palaeozoic sister group to modern crinoids had similar developmental stages.     

Divergent sympatric lineages of the Atlantic and Indian Ocean crinoid Tropiometra carinata

The shallow water comatulid crinoid Tropiometra carinata is native to both the Atlantic and Indian Oceans, a distribution anomalous among shallow water crinoids and many other broadcast spawning species. Given this species' short pelagic larval duration, the findings of previous work that suggest that the Benguela upwelling is a significant barrier to gene flow in broadcast spawning species, and T. carinata's unexpected geographic distribution, we predicted that the crinoids presently recognized as T. carinata consisted of a species complex. To test this prediction, we sequenced a portion of the mitochondrial cytochrome oxidase 1 gene from 30 individuals of T. carinata collected from Brazil, the Mozambique Channel, Madagascar, and Reunion Island. We found that nucleotide divergence ranged 0.02–3.10% among haplotypes. Moreover, while a Bayesian phylogenetic tree indicated that there were two substantially divergent genetic lineages, there was no evidence to support that T. carinata is comprised of a species complex due to isolation‐by‐distance. Surprisingly, both lineages were found in sympatry in both the Atlantic and Indian Oceans. Likewise, a 95% parsimony haplotype network revealed that identical haplotypes are found in both oceans, suggesting that a species complex may indeed exist, just not one caused by geographic isolation. We discuss possible explanations for this unexpected genetic structure, such as natural dispersal or human‐mediated movement, and how the genetic structure found here is relevant to other marine organisms and to cryptic speciation.     

Haemal and coelomic circulatory systems in the arms and pinnules ofFlorometra serratissima (Echinodermata: Crinoidea)

Summary The haemal and coelomic circulatory systems in arms and pinnules of a stalkless crinoid are described by transmission electron microscopy, and the coelomic topography is revealed by scanning electron microscopy of corrosion casts and peritoneal surfaces. In addition, the route of the coelomic circulation in the living crinoid is shown by injection of carmine particles, and sites of peritoneal phagocytosis are demonstrated by injection of latex beads. The most important morphological findings are: the controversial hyponeural circulation is haemal and not coelomic; peritoneal ciliation is general and not limited to the cells of the ciliated pits; and occur smooth muscle cells occur below the peritoneum. Carmine particles injected into the central body coelom rapidly travel outward toward the arm and pinnule tips via the aboral canals; the particles return to the central body via the subtentacular canals. Latex beads injected intracoelomically are taken up by peritoneal cells throughout the subtentacular, genital and aboral canals. The possible functions of the haemal and coelomic circulatory systems of crinoids are discussed.     

Ultrastructure of the coelom in the brachiopod Lingula anatina

The organization of the coelomic system and the ultrastructure of the coelomic lining are used in phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians. In the current study, the organization of the coelom of the lophophore in the brachiopod Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (= canals). The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium, alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and podocyte‐like cells. Connections between cells of the coelomic lining are provided by adherens junctions, tight‐like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges. The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated myoepithelium can be considered as the ancestral type of coelomic lining. The many different kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial cells and muscle cells.     

A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa,Echinodermata) and its bearing on the evolution of early crinoids

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well‐preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of Eumorphocystis suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.     

Development of Cyathidium foresti (Echinodermata: Crinoidea, Cyrtocrinida) from early attached larvae to adult-like juveniles

To date, knowledge about ontogenetic development of crinoids has been exclusively based on comatulid species, since early stages of other crinoid taxa have not been available so far. The authors now present a scanning electron microscopical and light microscopical study on a developmental series of young sessile individuals of the cyrtocrinid Cyathidium foresti. This species displays a developmental type of its own. In some aspects, the early stages resemble the early attached larva of comatulids (e.g. vestibulum, enteric sac, somatocoelomic cavity) but differ clearly in other (e.g. far oral position of hydrocoelomic primordium, pattern of podia formation, early splitting of the roof, absence of chambered organ). Therefore a specific term is proposed for this new kind of larva: cyathidula. Older juveniles are quite similar to adults; the developmental course is direct. Consequently the group of holopodid crinoids to which Cyathidium and Holopus belong, can be concluded to originate phylogenetically from neotenic larvae.     

