A model for the morphogenesis of strip reduction patterns in phototrophic euglenids: evidence for heterochrony in pellicle evolution

We propose a general developmental model that explains the evolutionary origin, diversification, and inheritance of pellicle strip patterns in phototrophic euglenids. Dividing cells of Euglena gracilis, E. viridis, and Phacus similis were observed with scanning electron microscopy in order to study the morphogenesis of posterior whorls of strip reduction. We found evidence that constant whorl numbers are maintained through cell division because of organized strip growth before and during cytokinesis. Alternating nascent strips form a new whorl of strip reduction at each of the anterior and posterior ends of daughter cells. Strips that terminated to form posterior whorls in the mother cell change in length during the development of daughter cells. In the mother cells of E. gracilis, the strips forming whorls I and II grow to become whorls II and III, respectively, in the daughter cells; the strips forming whorl III in the mother cell lengthen and meet with other strips already present at the posterior tip of daughter cells. This process of whorl morphogenesis during asexual reproduction is consistent with known variation in pellicle strip patterns and suggests that heterochrony played a major role in the ultrastructural evolution of phototrophic euglenids.     

Constraint and the square snail: life at the limits of a covariance set. The normal teratology of Cerion disforme

Two major developmental constraints, identified and assessed by the covariance structure of ontogenetic measures, strongly influence the form and variation of Cerion in every study I have ever undertaken on this protean genus: the jigsaw constraint of large-and few vs small-and-many whorls for shells reaching a similar final size; and the compensatory constraint of later heights balancing juvenile widths to bring adult shells into a limited range of final proportions.
This study explores a set of taxa (the Cerion dimidiatum complex of eastern Cuba) growing at one extreme of the compensatory constraint–maximally flat juvenile shells followed by abrupt allometric transition to orthogonal adult whorls. I show that the most curious of all Cerion taxa, the teratologously uncoiling C. disforme , owes its unique and distinctive form to growth at an extreme of the compensatory constraint: the juvenile cross-section goes beyond mere flatness to actual concavity; and the adult shell, after maximally abrupt allometric transition to downward growth, loses contact with the juvenile whorls and begins to unwind in teratologic fashion. I present several categories of evidence to demonstrate that C. disforme is the extreme in a morphocline of taxa, whose correlated features express increasing discordance (and correlated sequelae of size and shape) under the compensatory constraint: C. geophilus – C. dimidiatum – C. alberli – C.disforme. I provide data for clines in morphology, in amount and character of variation, and in mode of growth. This genre of constraint–Darwin's old "correlations of growth"–must be given equal weight with immediate adaptation in a unified explanation of growth and form.     

THE CONSEQUENCES OF BEING DIFFERENT: SINISTRAL COILING IN CERION

The overwhelming predominance of dextral coiling in gastropods is an outstanding and puzzling phenomenon. A few sinistral specimens (left coiling individuals) have been found in many dextral species. Only six sinistral shells have ever been found in Cerion; we base this analysis on the five available shells. We ask whether reversed symmetry is a simple either-or switch without further consequences for shell form, or whether sinistrality engenders associated effects, making left-coiling shells unlike their dextral deme-mates in other ways. All five sinistral shells differ in features of size and coiling late in growth, leading to relatively small apertures and a slight twist in the axis of coiling. We detect and measure this effect as follows: in multivariate morphospace, sinistrals occupy peripheral positions among their dextral deme-mates; in univariate analysis, sinistrals are consistently different for a set of characters involving covariance patterns never before seen in a decade of studies on ontogenetic and age-standardized variation in dextrals; a bootstrap procedure does not recover similar patterns in randomly constituted samples of dextrals matching the true sinistral distribution; direct x-ray measures of the coiling axis detect its slight twist in sinistrals. We discuss the implications of these unsuspected associations for the issues of developmental constraint upon the evolution of morphology.     

Morphological Features of Supralittoral Mollusks of the Genus Cecina (Gastropoda: Pomatiopsidae) from Peter the Great Bay, Sea of Japan

Amphibious mollusks of the genus Cecina in the littoral and supralittoral of the Sea of Japan are represented by three species: Cecina manchurica A. Adams, 1861; C. tatarica (Schrenck, 1867); and C. scarlatoi Prozorova, 1996. Additional data on the morphology of the shell and radula are provided. Indices for the identification of adult decollated and corroded shells of these species which retained only 3.0–2.5 last whorls are proposed: (1) the ratio of the width of the ultimate whorl (without aperture) to the length of the ultimate and penultimate whorls and (2) the ratio of the width of the third whorl (from the bottom) to the width of the ultimate whorl (without aperture).     

