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     

The sphaeridia of sea urchins: ultrastructure and supposed function (Echinodermata,Echinoida)

Summary Sphaeridia are minute skeletal appendages of the echinoid test which are considered to be sense organs, organs of equilibrium, according to their shape. The sphaeridium forms a functional unit with the tubercle to which it adheres. The tubercle is encircled by a basiepithelial nerve ring of the epidermis. A circle of regularly arranged myocytes stretches from the tubercle to the sphaeridium. The muscles are distant from each other. The myofibrillar processes enter the pore space of the sphaeridial skeleton to which they are anchored by tendons; tendons are absent in the tubercle region. The cell bodies of the myocytes lie opposite to the nerve ring outside the skeleton. In this region the muscle cells and the nerve ring are in contact with each other, their basal laminae fuse. Tensions of the various myocytes are dependent on the position of the top-heavy sphaeridium. The nerve ring contains neurones which are provided with a cilium which lies close to the contact region with the myocytes. This arrangement leads to the assumption that the nerve cells in question have a proprioceptor function. Unique filter cells are present in the pore spaces of the sphaeridium and the tubercle. They possibly detoxicate the extracellular fluid that surrounds the myocytes. Phagocytes loaded with spacious phagosomes are crowded in the adjacent pore spaces. They are possibly extruded via the epidermis. Filter cells and phagocytes have obviously to do with the metabolism within the sphaeridium-tubercle-system.     

Ultrastructural investigation of matrix-mediated biomineralization in echinoids (Echinodermata,Echinoida)

Summary The formation of echinoderm endoskeletons is studied using echinoid teeth as an example. Echinoid teeth grow rapidly. They consist of many calcareous elements each produced by syncytial odontoblasts. The calcification process in echinoderms needs (1) syncytial sclerocytes or odontoblasts, (2) a spacious vacuolar cavity within this syncytium, (3) an organic matrix coat in the cavity. As long as calcite is deposited, the matrix does not touch the interior face of the syncytium. The cooperation between syncytium, interior cavity and matrix coat during the mineralization process is discussed. The proposed hypothesis applies to the formation of larval skeletons, echinoderm ossicles and echinoid teeth.When calcite deposition ceases the syncytium largely splits up into filiform processes, and the skeleton is partly exposed to the extracellular space. However, the syncytium is able to reform a continuous sheath and an equivalent of the cavity and may renew calcite deposition.The tooth odontoblasts come from an apical cluster of proliferative cells, each possessing a cilium. The cilium is lost when the cell becomes a true odontoblast. This suggests that cilia are primitive features of echinoderm cells. The second step in calcification involves the odontoblasts giving rise to calcareous discs which unite the hitherto single tooth elements. During this process the odontoblasts immure themselves. The structures necessary for calcification are maintained until the end of the process.The mineralizing matrix is EDTA-soluble. The applied method preserves the matrix coating the calcite but more is probably incorporated into the mineral phase and dissolved with the calcite.Abbreviations A adhesive point (LNC) - B adaxial bag - bb basal body (ci) - CA calcareous deposits - cb cytoplasmic bladder (cp) - ce centriole - ci cilium - cp cable-like cell process - cv condensing vacuole - dp distal processes (sh) - E epithelium of the tooth - ex extracellular space - f extracellular fibrils - ga gasket (sh) - ic interior cavity - L lamellae (LNC) - LNC lamellae needle complex - m mitochondrium - mc matrix coat - MF main fold (P) - MI mitosis - mt microtubules - N nucleus - O odontoblast - P primary plate - Ph phagocyte - PR proliferative cell - pr prism - rb reserve body - RER rough endopl. reticulum - rl rootlet (ci) - RY relatively youngest plate - s satellite (bb, ce) - sh synplasmic sheath (O) - SP secondary plate - sv smooth-walled vesicle - TF transversal fold (P) - U umbo (P) - v Golgi vesicle - Y youngest tooth element     

Influence of temperature and salinity on embryonic development ofParacentrotus lividus (Lmk, 1816)

The combined effects of temperature and salinity on early development of the sea urchinParacentrotus lividus (Lmk, 1816) are reported. The optimal temperature-salinity combinations for development are 18 °–20 °C and 34–35‰; there is a significant temperature-salinity interaction. The optimal conditions found in the experiments are above the mean yearly values for the sampled population's environment (North Adriatic Sea), being more similar to those of the Tyrrhenian Sea. These results suggest that embryonic tolerances to temperature and salinity are under genetic and not environmental control.     

