Functional morphology of coronal and peristomeal podia in Sphaerechinus granularis (Echinodermata,Echinoida)

Summary Coronal podia of Sphaerechinus granularis are anchoring (adhering) appendages involved in either locomotion or capture of drift materials. Adhesion is not due to the presumed sucker action of the disc but relies entirely on secretions of the disc epidermis. Peristomeal podia function in wrapping together food particles or food fragments in an adhesive material thus facilitating their capture by the Aristotle's lantern. In both types of podia, the disc epidermis is made up of four cell types: non-ciliated secretory cells (NCS cells) that contain graules whose content is at least partly mucopolysaccharidic in nature, ciliated secretory cells (CS cells) containing granules of unknown nature, ciliated non-secretory cells (CNS cells) and support cells. The cilia of CS cells are subeuticular whereas those of CNS cells, although also short and rigid, traverse the cuticle and protrude in the outer medium. All these cells are presumably involved in an adhesive/de-adhesive process functioning as a duogland adhesive system. Adhesive secretion would be produced by NCS cells and de-adhesive secretion by CS cells. These secretions would be controlled through stimulations by the two types of ciliated cells (receptor cells) which presumably interact with the secretory cells by way of the nerve plexus. This model of adhesion/de-adhesion fits well with the activities of both coronal and peristomeal podia. The secretion of NCS cells would make up a bridge of adhesive material between a podium and the substratum (coronal podia) or would coat and gather food particles (peristomeal podia), respectively. The de-adhesive material enclosed in the granules of CS cells would allow the podia (either coronal or peristomeal) to easily become detached from the substratum and to always remain clear of any particles.Research Assistant, National Fund for Scientific Research (Belgium)     

Adaptations to benthic development: functional morphology of the attachment complex of the brachiolaria larva in the sea star Asterina gibbosa

The asteroid Asterina gibbosa lives all its life in close relation to the sea bottom. Indeed, this sea star possesses an entirely benthic, lecithotrophic development. The embryos adhere to the substratum due to particular properties of their jelly coat, and hatching occurs directly at the brachiolaria stage. Brachiolariae have a hypertrophied, bilobed attachment complex comprising two asymmetrical brachiolar arms and a central adhesive disc. This study aims at describing the ultrastructure of the attachment complex and possible adaptations, at the cellular level, to benthic development. Immediately after hatching, early brachiolariae attach by the arms. All along the anterior side of each arm, the epidermis encloses several cell types, such as secretory cells of two types (A and B), support cells, and sensory cells. Like their equivalents in planktotrophic larvae, type A and B secretory cells are presumably involved in a duo-glandular system in which the former are adhesive and the latter de-adhesive in function. Unlike what is observed in planktotrophic larvae, the sensory cells are unspecialized and presumably not involved in substratum testing. During the larval period, the brachiolar arms progressively increase in size and the adhesive disc becomes more prominent. At the onset of metamorphosis, brachiolariae cement themselves strongly to the substratum with the adhesive disc. The disc contains two main cell types, support cells and secretory cells, the latter being responsible for the cement release. During this metamorphosis, the brachiolar arms regress while post-metamorphic structures grow considerably, especially the tube feet, which take over the role of attachment to the substratum. The end of this period corresponds to the complete regression of the external larval structures, which also coincides with the opening of the mouth. This sequence of stages, each possessing its own adhesive strategy, is common to all asteroid species having a benthic development. In A. gibbosa, morphological adaptations to this mode of development include the hypertrophic growth of the attachment complex, its bilobed shape forming an almost completely adhesive sole, and the regression of the sensory equipment.     

