Relative motion inAmoeba proteus in respect to the adhesion sites

Summary The whole ectoplasmic layer of polytactic and heterotactic forms ofA. proteus behaves as self-contractile structure. Depending on the configuration of cell body and on the cell-to-substrate attachment conditions it continuously retracts from each distal cell projection toward its centre and/or from each free body end toward the actual adhesion sites. As in the monotactic forms, it leads to the withdrawal of the tail region behind the retraction center and may result in the fountain movement in front of it. In the long unattached pseudopodia of heterotactic forms the ectoplasm is retracted in the fountain form, with the velocity linearly increasing from the basis of pseudopodium up to its tip. In polytactic cells the fountain is often absent, if the advancing fronts immediately adhere to the substrate. When they develop in unattached condition, or are experimentally obliged to detach, the ectoplasmic cylinders of frontal pseudopodia are retracted backwards. On the substrates which do not offer firm points of support the cell periphery moves back as a whole,i.e., the principal ectoplasmic cylinder retracts together with the cylinders of lateral pseudopodia, and the direction and speed of movement in any spot is the resultant of forces produced by all other segments. The retraction of ectoplasmic gel layer is independent of the endoplasmic flow in such extent that a pseudopodium may be withdrawn as a whole in spite of the endoplasm streaming directed forwards in its interior. On the cell surface the particles attached by adhesion (glass rods) strictly follow the movements of the internal ectoplasmic structures, whereas the unattached particles flow forward in the direction of endoplasm streaming.Study supported by Research Project II. 1 of the Polish Academy of Science.     

Reorganization and translocation of the ectoplasmic cytoskeleton in the leech zygote by condensation of cytasters and interactions of dynamic microtubules and actin filaments

The formation and bipolar translocation of an ectoplasmic cytoskeleton of rings and meridional bands was studied in interphase zygotes of the glossiphoniid leech Theromyzon trizonare. Zygotes consisted of a peripheral organelle-rich ectoplasm and an internal yolk-rich endoplasm. After microinjection of labeled tubulin and/or actin, zygotes were examined by time-lapse video imaging, immunofluorescence and confocal microscopy. The rings and meridional bands were formed by condensation of a network of moving cytasters that represented ectoplasmic secondary centers of microtubule and actin filament nucleation. In some cases the network of cytasters persisted between the rings. The cytoskeleton had an outer actin layer and an inner microtubule layer that merged at the irregularly-shaped boundary zone. Bipolar translocation of the rings, meridional bands, or the network of cytasters led to accumulation of the cytoskeleton at both zygote poles. Translocation of the cytoskeleton was slowed or arrested by microinjected taxol or phalloidin, in a dose-dependent fashion. Results of drug treatment probably indicate differences in the degree and speed at which the cytoskeleton becomes stabilized. Moreover, drugs that selectively stabilized either microtubules or actin filaments stabilized and impaired movement of the entire cytoskeleton. Microtubule poisons and latrunculin-B failed to disrupt the cytoskeleton. It is concluded that the microtubule and actin cytoskeletons are dynamic, presumably cross-linked and resistant to depolymerizing drugs. They probably move along each other by a sliding mechanism that depends on the instability of microtubules and actin filaments.     

L'Organisation Ultrastructurale Corticale de la Gregarine Lecudina pellucida; ses Rapports avec l'Alimentation et la Locomotion