Antarctic associations: the parasitic relationship between the gastropod <Emphasis Type="Italic">Bathycrinicola tumidula</Emphasis> (Thiele, 1912) (Ptenoglossa: Eulimidae) and the comatulid <Emphasis Type="Italic">Notocrinus virilis</Emphasis> Mortensen, 1917 (Crinoidea: Notocrinidae) in the Ross Sea

The first case of parasitic association between an eulimid mollusc (Gastropoda, Ptenoglossa) and a comatulid (Echinodermata: Crinoidea) is reported for Antarctica. The mollusc involved in the association is Eulima tumidula Thiele, 1912, which has now been ascribed to the genus Bathycrinicola Bouchet & Warén, 1986, never recognized before in Antarctica. This genus is present only in the NE Atlantic Ocean and the Mediterranean Sea, and encompass species which are specific parasites of the sessile stalked crinoids of the family Bathycrinidae. However, in Antarctica, Bathycrinicola tumidula (Thiele, 1912) exploits the endemic vagile comatulid Notocrinus virilis Mortensen, 1917, and attains the largest known dimensions (∼1 cm) for a Bathycrinicola species. The absence of suitable Bathycrinidae host in modern Antarctic benthic assemblages, as well as the long paleontological history of the genus Notocrinus in Antarctica, suggest a possible ‘host-switch’ phenomenon. This event could reasonably have occurred when many species underwent considerable bathymetric shifts, during the dramatic climatic changes that affected Antarctica.     

Recolonization of the Himerometra robustipinna (Himerometridae, Crinoidea) by macrosymbionts: an in situ experiment

It is generally considered that symbiotic organisms colonize their hosts during their early stages of development. The main goals of the present study were to assess whether post-settled (juvenile and adult) symbionts were able to colonize comatulid crinoids, and whether a hosts’ spatial distribution may influence the colonization pattern through a series of field recolonization experiments. Three series of experiments on recolonization of the comatulid crinoid Himerometra robustipinna were conducted in the Nhatrang Bay, South-China Sea, Vietnam. Ten species of macrosymbiont, 1 polychaetes, 1 gastropods, 1galatheids, 1 ophiurids, and 6 shrimps were found to be associated with H. robustipinna host in the controls and in the 3 experimental series. We found that symbionts rapidly colonized depopulated crinoids in all the experimental series. The prevalence was lower in the experimental series than in the controls butthe abundance, species richness were not significantly different. The presence of post-settled juveniles and adults in experimental series indicated migration from neighboring hosts. Dispersal strategies of symbionts varied: some of them such as the polychaete Paradyte crinoidicola, the gastropod Annulobalcis vinarius, and the galatheid Allogalathea elegans were rapid colonizers. The shrimps Periclimenes commensalis, Pontoniopsis comanthi, and ophiuroid Gymnolophus obscura demonstrated low colonization rate. The 1 and 2 experimental series showed that there was movement of symbionts in dense hosts’ aggregations or over short distances. Unexpectedly, the infestation characteristics of crinoids in the spatially isolated site (series 3) didn’t differ from that of crinoids from aggregations (series 1 and 2), which indicates that long distance (tens meters) migrations of crinoid symbionts also occurs.     

Contractile tissues in the cirri of ancient crinoids: criteria for recognition

Stalked isocrinid and 'stalkless' comatulid crinoids are able to relocate by crawling on or swimming with their muscular arms. Reattachment is achieved using cirri containing contractile tissues which produce aboral flexure. The following cirral adaptations for active attachment were observed during a SEM study of two comasterid comatulids, Davidaster rubiginosa (Pourtalès) and D. discoidea (Carpenter): synarthrial articulations; fulcral ridges corresponding to short axes of ossicles; cirri flattened laterally; each cirrus with a claw at the tip and an aboral attachment pad; cirri serrated aborally and distally. Epizoans are only able to encrust single ossicles in actively motile cirri. These adaptations are associated with crinoids that have muscular arms which are used in relocation. Such a suite of characters is unknown in Palaeozoic crinoids; the ability to relocate only evolved in crinoids during the Mesozoic.□ Crinoids, cirri, comatulids, evoiution, functional morphology .     

Myzostomida from Madagascar, with the description of two new species

Four myzostomidan species were collected during a survey of echinoderms made on the Great Reef of Toliara, southwest Madagascar. The four species were associated with comatulid crinoids. Notopharyngoides aruensis infested the anterior part of the digestive lumen of Stephanometra indica. Comanthussp. aff.wahlbergiiwere infested by three myzostomidan species, two of them are new to science. Myzostoma polycyclus and Myzostoma pseudocuniculus n. sp. lived at the surface of crinoids. Myzostoma toliarense n. sp. live in soft cysts induced on crinoid arms. Cysts are always located close to the ambulacral grooves. They are each infested by one myzostomid.     