Structure of the organs of attachment of brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) (Echinodermata: Asteroidea)

The structure of the brachiolar arms and adhesive disk of the brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) was determined from light microscopy and from scanning and transmission electron microscopy. The structure of these organs was very similar in both species.The brachiolar arms are comprised of a stem region terminating in a crown of adhesive papillae which are made up of a variety of secretory cell types. Principal among these are elongated cells producing very electron-dense secretory particles, which are released at the free cell surface attached to cilia. Secretory particles appear to be important in temporary attachment of the brachiolar arms to the substratum. Ciliary sense cells, possibly used in the recognition of specific substrata are located at the tip of adhesive papillae.The adhesive disk is comprised of large cells packed with secretory droplets and elongated intracellular fibres. In the attached adhesive disk, secretory droplets are lost, having formed the cement that attaches the disk to the substratum. It appears that adhesive papillae lateral to the adhesive disk hold the disk in position close to the substratum during secretion and hardening of the cement. The intracellular fibres are the principal anchoring structures running from microvilli (locked into the attachment cement) on the surface of the disk to the underlying connective tissue of the attachment stalk.     

Functional morphology of the locomotory podia ofHolothuria forskali (Echinodermata,Holothuroida)

Summary The ventral surface ofHolothuria forskali (Holothuroida, Aspidochirotida) is almost completely covered by small-sized podia that are locomotory. Each podium consists of a stem that allows the podium to lengthen, to flex, and to retract, and this is topped by a disc that allows the podium to adhere to the substratum during locomotion. Podia ofH. forskali do not end in a sucker and their adhesion to the substratum thus relies entirely on the disc epidermal secretions. The disc epidermis is made of five cell types: non-ciliated secretory cells of two different types that contain granules whose content is either mucopolysaccharidic (NCS1 cells) or mucopolysaccharidic and proteinic in nature (NCS2 cells), ciliated secretory cells containing small granules of unknown nature (CS cells), cilitated nonsecretory cells (CNS cells), and support cells. The cilia ofCS cells are subcuticular whereas those ofCNS cells, although also short and rigid, traverse the cuticle and protrude in the outer medium. During locomotion, epidermal cells of the podial disc are presumably involved in an adhesive/de-adhesive process functioning as a duogland adhesive system. Adhesive secretions would be produced byNCS1 andNCS2 cells and de-adhesive secretion byCS cells. All these secretions would be controlled by stimulations of the two types of ciliated cells (receptor cells) which presumably interact with the secretory cells by way of the nerve plexus. The lack of suckers and the coexistence of two adhesive cell types in the disc epidermis give the locomotory podia ofH. forskali a compromise structure which would perhaps explain their ability to move as efficiently along soft and hard substrata.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea,Echinodermata) during the perimetamorphic period

Abstract

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea, Echinodermata) during the perimetamorphic period

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

Micro- and nanostructure of the adhesive material secreted by the tube feet of the sea star Asterias rubens

To attach to underwater surfaces, sea stars rely on adhesive secretions produced by specialised organs, the tube feet. Adhesion is temporary and tube feet can also voluntarily become detached. The adhesive material is produced by two types of adhesive secretory cells located in the epidermis of the tube foot disc, and is deposited between the disc surface and the substratum. After detachment, this material remains on the substratum as a footprint. Using LM, SEM, and AFM, we described the fine structure of footprints deposited on various substrata by individuals of Asterias rubens. Ultrastructure of the adhesive layer of attached tube feet was also investigated using TEM. Whatever the method used, the adhesive material appeared as made up of globular nanostructures forming a meshwork deposited on a thin homogeneous film. This appearance did not differ according to whether the footprints were fixed or not, and whether they were observed hydrated or dry. TEM observations suggest that type 2 adhesive cells would be responsible for the release of the material constituting the homogeneous film whereas type 1 adhesive cells would produce the material forming the meshwork. This reticulated pattern would originate from the arrangement of the adhesive cell secretory pores on the disc surface.     

Development of the starfish <Emphasis Type="Italic">Asterias amurensis</Emphasis> under laboratory conditions