SYNOPSIS. The structure of the cortical region (epicyte and ectoplasm) of the gregarine Lecudina pellucida , an intestinal parasite of the polychete worm Perinereis cultrifera was studied by electron microscopy.
The epicitary folds have 3 unit type membranes. Between the 1st and 2nd is a layer probably composed of fine longitudinal fibrils which has an arch-like or gutter-like structure at the crest of the folds. Inside these folds is cytoplasm without any noticeable differentiation or inclusion except for a granular (or finely fibrillar) layer under the limiting inner membrane and close to it.
The ectoplasmic zone of the entocyte is separated from the epicitary region by a lengthwise discontinuous cylindrical opaque layer, inwardly tangential to the folds. The ectoplasm lacks paraglycogen granules but has various organelles: apparently pinocytic vesicles against the wall between the folds, vesicles with myelinic membranes, opaque granules, a few mitochondria with blistered internal vesicles, and a few circular tubular fibers.
The superficial zone of the gregarine is supposed to contribute to nutrition, thru the extensive surface furnished by its folds and thru the pinocytic vesicles; but this alimentary intake is incomplete compared with that of the previously studied anterior region.
Insufficient mucus is discharged to account for locomotion. There are some circular ectoplasmic fibers, but locomotory myonemes are completely absent. However, there are deformations of the folds and corresponding waves that could account for locomotion by creeping or swimming. These movements of the folds might be due to the action of the contractile proteins and correspond with some of the layers seen in the wall.     

In vitro models of tail contraction and cytoplasmic streaming in amoeboid cells

We have developed a reconstituted gel-sol and contractile model system that mimics the structure and dynamics found at the ectoplasm/endoplasm interface in the tails of many amoeboid cells. We tested the role of gel-sol transformations of the actin-based cytoskeleton in the regulation of contraction and in the generation of endoplasm from ectoplasm. In a model system with fully phosphorylated myosin II, we demonstrated that either decreasing the actin filament length distribution or decreasing the extent of actin filament cross-linking initiated both a weakening of the gel strength and contraction. However, streaming of the solated gel components occurred only under conditions where the length distribution of actin was decreased, causing a self-destruct process of continued solation and contraction of the gel. These results offer significant support that gel strength plays an important role in the regulation of actin/myosin II-based contractions of the tail cortex in many amoeboid cells as defined by the solation-contraction coupling hypothesis (Taylor, D. L., and M. Fechheimer. 1982. Phil. Trans. Soc. Lond. B. 299:185-197). The competing processes of solation and contraction of the gel would appear to be mutually exclusive. However, it is the temporal-spatial balance of the rate and extent of two stages of solation, coupled to contraction, that can explain the conversion of gelled ectoplasm in the tail to a solated endoplasm within the same small volume, generation of a force for the retraction of tails, maintenance of cell polarity, and creation of a positive hydrostatic pressure to push against the newly formed endoplasm. The mechanism of solation-contraction of cortical cytoplasm may be a general component of the normal movement of a variety of amoeboid cells and may also be a component of other contractile events such as cytokinesis.     

Electron Microscopy of the Longitudinal Fibrillar Bundle and the Contractile Fibrillar System in Spirostomum ambiguum

SYNOPSIS. A study of the 'longitudinal fibrillar bundle' (LFB) and the 'contractile fibrillar system' (CFS) of a large protozoan ciliate, Spirostomum ambiguum , has been performed by means of an electron microscope. A system of sub-pellicular fibrils has been newly found and its function is discussed. Each LFB runs parallel with a longitudinal row of ciliary bases. It seems to be identical with the so-called kinetodesma. It is composed of tubular fibrils arranged in layered sheets, each of which contains 13 to 35 fibrils with the same diameter as the intra-ciliary fibrils and has a close connection to each of the ciliary bases. The CFS lies on a transitional plane between ectoplasm and endoplasm of the organism and forms a cobweb-like system of myofibrils as a whole. It stands in an intimate relationship with a characteristic vacuolar system. In a peristomial field, the fibrous structures are interrupted and somewhat thickened. A sub-pellicular system is composed of minute fibrils 20 to 26 mμ in diameter. The fibrils run parallel with each other in an antero-posterior direction, immediately beneath the inner pellicular membrane.     