The interpyramidal muscle of Aristotle's lantern: its myoepithelial structure and its growth (Echinodermata,Echinoida)

Summary The interpyramidal muscles of the lantern of Diadema setosum have been studied as an example of such muscles in regular echinoids. The light- and electron microscopic study proves that the interpyramidal muscle is nothing but a continuous, highly folded myoepithelium. Although it is a powerful and specialized comminator muscle its histological organization (a pseudostratified myoepithelium) is rather simple when compared with other echinoderm myoepithelia. It consists of only two cell types: 1) a single layer of well-developed myocytes and 2) monociliated adluminal cells that totally cover the myocytes and touch the basal lamina by thin basal processes. The interpyramidal muscle grows by addition of new folds to its upper region. Consecutive stages of the myoepithelial differentiation are found in each of the young folds. The origin of the cells which are necessarily added to the growing epithelium is unknown. The growth rate of the muscle is in accordance with the enlargement of the lantern ossicles. The respective data are discussed in detail.     

The lantern of Aristotle: organization of its coelom and origin of its muscles (Echinodermata,Echinoida)

Summary Comparative ultrastructural analyses of the muscles that work the lantern of Aristotle support the opinion that the muscles in question are myoepithelially organized or derivatives of myoepithelia. They are part of the epithelium of the peripharyngeal cavity (=lantern coelom). The coelom epithelium may become multiplelayered in certain regions and is composed of (1) a layer of muscle cells that vary in number and size, (2) nerve cells and their processes that are interspersed between the muscle layer and (3) monociliated adluminal cells that build a continuous cell lining and completely cover the muscle layer. According to their complexity, the lantern muscles exhibit consecutive stages of myoepithelial variations and may finally simulate subepithelial musculature. The results of this study support the hypothesis of a histological development of subepithelial musculature from simple myoepithelia, although both epithelial and mesenchymal musculature may occur in the Echinodermata. Detailed knowledge of the organization of the lantern's coelom space was a prerequisite for the present study. In contrast to previous examinations the lantern coelom is not a continuous space, but is subdivided into several cavities that are partially completely separated from each other. On the one hand, this subdivision is probably caused by the sophisticated arrangement of the lantern's ossicles and on the other by the septa that give rise to muscles that fulfill different functions. lanter's ossicles and on the other by the septa that give     

A spicule-reinforced contractile mesentery: organisation and mechanical behaviour of the exterior coelomic septum of Stylocidaris affinis (Echinodermata, Echinoida)

The exterior coelomic septum (ECS) is a mesentery-like structure that encloses the lantern of regular sea-urchins and connects it to the inner surface of the test. This paper describes the ultrastructure and microarchitecture of the ECS in Stylocidaris affinis (Cidaridae, Echinoida) and provides information on its contractile and passive mechanical properties. The ECS forms five interambulacral pouches each of which has adthecal (test-facing) and adambulacral (ambulacrum-facing) walls. The ECS wall comprises two coelothelia separated by a layer of connective tissue. The outer coelothelium is a single layer of monociliated cuboidal peritoneocytes and basally located axon-like processes. The inner coelothelium is a single layer of squamous peritoneocytes overlying axon-like processes and, in the adthecal regions only, parallel arrays of elongated myocytes orientated obliquely or horizontally. The intraseptal connective tissue consists mainly of collagen fibrils with sparsely distributed spherule cells and cells containing heterogeneous vesicles. In the adambulacral regions of the ECS hollow beaded microfibrils 20–23 nm in diameter form fibre-like aggregations. This layer also contains calcite spicules of variable size, shape, abundance and orientation. Isolated preparations of the ECS show concentration-dependent contractile responses to K⁺ ions and acetylcholine. The magnitude of the contractile force varies with the vertical position of the lantern (which determines the starting length of the ECS) in an unusual pattern. Cyclical loading-unloading tests indicate that, as the lantern is raised, the ECS shows low stiffness until the lantern reaches its normal resting position. It is concluded that the adthecal regions of the ECS help to set a limit to lantern retraction and that their contractility assists the protractor muscles in exerting a downward pull on the lantern. Accepted: 24 August 2000     