COMPUTER-SIMULATED SHELL SIZE AND SHAPE VARIATION IN THE CARIBBEAN LAND SNAIL GENUS CERION: A TEST OF GEOMETRICAL CONSTRAINTS

A computer graphical model of gastropod shell form is used to test a hypothesis of geometric constraint proposed to explain the disjunct distribution of shell forms observed in Cerion, a species-rich and geometrically varied genus of terrestrial gastropods. The mapping of computer-simulated forms into a morphospace of Cerion shells produces a continuum of sizes and shapes. Therefore, the absence of particular shell forms is not explained by geometric constraints. Two proposed modes of shell morphogenesis at extreme ranges in size (“dwarfs” and “giants”) previously were thought to be exclusive routes to the construction of high-spired (“smokestack”) forms. The present study shows that there are, in fact, multiple routes of transformation. In addition, these routes are geometrically reversible and interconnect the members of the shell-form continuum. Thus, the possible pathways followed during the course of evolution within this genus cannot be determined until an adequate phylogenetic hypothesis has been proposed.     

SYMMETRY IN EQUISETUM

Bierhorst , David W. (Cornell U., Ithaca, N. Y.) Symmetry in Equisetum. Amer. Jour. Bot. 46(3) : 170-179. Illus. 1959.—A total of 118 leaf whorls and corresponding nodes from a total of 9 species of Equisetum were studied in serial cross-section. The number of whorls having the same leaf number as the 2 adjacent ones was 67. The size of the arc measured in degrees of circumference occupied by each leaf, as well as its position relative to leaves of adjacent whorls were measured. The disposition of the internodal and trans-nodal vascular strands was determined for each of the nodes. The average per cent deviations from theoretical leaf size within whorls were: whorls not involved in change in leaf number, 3.9; those with fewer leaves than the one below, 9.5; those with more leaves than one above, 7.1; those with more leaves than one below, 9.4; those with different leaf number from the 2 adjacent ones, 12.5. In Equisetum, it is a general rule that (1) a leaf which falls directly above a leaf in the next whorl where the younger whorl possesses fewer leaves than the older one is usually the largest leaf within its whorl, (2) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses fewer leaves than the older one is usually the smallest leaf within its whorl, (3) a leaf which falls directly above a leaf in the next older whorl where the younger whorl possesses a greater number of leaves than the older one is usually the smallest leaf in its whorl, and (4) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses a greater number of leaves than the older one is usually the largest leaf within its whorl. The numerous variations in the disposition of vascular tissue associated with changes in leaf number are described. Double leaf traces which originate in various ways are common, as well as vertical strands traversing the nodes. The leaf trace system is considered to be determined by the number, position, and relative sizes of leaf primordia. The disposition of the trans-nodal protoxylem seems to be determined to a large extent by the proximity of the leaf traces above and below the node.     

Expression of AODEF,a B-functional MADS-box gene,in stamens and inner tepals of the dioecious species Asparagus officinalis L.

Garden asparagus (Asparagus officinalis L.) is a dioecious species with male and female flowers on separate unisexual individuals. Since B- and C-functional MADS-box genes specify male and female reproductive organs, it is important to characterize these genes to clarify the mechanism of sex determination in monoecious and dioecious species. In this study, we isolated and characterized AODEF gene, a B-functional gene in the development of male and female flowers of A. officinalis. Southern hybridization identified a single copy of AODEF gene in asparagus genome. Northern blot analysis showed that this gene was specifically expressed in flower buds and not in vegetative tissues. In situ hybridization showed that during early hermaphrodite stages, AODEFgene was expressed in the inner tepal and stamen whorls (whorls 2 and 3, respectively), but not in the outer tepals (whorl 1), in both male and female flowers. In late unisexual developmental stages, the expression of AODEF gene was still detected in the inner tepals and stamens of male flowers, but the expression was reduced in whorls 2 and 3 of female flowers. Our results suggest that AODEF gene is probably not involved in tepal development in asparagus and that the expression of AODEF gene is probably controlled directly or indirectly by sex determination gene in the late developmental stages.     