Mummified fossil of Keteleeria from the Late Pleistocene of Maoming Basin,South China,and its phytogeographical and paleoecological implications

Keteleeria is a small genus of Pinaceae now mainly restricted to eastern Asia. Although this genus has been documented with a wide distribution in the geologic record of Europe, North America, and Asia, its history in low‐latitude areas (including South China) has remained obscure. In this paper, a fossil wood of Keteleeria sp. is described from the Late Pleistocene (29–27 ka BP) of the Maoming Basin, South China. This wood is the most ancient megafossil evidence of Keteleeria within the modern distribution area of this genus. The fossil records of Keteleeria suggests that this thermophyllous genus migrated into South China by the Middle Pleistocene escaping from glacial cooling and became widespread over this region in the Late Pleistocene beginning from the interglacial stage preceding the Last Glacial Maximum. The analysis of growth rings in the fossil wood and its comparison with those of modern Keteleeria davidiana (Bertrand) Beissner indicates that in the Late Pleistocene of Maoming Basin (29–27 ka BP) there was a humid climate with less pronounced seasonality of precipitation than that seen in the subtropical monsoonal climate of modern northeastern Vietnam. Apparently, the Maoming Basin was influenced by interglacial regime with summer–monsoon circulation. The previously proposed method to distinguish between evergreen and deciduous conifers based on growth ring anatomy, is not reliable because of the wide variance and ambiguity in its results.     

Leaf surface development and the plant fossil record: stomatal patterning in Bennettitales

Stomata play a critical ecological role as an interface between the plant and its environment. Although the guard‐cell pair is highly conserved in land plants, the development and patterning of surrounding epidermal cells follow predictable pathways in different taxa that are increasingly well understood following recent advances in the developmental genetics of the plant epidermis in model taxa. Similarly, other aspects of leaf development and evolution are benefiting from a molecular–genetic approach. Applying this understanding to extinct taxa known only from fossils requires use of extensive comparative morphological data to infer ‘fossil fingerprints’ of developmental evolution (a ‘palaeo‐evo‐devo’ perspective). The seed‐plant order Bennettitales, which flourished through the Mesozoic but became extinct in the Late Cretaceous, displayed a consistent and highly unusual combination of epidermal traits, despite their diverse leaf morphology. Based on morphological evidence (including possession of flower‐like structures), bennettites are widely inferred to be closely related to angiosperms and hence inform our understanding of early angiosperm evolution. Fossil bennettites – even purely vegetative material – can be readily identified by a combination of epidermal features, including distinctive cuticular guard‐cell thickenings, lobed abaxial epidermal cells (‘puzzle cells’), transverse orientation of stomata perpendicular to the leaf axis, and a pair of lateral subsidiary cells adjacent to each guard‐cell pair (termed paracytic stomata). Here, we review these traits and compare them with analogous features in living taxa, aiming to identify homologous – and hence phylogenetically informative – character states and to increase understanding of developmental mechanisms in land plants. We propose a range of models addressing different aspects of the bennettite epidermis. The lobed abaxial epidermal cells indicate adaxial–abaxial leaf polarity and associated differentiated mesophyll that could have optimised photosynthesis. The typical transverse orientation of the stomata probably resulted from leaf expansion similar to that of a broad‐leaved monocot such as Lapageria, but radically different from that of broad‐leafed eudicots such as Arabidopsis. Finally, the developmental origin of the paired lateral subsidiary cells – whether they are mesogene cells derived from the same cell lineage as the guard‐mother cell, as in some eudicots, or perigene cells derived from an adjacent cell lineage, as in grasses – represents an unusually lineage‐specific and well‐characterised developmental trait. We identify a close similarity between the paracytic stomata of Bennettitales and the ‘living fossil’ Gnetum, strongly indicating that (as in Gnetum) the pair of lateral subsidiary cells of bennettites are both mesogene cells. Together, these features allow us to infer development in this diverse and relatively derived lineage that co‐existed with the earliest recognisable angiosperms, and suggest that the use of these characters in phylogeny reconstruction requires revision.     

The poorly illustrated crinoid

Donovan, S.K. 2011: The poorly illustrated crinoid. Lethaia, Vol. 44, pp. 125–135. Artistic licence is kept under firm control when restoring fossil tetrapods and their natural environments, but the same care is not always applied to reconstructions of ancient invertebrates. Selected renderings of fossil crinoids illustrate grossly inaccurate skeletal geometry and outmoded ideas of palaeoautecology. Together, these combine to give the public an incorrect impression of what a fossil crinoid looked like and how it lived, in some instances inferior to what was possible in the 19th century. The reason why crinoids receive such poor service from artists is, in part, probably due to their being exotic; humans and other tetrapods are a common part of our experience, whereas stalked crinoids, although extant, can only be seen in life with the aid of a research submersible in deep water (>100 m). Crinoids also have a complex endoskeleton that requires care in illustration. They are unusual creatures in an alien environment even at the present day. But echinoderm palaeontologists need to be more energetic in promoting the correct depiction of ancient species. □Crinoidea, illustration, morphology, palaeoenvironments, reconstruction.     