The Cytology of Sheep Rumen Ciliates. II. Ultrastructure of Eudiplodinium maggii1

The anterior adoral zone of syncilia (AZS) of Eudiplodinium maggii is mounted on an extrusible peristome within a vestibulum. The peristome contains cytopharyngeal components derived from the infraciliature. These components include a crescent-shaped palisade of nematodesmata, two types of sub-membrane cytopharyngeal ribbons, and an ensheathing fibrous layer enclosing a phagoplasmic zone containing the other components. A convoluted esophagus is continuous with and extends from the posterior of the cytopharynx adjacent to the macronucleus. A posterior cytoproct has specialized cytoplasm around it and associated myoneme-like elements. The skeletal plate is composed of finely granular platelets and lies under the cortex ventral to the macronucleus. The endoplasm is separated from the ectoplasm by a fibrous boundary layer. The cortex has an external glycocalyx, a membranous layer, epiplasm, and microtubular and microfilament layers. The AZS infraciliature is of the usual cntodiniomorph type, kinetosomes linked by a sub-kinetosomal rod and with associated bifurcated kinetodesma, postciliary and transverse microtubules-the latter extending into the cytopharynx—nematodesmata, and a fibrous reticulum. A possible vestigial, somatic infraciliature consisting of short, barren kinetosomes with associated basal and cortex-directed microtubules and a periodic incomplete fiber, is found subcortically throughout the cell.     

Ultrastructure of the endoplasm of ophryoscolescides]

The digestive system of the Ophryoscolescides includes different parts : the adoral ciliary zone, the cytostome, the oesophagus, the endoplasm, the rectum and the anus. The endoplasm appears as a digestive sack surrounded by the mesoplasm. This fibrillary envelop separates it from the ectoplasm. It's in the endoplasm that the mechanism of the digestion develops. The endoplasm contains many different structures. It includes macrovesicles of three types, bacteria, plant-particles, microvesicles of two types, ergastoplasm, polysaccharides, the Golgi apparatus and fibrils. The macrovesicles of the first and the second type are not to be found simultaneously in the same endoplasm. The macrovesicles of the third type, very few in number, are different of the previous ones by their double membrane and their denser granulations. Large-sized vesicles contain bacteria at different stadia of digestion. At the end of the digestion, the bacteria are completely or partially degraded. The plant-particles ingested by the Ciliate are to be found in the endoplasm, surrounded by a vesicle. A first type of microvesicles are found in the oesophagus and in the rectum. They are distinguished by a deep black coloration of variable shape. Another type of microvesicles is also present in the endoplasm. They are small bright vesicles that can bud from large-sized vesicles. In the endoplasm the ergastoplasmic lamellae and rings are easily recognizable thanks to the ribosomes, that edge their external membrane. The ribosomes are also situated along the mesoplasm, around polysaccharidic grains and in small groups between the other endoplasmic elements. The polysaccharidic grains are netting-needle shaped bodies. Noirot-Timothee confers them a reserve function. The Golgi apparatus includes eight to eleven saccules. The extremities of those saccules present swellings or golgian ampullae. The endoplasm is also strewn with fibrillary structures that, cross-section are arranged in groups of four or five tubules. This type of endoplasm is found in Ophryoscolescides taken from a ruminant that has been fasting for several hours. In some individuals the endoplasm can have a totally different aspect. It is filled with vesicles of quite different sizes and shapes. The localization test of the acid phosphatase reveals, in this type of endoplasm, lead deposits. These vesicles are probably autophagic vesicles.     

Ultrastructure of a colonial radiolarian Collozoum inerme and a cytochemical determination of the role of its zooxanthellae

Collozoum inerme (Müller) is a colonial Radiolarian containing numerous cells bound in a common gelatinous matrix. The cells do not possess a skeleton as observed in many unicellular Radiolaria, but the cytoplasmic organization is similar. The cells are multinucleate and a complex system of cellular processes containing mitochondria, Golgi, and numerous vacuoles radiate out from the nuclear region. The endoplasm is connected to the ectoplasm across a double membrane boundary by thin cytoplasmic strands called fusules whose structure resemble those in unicellular Radiolaria. The ectoplasm contains a lacy network of vacuoles containing an osmiophilic substance. Rhizopodia emerge from the ectoplasmic sheath. Some are thin and densely granular. Larger diameter rhizopodia, containing less dense cytoplasm, sequester the zooxanthellae which present a typical dinoflagellate fine structure. Some of the zooxanthellae are apparently cultivated since they are sometimes observed dividing and persist in large numbers when colonies are cultivated under illumination for several weeks in the laboratory. However, colonies maintained in the dark have a decline in number of zooxanthellae and light microscopic examination shows they are being drawn into the ectoplasm of the radiolarian cells. Electron microscopic examination of zooxanthellae drawn into the ectoplasm sheath indicates they are digested. C. inerme is a remarkable example of a simple cellular aggregate that has exploited its colonial habit to culture algae and use them as food thus possibly enhancing the viability of the colony.     