How do gastropods grow synchronized shell sculpture? Effect of experimental varix manipulations on shell growth by Ceratostoma foliatum (Muricidae: Ocenebrinae)

Gastropod shells display striking patterns in both color and sculpture, but rather little is known about the developmental mechanisms that produce those patterns. Here, we tested a physical feedback hypothesis for how snails control spatial patterning of shell sculpture. Varices—a form of synchronized, blade‐like axial sculpture—are produced at regular intervals around the shell and often aligned closely between adjacent whorls. Older varices were believed to provide a spatial cue about where to position a new varix. To test this hypothesis, we manipulated physical cues by cutting off varices or attaching new ones to the body whorl of individuals of Ceratostoma foliatum, and then allowing snails to grow a new varix. We found that previous varices on the shell were neither necessary nor sufficient to induce a new varix at a particular location. However, the position of older varices did appear to affect the fine tuning of subsequent varix placement. The results of our experiments therefore suggest that varix synchrony arises mainly from some internal mechanism that yields a standardized amount of spiral growth per growth spurt. We also found that shell damage can induce varix production in unusual or aberrant locations during subsequent shell growth.     

North American black-fruited hawthorns. II. Floral development of 10- and 20-stamen morphotypes in Crataegus section Douglasii (Rosaceae: Maloideae)

Crataegus section Douglasii exhibits variation in stamen number per flower typical for the genus throughout North America. To understand the developmental basis for this variation we studied the early floral ontogeny of the three taxa in section Douglasii: C. douglasii (both Pacific northwest and the upper Great Lakes basin), C. rivularis, and C. suksdorfii. Crataegus suksdorfii, like all known diploid Crataegus, has ≈20 stamens; the two other taxa have ≈10 stamens, a condition associated only with polyploidy. In all taxa petal primordia and a whorl of five pairs of stamen primordia develop from five common primordia. The 10-stamen∗∗∗ condition results from loss of two whorls of five stamens that are subsequently formed in C. suksdorfii. Loss of these two whorls in the 10-stamen taxa is the result of neither a smaller floral apex at initiation, nor a smaller flower at anthesis. Stamen number variability, particularly in C. douglasii and C. rivularis, is the result predominantly of fewer than two stamen primordia developing between adjacent petal primordia. Pollen production in C. douglasii is half that in C. suksdorfii because of the reduction in stamen number. The results are presented and discussed in terms of morphogenetic explanations of meristic variation.     

FLORAL DEVELOPMENT IN MANGROVE RHIZOPHORACEAE

The flowers of mangrove Rhizophoraceae (tribe Rhizophoreae) are adapted to three different pollination mechanisms. Floral development of representative species of all four genera suggests that the ancestral flower of the tribe was unspecialized, with successively initiated whorls of separate sepals, petals, antisepalous stamens, and antipetalous stamens; at its inception, the gynoecium had a united, half-inferior ovary and separate stigmatic lobes. This developmental pattern is found in Rhizophora mangle (wind-pollinated) and Ceriops decandra (insect-pollinated). In Kandelia, all floral organs distal to the sepals are initiated simultaneously, and there has apparently been an evolutionary amplification in the number of stamens to about six times the number of petals. Explosive pollen release evolved independently in C. tagal and in Bruguiera. In the former, all stamens belong to one whorl and arise simultaneously upon a very weakly differentiated androecial ring primordium. In Bruguiera, the androecial ring is pronounced, and two whorls of stamens arise upon it; the primordia of the antisepalous whorl arise first but are closer to the center of the apex than the antipetalous stamen primordia. The antisepalous stamens bend toward and are enclosed by the petals early in development. In all genera, the inferior ovary develops by zonal growth of receptacular tissue; additional intercalary growth above the placenta occurs in Bruguiera. In general, floral specialization is accompanied by an increase in the width of the floral apex compared to the size of the primordia, increasing fusion of the stylar primordia, and decreasing prominence of the superior portion of the ovary. Apparent specializations of petal appendages for water storage, including the presence of sub-terminal hydathodes (previously unreported in any angiosperm), were found in two species in which flowers remain open during the day but were absent from two species normally pollinated at night or at dawn. Distinctive tribal characteristics that may aid in phylogenetic analysis include the mode of development of the inferior ovary; the aristate, bifid, usually fringed petals that individually enclose one or more stamens; the intrastaminal floral disc; and the initially subepidermal laticiferous cell layer in the sepals and ovary.     