Under laboratory conditions the development of the starfish Asterias amurensis Lütken from Vostok Bay (Sea of Japan) was studied at 14 and 17°C. At 14°C and a salinity of 31.6–32.6, ciliated coeloblastulae hatched from egg envelopes 19 h after fertilization. At this temperature the development proceeded slowly and stopped at the stage of bipinnaria. At 17°C and normal salinity of seawater, the development of A. amurensis was successful. The swimming blastula appeared in 14 h. It took 30.5 h for the embryos to reach the gastrula stage. The larvae began swimming in a horizontal position with the apical tip ahead. The dipleurula appeared at 60 h. These larvae began feeding. At 71 h after the beginning of development, the early bipinnaria has developed. In the larva, the edged ciliated band, the preoral plate, and the anal plate were already formed. At the age of 4.2 days, the larvae reached the stage of bipinnaria and the brachiolaria stage developed by 26–28 days after fertilization. The larvae had three identical brachiolar arms with attachment papillae on their tips and an attachment disk. In 37–44 days (at 17°C) the pelagic phase of A. amurensis development was completed by the attachment of larvae to the bottom plates and termination of metamorphosis. Most likely, the specificity to a substrate is not expressed in the brachiolaria of A. amurensis. They can settle on almost any hard substrate which is coated with a bacterial film. The newly settled juvenile starfish had five well-developed arms and moved using their ambulacral podia.Original Russian Text Copyright © 2005 by Biologiya Morya, Kashenko.     

Ultrastructure of the Penicillate Podia of the Spatangoid Echinoid Echinocardium cordatum (Echinodermata) with Special Emphasis on the Epidermal Sensory-Secretory Complexes

The spatangoid echinoid Echinocardium cordatum possesses specialized penicillate podia that handle sediment particles during burrowing and feeding. Epidermal complexes, which occur on podial surfaces directly contacting the sediment, each comprise four cells: a non-ciliated secretory cell containing granules rich in mucopolysaccharides (NCS cell), a ciliated secretory cell containing granules of unknown composition (CS cell), and two ciliated non-secretory cells (CNS cells). The cilium of the CS cell is subcuticular whereas that of each CNS cell traverses the cuticle. We propose that these four cells constitute a sensory-secretory complex wherein the ciliated cells are sensory cells and the secretory cells function for adhesion and de-adhesion. More exactly, an NCS cell adhesive and a CS cell de-adhesive would be sequential and would be initiated by two successive stimulations transduced by cilia when the podium touches the sediment. Cilia that first contact the sediment are those protruding through the cuticle from the CNS cells. Their stimulation would result in the secretion of an adhesive material by the NCS cells. Subsequently, the subcuticular cilia of CS cells would be stimulated when the podial digitations closely squeeze the substrate, and this would induce the secretion of a de-adhesive. These two antagonistic secretions would allow the podium to pick up and discharge sediment repetitively during burrowing and feeding.     

Fine structure of the dorsal papillae in the holothurioid Holothuria forskali (Echinodermata)

The dorsal surface of the holothurioid Holothuria forskali bears several longitudinal rows of modified podia called papillae. Each papilla consists of a conical stem topped by an hemispherical bud. Their gross tissue stratification is the same all along the papilla being made up of four tissue layers, viz. an inner mesothelium, a connective tissue layer, a nerve plexus and an outer epidermis. The latter is differently organized according to whether it belongs to the stem or to the bud. The epidermis of the bud is built up by ciliated cells that intimately contact the nerve plexus and have the classical structure of echinoderm sensory cells. The papillae are thus sensory organs involved in mechanoreception and possibly chemoreception.     

Characterisation of the Carbohydrate Fraction of the Temporary Adhesive Secreted by the Tube Feet of the Sea Star <Emphasis Type="Italic">Asterias rubens</Emphasis>

In sea stars, adhesion takes place at the level of a multitude of small appendages, the tube feet. It involves the secretion of an adhesive material which, after tube foot detachment, remains on the substratum as a footprint. It was previously reported that the two main organic components of this material are proteins and carbohydrates. The carbohydrate moiety of the adhesive secretion of Asterias rubens was investigated using a set of 16 lectins which were used on sections through tube feet, on footprints, and on the proteins extracted from these footprints. After gel electrophoresis, these proteins separate into eight protein bands which were named sea star footprint proteins (Sfps). Eleven lectins label the tube foot epidermis at the level of the adhesive cells, four react with footprints, and eight with two of the extracted footprint proteins, which are therefore classified as glycoproteins. Sfp-290 appears to bear mostly N-linked oligosaccharides and Sfp-210 principally O-linked oligosaccharides. The outer chains of both glycoproteins enclose galactose, N-acetylgalactosamine, fucose, and sialic acid residues. Another part of the carbohydrate fraction of the footprints would be in the form of larger molecules, such as sialylated proteoglycans. These two types of glycoconjugates are presumably key components of the sea star temporary adhesive providing both cohesive and adhesive contributions through electrostatic interactions by the polar and hydrogen-bonding functional groups of their glycan chains.     