Ultrastructure of the cytoplasm of the lower holotrichous infusorian Tracheloraphis prenanti]

The ciliature of T. prenanti Dragesco 1960 (forma oligocineta Raikov et Kovaleva, 1968) consists of 14-18 ventral and lateral longitudinal kineties with paired kinetosomes, carrying either two cilia or one cilium per kinetosome pair (in the latter case, the nonciliated kinetosome is always the posterior one). The ectoplasmic fibrillar system belongs to the postciliary type. A pair of kinetosomes shares a common basal plate. The anterior kinetosome gives rise to a short ribbon of transverse microtubules, the posterior one, to a poorly developed kinetodesmal filament and to a strong ribbon of postciliary microtubules. The latter proceeds backwards along 8 to 12 kinetosome pairs, being incorporated into a laminated postciliodesma which accompanies each kinety on its right side. Rows of Golgi elements, sending secretory vesicles and channels towards the body surface, exist beneath the kinetosome bases. Each kinety is accompanied on its left by a microfibrillar myoneme, surrounded by perimyary vesicles and underlain by a row of mitochondria. The median part of the dorsal surface is nonciliated; the cytoplasm here is rich of membrane systems, contains peripheral, electron-dense, extrusible inclusions and sometimes also bacteria. The electron-dense inclusions develop in the endoplasm, in close contact with mitochondria. The endoplasm contains also large microfibrillar spheres of unknown nature.     

Cortical organellar complexes,their structure,formation, and bearing upon cell shape in a ciliate,Dileptus

Summary Ciliary complexes termed the kinetids, contain fibres of several kinds attached to the proximal end of a basal body. One of these fibres, the long microtubular fibre running in the endoplasm ofDileptus, is of special interest for this study. The fibres when attached to oral kinetids are orientated towards the cell posterior, and are numerous in the proboscis endoplasm. The fibres anchored at locomotor kinetids are orientated towards the cell anterior and penetrate the endoplasm of the tail. The endoplasm of both proboscis and tail appears transparent when viewed in the light microscope, and is deprived of many organelles: nuclei, lipid droplets, and food vacuoles. During regeneration proboscis and tail reconstitution is simultaneous, with an increase in transparency and in the density of microtubular fibres within the regenerating region. In posterior fragments ofDileptus which contain locomotor kinetids only, oral kinetids form as an offspring of locomotor ones. During differentiation of oral structures oral kinetids rotate until their endoplasmic fibres point posteriorly. It is this rotation that supplies the cell with a posteriorly directed set of endoplasmic fibres. The possibility that the translocation of endoplasmic organelles along the microtubular fibres may be one of mechanisms in shaping cells is discussed. Since the direction of endoplasmic translocation depends upon fibre orientation, the MTOCs which govern this orientation are likely candidates to be bearers of information concerning cell shape inDileptus.     

Ontogenesis of Dileptus terrenus and Pseudomonilicaryon brachyproboscis (Ciliophora, Haptoria)