History as a cause of area effects: an illustration from Cerion on Great Inagua, Bahamas

The two parts of this paper work towards the common aim of setting contexts for and documenting explanations based on historical contingencies. The first part is a review of area effects in Cepaea. We discuss the original definitions and explanations, emphasizing the debate of adaptationist vsstochastic approaches, but arguing that the contrast of historical contingency vs. selective fit to environment forms a more fruitful and fundamental context in discussing the origin of area effects. We argue that contingencies of bottlenecks and opening of formerly unsuited habitats may explain the classic area effects of Cepaea better than selectionist accounts originally proposed. The second part is a documentation of an area effect within Cerion columnaon the northern coast of Great Inagua, Bahamas. Historical explanations are often plagued by insufficiency of preserved information, but the Inagua example provides an unusual density of data, with several independent criteria all pointing to the same conclusion. Shells in the area effect are squat and flat-topped in contrast with typical populations of long, thin, tapering shells living both east and west of the area effect. The flat-topped area effect is a result of introgression with a propagule of the C. dimidiatum stock (living on nearby Cuba, and most apically flattened of all Cerion). Fossils of this propagule were found fully cemented into highly indurated fossil soil crusts within the region of the current area effect. Multivariate morphometry, based on complex patterns of covariation, not just intermediacy in single characters, identifies the area effect samples as hybrids between this propagule and typical C. columna. Genetic analysis has identified three unexpected alleles in area effect samples only, and in no other snails of any other Cerion taxon anywhere else on Inagua. We hypothesize that the flattopped area effect did not arise as a selective response to local environments within C. columna, but by introgression from a fortuitously introduced propagule of the C. dimidiaium complex. The unexpected alleles therefore represent genetic phantoms of C. dimidiatum's former presence or are hybrizymes—novel alleles produced by interspecific hybridization     

Within and between Whorls: Comparative Transcriptional Profiling of Aquilegia and Arabidopsis

Background

The genus Aquilegia is an emerging model system in plant evolutionary biology predominantly because of its wide variation in floral traits and associated floral ecology. The anatomy of the Aquilegia flower is also very distinct. There are two whorls of petaloid organs, the outer whorl of sepals and the second whorl of petals that form nectar spurs, as well as a recently evolved fifth whorl of staminodia inserted between stamens and carpels.

Methodology/Principal Findings

We designed an oligonucleotide microarray based on EST sequences from a mixed tissue, normalized cDNA library of an A. formosa x A. pubescens F2 population representing 17,246 unigenes. We then used this array to analyze floral gene expression in late pre-anthesis stage floral organs from a natural A. formosa population. In particular, we tested for gene expression patterns specific to each floral whorl and to combinations of whorls that correspond to traditional and modified ABC model groupings. Similar analyses were performed on gene expression data of Arabidopsis thaliana whorls previously obtained using the Ath1 gene chips (data available through The Arabidopsis Information Resource).

Conclusions/Significance

Our comparative gene expression analyses suggest that 1) petaloid sepals and petals of A. formosa share gene expression patterns more than either have organ-specific patterns, 2) petals of A. formosa and A. thaliana may be independently derived, 3) staminodia express B and C genes similar to stamens but the staminodium genetic program has also converged on aspects of the carpel program and 4) staminodia have unique up-regulation of regulatory genes and genes that have been implicated with defense against microbial infection and herbivory. Our study also highlights the value of comparative gene expression profiling and the Aquilegia microarray in particular for the study of floral evolution and ecology.     