Characterization of the lecithotrophic larval development of the temperate New Zealand asterinid Stegnaster inflatus

In the family Asterinidae, development through a planktonic lecithotrophic brachiolaria larva is common and has evolved independently several times. Here, we describe the lecithotrophic development of the asterinid Stegnaster inflatus, a species endemic to New Zealand. Early development through the blastula and gastrula stages is short, with hatching at the brachiolaria stage occurring within 48 hr. After hatching, larvae are negatively buoyant, and without aeration remain near the bottom of the culture containers. The settled benthic juvenile stage was reached in ~2 weeks. The brachiolaria of S. inflatus shares common characteristics with the planktonic brachiolaria of other asterinids in that the brachiolar attachment apparatus comprises three brachia and a central adhesive disc, although the latter is thin and appears to be reduced. Mortensen (1925, Videns kabelige Meddelelser fra Dansk naturhistorisk Forening i København, 79 (15), 261‐420) had hypothesized that individuals of S. inflatus might brood within the “cave” formed in the interambulacral space between the arms. We found no evidence for brooding, but hypothesize that S. inflatus may have demersal development, on or near the bottom, which has implications for larval dispersal and population structure.     

The structure of the larval nervous system of Pisaster ochraceus (Echinodermata: Asteroidea)

Ultrastructural observations and glyoxilic acid-induced fluorescence of catecholamines indicate that tracts of axons lie at the base of the ciliary bands and run throughout their length in bipinnaria and brachiolaria larvae of Pisaster ochraceus. Two types of nerve cells occur at regular intervals within the ciliary bands. Type I nerve cells are associated with the axonal tracts, and type II nerve cells, which are ciliated, occur along the edge of the ciliary bands. Two prominent ganglia, which appear as accumulations of nerve cells and neuropile, occur on the lower lip of the larval mouth. Smaller ganglia occur irregularly throughout the ciliary band. Synapses were never clearly identified and were assumed to be unspecialized. Nervous tissues were also found associated with the esophageal muscles, the attachment organ, and the larval arms. Organization of the nervous system and its association with effectors suggest it controls swimming and feeding. Several similarities exist between the nervous systems of larval asteroids, larval echinoids, and adult echinoderms.     

Adhesive papillae on the brachiolar arms of brachiolaria larvae in two starfishes, Asterina pectinifera and Asterias amurensis, are sensors for metamorphic inducing factor(s)

It has been hypothesized by Barker that starfish brachiolaria larvae initiate metamorphosis by sensing of metamorphic inducing factor(s) with neural cells within the adhesive papillae on their brachiolar arms. We present evidence supporting Barker's hypothesis using brachiolaria larvae of the two species, Asterina pectinifera and Asterias amurensis. Brachiolaria larvae of these two species underwent metamorphosis in response to pebbles from aquaria in which adults were kept. Time-lapse analysis of A. pectinifera indicated that the pebbles were explored with adhesive papillae prior to establishment of a stable attachment for metamorphosis. Microsurgical dissections, which removed adhesive papillae, resulted in failure of the brachiolaria larvae to respond to the pebbles, but other organs such as the lateral ganglia, the oral ganglion, the adhesive disk or the adult rudiment were not required. Immunohistochemical analysis with a neuron-specific monoclonal antibody and transmission electron microscopy revealed that the adhesive papillae contained neural cells that project their processes towards the external surface of the adhesive papillae and they therefore qualify as sensory neural cells.     

Is the adhesive material secreted by sea urchin tube feet species‐specific?