ABSTRACT. Dileptids are haptorid ciliates with a conspicuous proboscis belonging to the oral apparatus and carrying a complex, unique ciliary pattern. We studied development of body shape, ciliary pattern, and nuclear apparatus during and after binary fission of Dileptus terrenus using protargol impregnation. Additional data were obtained from a related species, Pseudomonilicaryon brachyproboscis . Division is homothetogenic and occurs in freely motile condition. The macronucleus is homomeric and condenses to a globular mass in mid-dividers. The proboscis appears in late mid-dividers as a small convexity in the opisthe's dorsal brush area and maturates post-divisionally. The oral and dorsal brush structures develop by three rounds of basal body proliferation. The first round generates minute anarchic fields that will become circumoral kinetofragments, while the second round produces the perioral kinety on the right and the preoral kineties on the dorsal opisthe's side. The dorsal brush is formed later by a third round of basal body production. The formation of various Spathidium -like body shapes and ciliary patterns during ontogenesis and conjugation of Dileptus shows a close relationship between spathidiids and dileptids. On the other hand, the peculiarities of the dileptid morphology and ontogenesis indicate a long, independent evolution.     

Relationship between endoplasmic and ectoplasmic oscillations during chemotaxis ofPhysarum polycephalum

Summary The plasmodium ofPhysarum polycephalum usually migrates coordinately as one whole body even in a complicated environment. By measuring oscillation phenomena in endoplasm and ectoplasm separately during chemotactic process, we studied the mechanism of information processing to achieve such a coordination. (1) The interaction between endoplasmic oscillators was long-range, competitive according to the length of period, and fast (18 cm/min). Ectoplasmic one was short-range. (2) After a partial stimulation of attractant to the organism, the period at the stimulated portion decreased first, and a global phase gradient appeared in endoplasm. Then ectoplasm at the non-stimulated portion was entrained to the endoplasmic pattern, and the migration direction at each part changed in accordance with the phase gradient as a whole body. (3) When the endoplasmic interaction was interrupted, the above coordinated response was not observed. These facts suggest that two-layer coupled oscillator system composed of endoplasm and ectoplasm play important roles for such an information integration.     

The Cytology of Sheep Rumen Ciliates. I. Ultrastructure of Epidinium caudatum Crawley1

Epidinium caudatum has an anterior vestibulum containing the adoral zone syncilia (AZS) on an extrusible peristome. The cytopharyngeal structures include a funnel-shaped arrangement of nematodesmata, longitudinal and transversely oriented microtubular ribbons all of which are located in the peristome, a structure which also contains filamentous phagoplasm. The origins of the microtubular ribbons indicate affinities to the rhabdos type of cytopharynx. The peristomal base is continuous with the tubular esophagus, the region connecting the two being ensheathed by a fibrous layer and low density cytoplasm. The esophagus has a microtubular/microfilamentous wall. A distinct cytoproct with associated myonemal structures occurs posteriorly. The skeletal plates consist of a large number of interconnected, variably shaped platelets and may have dual skeletal and storage functions. The endoplasm is more vesicular than the ectoplasm, the two separated by a fibrous boundary layer. The five-layered cortex has an external glycocalyx, a plasma membrane with two subtending membranes, homogeneous, microtubular, and microfilamentous layers. The syncilia of the AZS are mounted in a U-shaped band on the peristome with transversely oriented kinetics consisting of kinetosomes linked by a sub-kinetosomal rod. There is a bifurcated kinetodesma, dense support material forming a lateral spur with associated transverse microtubules, and postciliary, interkinetal, and occasional basal microtubules, nematodesmata, and a subciliary reticulum. A barren, possibly vestigial, somatic infraciliature consists of non-ciliated kinetosomes and a basal striated fiber with associated basal and perpendicular (cortical) microtubules.     

Relative motion inAmoeba proteus in respect to the adhesion sites. I. Behavior of monotactic forms and the mechanism of fountain phenomenon