The complete mitochondrial genomes of the sea lily Gymnocrinus richeri and the feather star Phanogenia gracilis: signature nucleotide bias and unique nad4L gene rearrangement within crinoids

Complete DNA sequences have been determined for the mitochondrial genomes of the crinoids Phanogenia gracilis (15892 bp) and Gymnocrinus richeri (15966 bp). The mitochondrial genetic map of the stalkless feather star P. gracilis is identical to that of the comatulid feather star Florometra serratissima (Scouras, A., Smith, M.J., 2001. Mol. Biol. Evol. 18, 61-73). The mitochondrial gene order of the stalked crinoid G. richeri differs from that of F. serratissima and P. gracilis by the transposition of the nad4L protein gene. The G. richeri nad4L mitochondrial map position is unique among metazoa and is likely a derived feature in this stalked crinoid. Nucleotide compositional analyses of protein genes encoded on the major sense strand confirm earlier conclusions regarding a crinoid-distinctive T over C bias. All three crinoids exhibit high T levels in third codon positions, whereas other echinoderm classes favor A or C in the third codon position. The nucleotide bias is reflected in the relative synonymous codon usage patterns of crinoids versus other echinoderms. We suggest that the nucleotide bias of crinoids, in comparison to other echinoderms, indicates that a physical inversion of the origin of replication has occurred in the crinoid lineage. Evolutionary rate tests support the use of the cytochrome b (cob) gene in molecular phylogenetic analyses of echinoderms. A consensus echinoderm tree was generated based on cytochrome b nucleotide alignments that placed the asteroids as a sister group to a clade containing the ophiuroids and the (echinoids+holothuroids) with the crinoids basal to the rest of the echinoderm classes: [Crinoid,(Asteroid,(Ophiuroid,(Echinoid,Holothuroid)))].     

Myoanatomy of Myzostoma cirriferum (Annelida,Myzostomida): Implications for the evolution of the myzostomid body plan

Studies of rare genomic marker systems suggest that Myzostomida are a subgroup of Annelida and phylogenomic analyses indicate an early divergence of this taxon within annelids. However, adult myzostomids show a highly specialized body plan, which lacks typical annelid features, such as external body annulation, coelomic cavities with metanephridia, and segmental ganglia of the nervous system. The putative loss of these features might be due to the parasitic/symbiotic lifestyle of myzostomids associated with echinoderms. In contrast, the larval anatomy and adult locomotory system resemble those of annelids. To clarify whether the myoanatomy of myzostomids reflects their relationship to annelids, we analyzed the distribution of f‐actin, a common component of muscle fibers, in specimens of Myzostoma cirriferum using phalloidin‐rhodamine labeling in conjunction with confocal laser‐scanning microscopy. Our data reveal that the musculature of the myzostomid body comprises an outer circular layer, an inner longitudinal layer, numerous dorsoventral muscles, and prominent muscles of the parapodial complex. These features correspond well with the common organization of the muscular system in Annelida. In contrast to other annelids, however, several elements of the muscular system in M. cirriferum, including the musculature of the body wall, and the parapodial flexor muscles, exhibit radial symmetry overlaying a bilateral body plan. These findings are in line with the annelid affinity of myzostomids and suggest that the apparent partial radial symmetry of M. cirriferum arose secondarily in this species. Based on our data, we provide a scenario on the rearrangements of muscle fibers that might have taken place in the lineage leading to this species. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Development of the coelomic cavities in larvae of the living isocrinid sea lily Metacrinus rotundus

Coelomogenesis in the isocrinid sea lily, Metacrinus rotundus, is described through the swimming larval stages. After the late gastrula stage, the archenteron separates from the ectoderm to form an archenteral sac, which develops into a dumbbell shape consisting of anterior and posterior lobes, and a middle part connecting both lobes. The anterior and posterior lobes, and the middle part, become separated into an axo-hydrocoel, the left and right somatocoels and an enteric sac, respectively. The hydrocoel forms from the left lower edge of the axo-hydrocoel and becomes separated from the axocoel by the late dipleurula stage, when chambered organs and coelom X bud off from the anterior tip of the right and left somatocoels, respectively. Coelom X does not occur in comatulid crinoids (feather stars), and its fate is unclear. The pore canal extends from the axocoel. The hydrocoel differentiates into a crescent shape at the overtime semidoliolaria stage, a few days after the semidoliolaria becomes competent to settle. Coelomogenesis in M. rotundus is much simpler than in the comatulids and probably represents the ancestral mode of the crinoids. As each portion of the dumbbell sac differentiates almost in situ into each coelom, presumptive fates in the sac are easily followed in M. rotundus.