Macrostructure and evolution of the digestive system in Echinoida (Echinodermata)

The structure of the digestive system in Echinoida has long been puzzling since comparative studies have suggested that a derived structure, the siphon, has apparently evolved twice independently. New observations on the digestive system in five species of Cidaroida, four species of the Diadematoida and three species of Echinothurioida are presented. The results show that the four diadematoid species have a siphon and the three species of Echinothurioida have a siphonal groove, contrary to previous assertions. These observations make the macrostructure of the echinoid digestive system fully consistent with more recent phylogenetic hypotheses based on molecular and general morphological data, and support the idea that a siphon has evolved only once, in the stem lineage of the Acroechinoidea.     

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)     

Functional morphology of regular echinoid tests (Echinodermata,Echinoida): a finite element study

Functional morphology of the calcareous test ofEchinus esculentus was investigated by parametric finite element analysis, an engineering technique developed for numerical analysis of the behaviour of complex structures responding to external forces. Finite element models of the test were generated by methods of Computer Aided Geometric Design (CAGD) to calculate the mechanical responses to different types of loading. The load cases included vertical, concentrated load at the apex, vertical, distributed load on the upper third of the test, internal pressure and tensile forces as introduced into the test by tube feet activity. The objectives were the shape of the test, the distribution of material and the alternating zones of porous and non-porous plates within the test.—Echinoid tests resist external loading without showing any specific points of failure. The thickened margins of the periproct and peristome apertures account for load-bearing capacity as well as the thickned meridional structures which carry a greater portion of stress than the thinner parts of the test. Distribution of material is not a response to concentrated loads on the apex nor to self-weight. Taken strictly, echinoid tests are not thin (or membrane) shells. Under loading, bending moments occur which influence the stress state in the entire test. The pneu hypothesis could not be confirmed. Adaptation of the test shape or of the distribution of material as a response to internal pressure does not exist. Tests of regular echinoids are especially well adapted to the mechanical activity of the ambulacral tube feet, i.e. the shape of the test, its flattening towards the substrate, the outward bulge of the ambulacra and the differential distribution of material within the test.     

Morphological and mechanical aspects of fission in Ophiocomella ophiactoides (Echinodermata,Ophiuroida)

Summary During fission the ophiuroid Ophiocomella ophiactoides splits across the disc into two halves each of which regenerates to form a complete individual. This paper describes the gross anatomy of the fission plane and the histology, ultrastructure, and mechanical properties of key structures transected during fission.Rupture of the disc integrument appears not to be facilitated by a pre-determined plane of weakness. Comparison of naturally split and artificially split animals showed that at fission a mechanism operates which restricts breakage to the interradial plane of two jaws. The interradial plane is subtended mainly by collagenous ligaments and by muscles linked to the skeletal components by basal lamina-like tendinous fibres. No fission-related adaptations could be identified in the oesophagus, circumoral nerve ring, or circumoral water vascular canal.On the basis of creep tests on isolated preparations of the disc integrument and jaw-frame and the responses of these preparations to ionic manipulation, it is suggested that their behaviour is dominated by collagenous elements that can undergo actively controlled changes in their mechanical properties. A hypothesis is presented which proposes a role for such a mechanism in the initiation and facilitation of fission.Contribution 318 of the Discovery Bay Marine Laboratory, University of the West Indies     