Sea urchin adoral tube feet are highly specialized organs that have evolved to provide efficient attachment to the substratum. They consist of a disk and a stem that together form a functional unit. Tube foot disk tenacity (adhesive force per unit area) and stem mechanical properties (e.g., stiffness) vary between species but are apparently not correlated with sea urchin taxa or habitats. Moreover, ultrastructural studies of sea urchin disk epidermis pointed out differences in the internal organization of the adhesive secretory granules among species. This prompted us to look for interspecific variability in the composition of echinoid adhesive secretions, which could explain the observed variability in adhesive granule ultrastructure and disk tenacity. Antisera raised against the footprint material of Sphaerechinus granularis (S. granularis) were first used to locate the origin of adhesive footprint constituents in tube feet by taking advantage of the polyclonal character of the generated antibodies. Immunohistochemical assays showed that the antibodies specifically labeled the adhesive secretory cells of the disk epidermis in the tube feet of S. granularis. The antibodies were then used on tube foot histological sections from seven other sea urchin species to shed some light on the variability of their adhesive substances by looking for antibody cross‐reactivity. Surprisingly, no labeling was observed in any of the species tested. These results indicate that unlike the adhesive secretions of asteroids, those of echinoids do not share common epitopes on their constituents and thus would be “species‐specific.” In sea urchins, variations in the composition of adhesive secretions could therefore explain interspecific differences in disk tenacity and in adhesive granule ultrastructure. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

Fine structure of the photogenous areas in the bioluminescent ophiuroidAmphipholis squamata (Echinodermata,Ophiuridea)

 Amphipholis squamata is a small bioluminescent ophiuroid whose arms are the only body part to produce light. The morphology of the arms was described paying particular attention to the spinal ganglia, viz the areas of most intense luminescence. Spinal ganglia consist of five different cell types (A–E) which were studied at different stages of the photogenous reaction. Type D cells have numerous irregularlyshaped vacuoles, widespread Golgi apparatus and well-developed rough endoplasmic reticulum (RER) that show obvious ultrastructural changes after luminescence. Type D cells appear, therefore, to be the best photocyte candidate. Type B and C cells were frequently observed in the nervous system outside spinal ganglia. Type A and E cells have not been described before. Type A cells are ciliated cells and type E cells extend long processes which are intimately associated with type D cells and epidermal ciliated cells. Both type A and type E cells could take part to the stimulation pathway that triggers luminescence. Accepted: 1 September 1996     

Colony–Substratum Relations in Scrupocellariidae (Bryozoa, Cheilostomata)

In the Bryozoa in general the colony is attached by means of the primary zooid, the ancestrula, which is permanently cemented to the substratum. The attachment is brought about, in the marine bryozoans, by the larva everting its interior sac into a basal adhesive disc secreting a thin layer of hardening mucus. In Scrupocellaria reptans no adhesive disc was found. The metamorphosing larva is fixed to the substratum by a column of loose, sticky secretion. This primary fixation is ephemeral and replaced by a secondary, permanent fixation by one pair of rootlets. Thus, the ancestrula body proper and the colony arising from it become permanently free from the substratum but anchored to it by rootlets, the primary pair and series of secondary rootlets. This unique and certainly secondarily evolved type of attachment is apparently realized in the Scrupocellariidae in general, to a more or less perfect degree. It appears as one of several possible models to meet efficiently with environmental disturbances.     

Larval growth and perimetamorphosis in the echinoid <Emphasis Type="Italic">Echinocardium cordatum</Emphasis> (Echinodermata): the spatangoid way to become a sea urchin