GEOGRAPHIC VARIATION AND HETEROCHRONY IN TWO SPECIES OF COWRIES (GENUS CYPRAEA)

Geographic variation in the marine, Indo-Pacific cowry, Cypraea caputserpentis, involves clinal variations that parallel the ontogenetic development of adult shell characteristics. Cypraea caputdraconis, a closely related species endemic to Easter Island and Sala y Gómez, is morphologically similar to juvenile C. caputserpentis. Using multivariate measures of size and shape, I examine these patterns as a possible outcome of heterochrony, or changes in the timing of developmental events in ontogeny. Whorl-expansion rates of juvenile shells are significantly higher in C. caputdraconis when compared to C. caputserpentis and are negatively correlated with surface seawater temperatures among populations of C caputserpentis. High expansion rates, often associated with slow growth, result in a delay in the onset of lateral callus development and subsequent paedomorphosis. Ontogenetic trajectories calculated from growth series of adult and preadult shells indicate that paedomorphosis results from the combined effects of neoteny and post-displacement. Paedomorphosis among cowries may result from the advantages of larger body size relative to shell size under reduced predation intensities and associated increases in fecundity.     

Shell structure of two polar pelagic molluscs, Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica

The purpose of this study was to investigate shell growth performance in two thin-shelled pelagic gastropods from cold seawater habitats. The shells of Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica are very thin, approximately 2–9 μm for shells of 0.5–6 mm in diameter. Many axial ribbed growth lines were observed on the surface of the shell of both Limacina species. Distinct axial ribs were observed on the outermost whorl, while weak or no rib-like structures were observed on the inner whorls in the larger shell of L. helicina antarctica forma antarctica. For L. helicina, no ribs were observed on small individuals with three whorls, while larger individuals had distinct ribs on the outer whorls. Shell microstructure was examined in both species. There is an inner crossed-lamellar and extremely thin outer prismatic layer in small individuals of both species, and a distinct thick inner prismatic layer was observed beneath the crossed-lamellar layer in large Antarctic individuals. Various orientations of the crossed-lamellar structure were observed in one individual. Shell structure appeared to be different between the Antarctic and Arctic species and among shells of different size.     

UNIDIRECTIONAL ORGAN INITIATION IN LEGUMINOUS FLOWERS

The order of initiation of floral organs is compared in several legumes. In Bauhinia fassoglensis, a caesalpinioid, the sepals are initiated helically, with the first one forming abaxially. In Genista tinctoria and Lupinus affinis (both papilionoids) the sepals are initiated unidirectionally, with the first forming on the abaxial side of the floral apex and subsequent sepals initiating laterally and then adaxially. All three taxa show unidirectional order of initiation for petals, first-whorl stamens, and second-whorl stamens. In each whorl, the first member or members form on the abaxial side, next to the subtending bract, then the lateral ones, and lastly the member(s) on the adaxial side, next to the axis. In Lupinus and Genista there are overlaps in time of initiation between organs in different whorls; for instance, the first stamens begin initiating before the last petals appear. Size differences among members of a whorl are evident in early stages, but may disappear after organogeny ceases, when the members become equal in size in each whorl. This precocious onset of dorsiventrality in floral development is viewed as a specialized feature.     

CaMADS1, an AGAMOUS homologue from hazelnut, produces floral homeotic conversion when expressed in Arabidopsis

CaMADS1 is a floral-specific MADS box gene of hazelnut (Corylus avellana) which, according to its sequence and expression pattern, belongs to the AGAMOUS gene sub-family. To investigate whether CaMADS1 plays a role in specifying stamen and carpel identity, this gene was ectopically expressed in Arabidopsis. The constitutive expression of CaMADS1 in transgenic plants produced the homeotic conversion of first and second whorl organs: the first whorl exhibited carpelloid sepals and the second whorl showed staminoid features. This was expected on the basis of the ABC model, according to which ectopic expression of a functional AGAMOUS (a gene of class C) orthologue would suppress the A class homeotic function in the first and second whorls, leading to transformation of these whorls into carpels and stamen, respectively. These results indicate a functional equivalency between AGAMOUS and CaMADS1, for which CaMADS1 might behave like a class C homeotic gene, controlling the determination of stamen and carpel identity in hazelnut Received: 31 July 2000 / Revision accepted: 28 September 2000     

THE EVOLUTION OF SOCIAL SIGNALS: MORPHOLOGICAL,FUNCTIONAL, AND GENETIC INTEGRATION OF THE SEX PHEROMONE IN NAUPHOETA CINEREA