Fine structure and presumed functions of the pedicellariae of Echinocardium cordatum (Echinodermata,Echinoida)

Summary Tridactylous, trifoliate, and globiferous pedicellariae occur on the body surface of Echinocardium cordatum. Tridactyles have three forms: the typical, the rostrate, and the large forms. Both typical and rostrate tridactyles and trifoliates occur all around the echinoid body (trifoliates are, however, 4 times more numerous than tridactyles). Large tridactylous and globiferous pedicellariae are restricted to the peribuccal area.As a general rule tridactyles and trifoliates are similar in morphology. The distal part of the valves forms an open blade and bears lateral teeth and/or denticles (single or in combs). The stalk consists of a rigid proximal part supported by an axial rod and a flexible distal part which includes an axial fluid-filled cavity. The cavity is surrounded by muscle fibers and acts as an hydroskeleton, allowing the undulating-coiling movements of the flexible part of the stalk. Trifoliates are always active while tridactyles react only to direct or indirect mechanical stimulation.The valves of the globiferous pedicellariae have a tubular distal part whose upper opening is surrounded by teeth. There is no differentiated venom gland but a cluster of epithelial glandular cells located at the level of the valve upper opening. A small ciliary pad occurs just below the glandular cluster. Globiferous stalks are not flexible, being supported for their full length by an axial rod. Globiferous pedicellariae appear to be sensitive only to chemical stimulation.The presumed functions of E. cordatum pedicellariae are (1) cleaning of the body surface and ciliary structures (trifoliates), (2) protection against sedimenting particles (tridactyles), and (3) defense of the peribuccal area against potential small predators (globiferous pedicellariae).     

Functional anatomy of the valves in the ambulacral system of sea urchins (Echinodermata,Echinoida)

Summary The water vascular system of sea urchins is examined with special reference to the valves positioned between the radial vessel and the ampullae of the tube feet. The lips of the valve protrude into the ampulla. Thus the valve functions mainly like a check valve that allows the unidirectional flow of fluid towards the ampulla. Each ampulla-tube foot compartment acts as a semi-autonomous hydraulic system. The lumina of the ampulla and the tube foot are lined with myoepithelia except for the interconnecting channels that pierce the ambulacral plate. The contraction of the ampulla results in an increasing hydraulic pressure that protrudes the tube foot, provided that the valve is closed. The retraction of the tube foot results in a backflow of fluid independent of the condition of the valve. The lips of the valve are folds of the hydrocoel epithelium. The pore slit lies in the midline. The perradial faces of the lips are covered with the squamous epithelium of the lateral water vessel. The ampullar faces are specialized parts of the ampulla myoepithelium. Turgescent cells which form incompressible cushions take the place of the support cells. The valve myocytes run parallel to the pore slit and form processes that run along the base of the ampulla and the perradial channel up to the podial retractor muscle. The findings lead to the hypothesis of multiple control of the ampulla-tube foot system: (1) The mutual activity of the ampulla and the tube foot is indirectly controlled by the lateral and podial nerves which release transmitter substances that diffuse through the connective tissue up to the muscle layers. (2) A muscle-to-muscle conduction causes the simultaneous contraction of the ampulla or the podial retractor muscles. (3) The valve muscles are directly controlled by the processes of the valve myocytes which make contact with the podial retractor. In extreme conditions a backflow of hydrocoel fluid towards the radial water vessel occurs.     