The prejuvenile development of Echinocardium cordatum (Echinoidea) was investigated by means of scanning electron, confocal and light microscopes, aiming to illustrate the early life history of a spatangoid representative and to compare it with the other major echinoid groups. During the larval development of E. cordatum, two periods follow one another. The first one takes 12 days; it ends with the formation of a complete echinopluteus with twelve elongated larval arms. The second lasts from 3 to 12 days; it is entirely devoted to the building of the echinid rudiment and ends with the acquisition of larval competence. No appendage other than arms develops at the larva’s outer surface. Competent larvae are demersal. They settle onto the substratum and test it for suitability using the five rudiment podia that protrude through the vestibule opening. Metamorphosis is a rapid event that lasts less than an hour. The rudiment does not everse and its spines and podia actively tear up the larval epidermis which is progressively covered by the growing vestibular epidermis. The resulting postlarva is short-lived and morphologically similar to both the late rudiment and the early juvenile, which, however, is exotrophic. Late rudiments in E. cordatum show basic spatangoid features being bilaterally symmetric and having clavulae and sphaeridia. More importantly, they already have the convex shape and the appendage cover of early juveniles. Metamorphosis in E. cordatum appears to be less complex, i.e. no rudiment is everted, and more complete, since, in contrast to “regular” echinoids, no transitory appendages are seen. Metamorphosis/development of E. cordatum, thus, is closer to that of clypeasteroids, since the rudiment of the latter already bears juvenile definitive appendages, when everted during metamorphsis.     

Integument and epidermal sensory structures of Myzostoma cirriferum (Myzostomida)

Summary The fine structure of the integument of Myzostoma cirriferum is described with special attention to the integument sensory areas. Hypotheses about the function and a functional model of these are proposed. The integument consists of an external pseudostratified epithelium with cuticle (the epidermis) covering a parenchymo-muscular layer (the dermis). The dermis includes two types of cells: muscular fibers of the double obliquely striated type and parenchymal cells. Differences occur in the epidermis, which consists either of a large non-innervated myoepithelial area (viz. the regular epidermis). or of several rather localized sensory-secretory areas associated with discrete nerve proceses (viz. the sensory epidermis). The regular epidermis is made up of three types of cell: covering cells, ciliated cells and myoepithelial cells. The sensory epidermis shows small or marked structural variations from the regular epidermis. Small variations occur in the cirri, the buccal papilla, the body margin, the parapodia and the parapodial folds where nerve processes insinuate between epidermal cells. They are thought to be mechanoreceptor sites that could give information on the structural variations of the host's integument and participate in the recognition of individuals of the same species. The sensory epidermis differs markedly from the regular eidermis in the four pairs of lateral organs. Each lateral organ consists of a villous and ciliated dome-like central part, surrounded by a peripheral fold. The epidermis of the fold's inner part (viz. the part facing the central dome) is made up of secretory cells, while that of the fold's outer part is similar to the regular epidermis. The epidermis of the dome includes vacuolar cells, sensory cells and a different type of secretory cell. Lateral organs are presumed to be both chemoreceptors and mechanoreceptors. They could allow the myzostomids to recognize the host's integument and prevent them from shifting on the surrounding inhospitable substrate.     

From competent larva to exotrophic juvenile: a morphofunctional study of the perimetamorphic period of Paracentrotus lividus (Echinodermata, Echinoida)

 The perimetamorphic period in Paracentrotus lividus lasts for 8–12 days. It starts from the acquisition of larval competence, includes the change in form (metamorphosis) and the endotrophic postlarval life, and stops with the appearance of the exotrophic juvenile. All major postlarval appendages already occur in competent larvae being either grouped into the echinoid rudiment (terminal plates, early spines and primary podia) or scattered within the larval integument (genital plates and sessile pedicellariae). Competent larvae show particular behaviour which brings them close to the substratum. The latter is tested by primary podia protruding through the vestibular aperture of the larva. Primary podia are sensory–secretory appendages that are deprived ampullae. They are able to adhere to the substratum in order to allow evagination of the echinoid rudiment (i.e. metamorphosis) and substatum adhesion of the postlarva. Particular spines are borne by the postlarva; these are multifid non-mobile appendages forming a kind of protective armour. Like those of the larva, all characteristic structures of the postlarva (primary podia, multified spines and sessile pedicellariae) are transitory and regress either at the end of postlarval life (primary podia) or during early juvenile life (multifid spines and sessile pedicellariae). Other appendages that develop during postlarval life (i.e. podia with ampulla, point-tipped spines and sphaeridiae) are similar to those borne by the adults and become functional when the individual enters its juvenile life. Thus, the perimetamorphic period appears to be a fully fledged period in the life-cycle of P. lividus, and presumably in the life-cycle of any other sea-urchin species. Accepted: 7 October 1997