Social signals that mediate intraspecific interactions can be complex, conveying considerable information concerning the probable behavior of individuals and minimizing overt aggression and wasted energy. In the cockroach Nauphoeta cinerea, male-male competition and female mate choice are mediated by a multicomponent male-produced sex pheromone. In this study, I examine variation in this pheromone. First I measure differences among males in both individual pheromone compounds and the overall composition of the pheromone. Principal component analysis is used to quantify and describe pheromone composition. Next, I explore some of the causes and consequences of this variation by examining the pheromone of males with different social experiences. Compared to subordinate males, dominant males have significantly less variable quantities of the individual pheromone compounds and are significantly less variable in the composition of their pheromone. Because of an association between status and mating success, male-male competition can result in stabilizing sexual selection on the sex pheromone. Finally, I test the hypothesis that the pheromone compounds evolve in a manner consistent with their function. As predicted for morphologically integrated characters, the patterns of phenotypic, genetic, and environmental correlations among my measures of pheromone compounds and composition match functional patterns suggested by this study and the developmental patterns demonstrated in my previous studies. Based on these studies of the N. cinerea sex pheromone, I argue that stabilizing sexual selection shapes the evolution of pheromonal communication involved in social interactions among male N. cinerea. Further, I argue that coordinated evolution of social signals may be possible due to the morphological integration of their multiple compounds.     

Evolution of petaloid sepals independent of shifts in B-class MADS box gene expression

Attractive petals are an integral component of animal-pollinated flowers and in many flowering plant species are restricted to the second floral whorl. Interestingly, multiple times during angiosperm evolution, petaloid characteristics have expanded to adjacent floral whorls or to extra-floral organs. Here, we investigate developmental characteristics of petaloid sepals in Rhodochiton atrosanguineum, a close relative of the model species Antirrhinum majus (snapdragon). We undertook this in two ways, first using scanning electron microscopy we investigate the micromorphology of petals and sepals, followed by expression studies of genes usually responsible for the formation of petaloid structures. From our data, we conclude that R. atrosanguineum petaloid sepals lack micromorphological characteristics of petals and that petaloid sepals did not evolve through regulatory evolution of B-class MADS box genes, which have been shown to specify second whorl petal identity in a number of model flowering plant species including snapdragon. These data, in conjunction with other studies, suggests multiple convergent pathways for the evolution of showy sepals.     

NORMAL FLORAL ONTOGENY AND COOL TEMPERATURE-INDUCED ABERRANT FLORAL DEVELOPMENT IN GLYCINE MAX (FABACEAE)

Floral onset in soybean (Glycine max cv. Ransom) is characterized by precocious initiation of axillary meristems in the axils of the most recently initiated leaf primordium. During floral transition, leaf morphology changes from trifoliolate leaf with stipules, to a three-lobed bract, to an unlobed bract. Soybean flowers initiated at 26/22 C day/night temperatures are normal, papilionaceous, and pentamerous. Sepal, petal, and stamen whorls are initiated unidirectionally from the abaxial to adaxial side of the floral apex. The median sepal is located abaxially and the median petal adaxially on the meristem. The organogeny of ‘Ransom’ flowers was found to be: sepals, petals, outer stamens plus carpel, inner stamens; or, sepals, petals, carpel, outer stamens, inner stamens. The outer stamen whorl and the carpel show possible overlap in time of initiation. Equalization of organ size occurs only within the stamen whorls. The sepals retain distinction in size, and the petals exhibit an inverse size to age relationship. The keel petals postgenitally fuse along part of their abaxial margins; their bases, however, remain free. Soybean flowers initiated at cool day/night temperatures of 18/14 C exhibited abnormalities and intermediate organs in all whorls. The gynoecium consisted of one to ten carpels (usually three or four), and carpel connation varied. Fusion of keel petals was often lacking, and stamen filaments fused erratically. Multiple carpellate flowers developed into multiple pods that were separate or variously connate. Intermediate type organs had characteristics only of organs in adjacent whorls. These aberrant flowers demonstrate that the floral meristem of soybean is not fixed or limited in its developmental capabilities and that it has the potential to produce alternate morphological patterns.     

Shell morphology and microstructure of Neogene <Emphasis Type="Italic">Melanopsis</Emphasis> Ferussac (Gastropoda)

Representatives of the genus Melanopsis have an extraconch, a shell structure unusual for gastropods, which envelops several whorls, including the last whorl and up to two preceding whorls. Microstructural features of the shell are studied for the first time, using M. impressa Krauss as an example. It is shown that the extraconch is formed by the outer crossed-lamellar layer.