Structure of the digestive tract of the pluteus larva of Dendraster excentricus (Echinodermata: Echinoida)

Summary Histological and ultrastructural observations of the digestive tract of eight-armed plutei of Dendraster excentricus are reported. The esophagus is divided into two regions. The uppermost is a narrow tube comprised of ciliated cells that assist in transporting food to the more bulbous lower esophagus where food particles are formed into a bolus prior to entering the stomach. The esophagus is surrounded by a network of smooth muscle fibers that are predominantly oriented circumferentially in the upper esophagus, and longitudinally in the lower esophagus. The musculature of the upper esophagus produces peristaltic contractions, whereas contractions of the muscle of the lower esophagus open the cardiac sphincter and force food from the lower esophagus into the stomach. Axons are associated with the ciliated cells and the muscles of the upper esophagus. The cardiac sphincter consists of a ring of myoepithelium, with cross-striated myofibrils oriented around the bases of the cells. The gastric epithelium is comprised of two cell types. Type I cells, which predominate, absorb and store nutrients, and may be the source of secreted digestive enzymes. Type II cells apparently phagocytize and intracellularly digest whole algal cells. The intestine is comprised of relatively unspecialized cells and probably functions primarily as a conductive tube for the elimination of undigested materials.     

Ultrastructure and organization of the epineural canal and the nerve cord in sea urchins (Echinodermata,Echinoida)

Summary The radial nerve cord ofMespilia globulus has been examined as an example of echinoid nerve cords. In the radius of echinoids only the ectoneural component of the nerve cord is present which is a derivative of the ectoderm. The nerve cord runs in the interior of the body and is accompanied by the epineural canal. In echinoids, the neuroepithelium makes up the upper and side walls of the epineural canal. Each lateral branch of the nerve cord forms a sort of neural tube. It encloses a branch of the epineural canal which represents an open connection with the sea water. Thus, the epineural canal exhibits numerous openings which probably allow sea water to flow back and forth. This organization is unique in echinoderms. — The neuroepithelium exhibits the organization of an epidermis with well-developed nervous elements. Glial cells are not present. The support cells are the true epithelial cells. Their monociliated cell bodies border the lumen and, by means of cytoplasmic stems that contain a bundle of filaments, they reach up to the basal lamina. The nerve cells and their trunk of nerve fibres fill the spaces between the support cells. — Three types of nerve cells can be distinguished according to their polarity: (1) Primary sensory cells that project a cilium into the epineural canal, the axon hillock region is at the opposite pole. (2) Subluminal cells whose cilium originates in the axon hillock region. (3) Neurones that lie within the trunk of nerve fibres. They are highly stretched in the direction of the nerve cord and are also provided with a cilium. Types 2 and 3 may be homologized with the basal nerve cells of the epidermis. They are possibly multipolar. — The lateral nerve cords make contact with the ampulla and pass the ambulacral plate parallel to the channel that connects the ampulla and the tube foot. The activity of the tube foot-ampulla system is possibly controlled by means of transmitter substances that diffuse through the connective tissue layer between the nerve cord and the myoepithelia of the ampulla and the tube foot respectively.     

Four lines of spermatid development and dimorphic spermatozoa in the sea urchin Anthocidaris crassispina (Echinodermata, Echinoida)

 The process of sperm development in the sea urchin Anthocidaris crassispina was studied by light and electron microscopy. Similar to other echinoids studied, a single flagellum, striated rootlet and nuage-like materials were present in spermatogonia of A. crassispina. Spermatocytes near the diplotene stage showed intracellular localization of the axoneme which appeared to be a retracted flagellum prior to cell division. Fibrous filaments were associated with a proximal centriole in spermatocytes and spermatids and might be involved in movement of the proximal centriole. An acrosomal vesicle was developed and a residual body was formed in spermatids. The special development patterns in A. crassispina attributed to the presence of two patterns of tail development and two patterns of mitochondrial development during spermiogenesis. These four lines of spermiogenesis resulted in the formation of four morphological types of sperm cell, i.e. sperms with: (1) a symmetrical midpiece and posterior tail, (2) an asymmetrical midpiece and posterior tail, (3) a symmetrical midpiece and bent tail and (4) an asymmetrical midpiece and bent tail. Sperm cells with bent tails (type 3+4) were probably still at the late spermatid stage because results of scanning electron microscopy demonstrated gradual detachment and eventual straightening of the bent tail, and their percentage occurrence in the sperm population decreased significantly (P<0.05) towards the spawning season of A. crassispina. Spermatozoa with a symmetrical midpiece were dominant (averaging 70% occurrence in the sperm population) over those with an asymmetrical midpiece. The dimorphic spermatozoa in A. crassispina (types 1, 2) are both considered to be euspermatozoa as their morphology is typical for Echinoida. Accepted: 4 May 1998     

The aboral ring and the development of early gonads in the echinoid Paracentrotus lividus (Echinodermata, Echinoidea)

Abstract. The development of the genital apparatus is described for the echinoid Paracentrotus lividus. This apparatus derives from the aboral ring, an annular structure that includes an inconspicuous coelom and, in juveniles, the germinal rachis. The germinal epithelium grows out from the germinal rachis, and the gonadal wall and coelom in early (tubular) gonads share similarities with their equivalents in the aboral ring. The original germinal rachis regresses to form a genital cord one cell wide in late juveniles. A genital cord was observed in a few field-collected adult individuals (>40 mm test diameter).     

On the ultrastructure and the supposed function of the mineralizing matrix coat of sea urchins (Echinodermata,Echinoida)

Summary Echinoderm ossicles are part of the mesenchyme. Their formation and growth, with respect to the underlying tissues, is studied using echinoid spines and teeth and applying different methods of fixation. The calcification process in echinoderms is strictly intracellular and needs (1) syncytial sclerocytes which completely enclose (2) a vacuolar cavity which in turn contains (3) an organic matrix coat. Strictly speaking, each ossicle is nothing but the calcified vacuolar space of a single syncytium of sclerocytes. In fully grown parts, however, the continuous sheath may split open and the matrix-coated mineral may come into contact with the extracellular space. According to biochemical analyses the matrix consists of insoluble components, but most (95%) of its constituents are soluble in EDTA or weak acids. If routine transmission electron microscope methods are used the soluble components are lost and the matrix at best looks electron light. If tannic acid is added to the fixative the soluble matrix components are preserved and reveal further ultrastructural details of the biomineralization process in echinoderms. The matrix coat looks extremely electron dense. Further soluble material is to be found within the vacuolar space or attached to the vacuolar surface of the cytoplasmic sheath. The results lead to the opinion that the matrix coat consists of a hydrophobic framework of insoluble components that contains soluble components which guide the Ca through pores in the hydrophobic layers into the interior of the matrix-coated space. It is only within this space that the mineral is deposited.     

Variable tensility of the oral arm plate ligaments of the brittlestar Ophiura ophiura (Echinodermata: Ophiuroidea)

The oral arm plates of the brittlestar Ophiura ophiura L. are connected to lateral arm plates at distal and proximal ligamentous junctions. The distal junction is mobile and is disrupted during arm autotomy; the proximal junction is more rigid and does not participate in autotomy. Aspects of the morphology and mechanical properties of the distal and proximal oral arm plate ligaments have been investigated in order to determine if their tensility is under physiological control. By means of creep tests it was found that elevation of the external potassium (K⁺) ion concentration causes a decrease in the viscosity of the distal ligament which is either transient or continues until rupture intervenes. In forced vibration tests the distal ligament often shows a biphasic stiffening then softening response to excess K ^- ions. Anaesthetics block the softening phase but enhance the stiffening component of this response. This ligament is also softened by repetitive electrical stimuli but stiffened by excess calcium ions and by acetylcholine. The proximal ligament appears to have the capacity for only transient changes in mechanical properties. Both ligaments are penetrated by the processes of juxtaligamental cells whose perikarya are arranged in clusters innervated by hyponeural nerves. These cells are thought to modulate the interfibrillar cohesion of the ligaments. It is concluded that the distal and proximal ligaments are mutable collagenous structures which in their stiffened condition help to maintain arm posture without the need for continuous muscular activity, and that at autotomy the distal ligament undergoes a profound loss of tensile strength which facilitates arm detachment.