Summary The unbranched ectoplasmic cylinder of monotacticA. proteus is always retracted toward the cell-substrate attachment sites. The retraction velocity increases from the adhesion sites toward any free distal body end in a linear way, which indicates the uniform contractility of the whole cylinder. Therefore, in the cells frontally attached all the ectoplasm moves forward, and in those adhering by the tail the whole ectoplasmic tube moves backward producing the full fountain phenomenon. With cell attachment at the middle body regions, which is most typical for normal locomotion, the whole ectoplasm is centripetally retracted from both body poles toward the adhesion zone, producing then the tail retraction in the posterior and incomplete fountain in the anterior body part. In unattached amoebae the whole peripheral tube is retracted toward its geometrical centre which coincides with its posterior closed end, producing therefore also a full fountain. It is generalized that the fountain arises always between an unattached front and the nearest attachment point behind its manifestation zone. The photographic records of movement and longitudinal velocity profiles of ectoplasmic retraction are identical on both sides of the attachment points, suggesting the same mechanism for the fountain movement as for the tail withdrawal. It is concluded therefore that not the axial endoplasmic arm of the fountain is active, but its peripheral arm built of the ectoplasm.All elements complicating the cell contour, as the constriction rings and ephemeral lateral pseudopodia, do not change their position in respect to the ectoplasmic material, but move together with it in respect to the substrate, i.e., the cytoskeleton moves as a whole. Loose glass rods attached by adhesion to cell surface also precisely follow the cytoskeleton movements, being transported toward the main locomotory adhesion zone established on the firm substrate, although the cell membrane as such behaves differently. It suggests a direct connection between the adhesion sites and the cytoskeleton.Study supported by Research Project II. 1 of the Polish Academy of Science.I dedicate this paper to the memory of Reginald J. Goldacre, deceased in December 1983, who twenty years ago introduced me to the study of amoebae.     

Ultrastructure of sperm development in the free-living marine nematode Metachromadora itoi (Chromadoria, Desmodorida)

The spermatogenesis of the free‐living marine nematode Metachromadora itoi was studied with electron microscopy. Spermatocytes and early spermatids have no cytoplasmic components specific for nematodes, i.e. membranous organelles (MO) and fibrous bodies (FB). The late spermatids are subdivided into the residual body and the main cell body with a centrally located nucleus devoid of a nuclear envelope. A pair of 9 × 2 centrioles is associated with the nuclei of spermatids and spermatozoa. The nucleus of the mature spermatid is surrounded by a thick mass of radially arranged FB delimited externally by a discontinuous layer of mitochondria, which underlie a thin ectoplasm. Sperm development is accompanied by transfer of FB matter through the mitochondrion layer into the ectoplasm. The immature spermatozoa from the testis have the centrally located nucleus surrounded by a transparent halo with remnants of FB. The halo is delimited by a sphere of mitochondria that underlie the thick fibrous ectoplasm, a derivative of the FB. In the mature spermatozoa the ectoplasm is transformed into the prominent unpolarized pseudopod. The central nucleus is surrounded by a transparent halo and a sphere of mitochondria, which underlie the pseudopod. MO were not found throughout spermatogenesis. In general, spermatogenesis in M. itoi differs from that observed in many nematodes but resembles in some details the sperm development in some chromadorid and tylenchomorph nematodes. The phylogenetic importance of this sperm development is discussed.     

The contractile basis of amoeboid movement : IV. The viscoelasticity and contractility of amoeba cytoplasm in vivo

The viscoelasticity and contractility of amoeba cytoplasm has been studied in vivo and in vitro. A gradient of increasing viscoelasticity and contractility was identified in the endoplasm of intact cells from the uroid (tail) to the fountain zone (tip of advancing pseudopod). Anterior endoplasm, as well as all of the ectoplasm, contracted in response to the microinjection of a threshold calcium ion concentration (ca 7.0 × 10⁻⁷ M). In contrast, there were only delayed weak contractions in the uroid endoplasm upon the microinjection of a threshold calcium ion concentration. Contractions induced in the ectoplasm by microinjecting the contraction solution readily caused the endoplasm to stream. However, the endoplasm at the tips of the extending pseudopods were also contractile and transmitted applied tensions. Furthermore, the microinjection of subthreshold calcium ion concentrations caused the loss of distinct endoplasmic structure and the cessation of streaming in both the uroid and the anterior third of the cell. In addition, the relationship between contractility and cytoplasmic streaming was characterized in “relaxed” cytoplasm placed in a gradient of calcium ion concentration inside quartz capillaries. The results of these experiments demonstrated that the mechanochemical conversion of endoplasm to ectoplasm caused the cytoplasm to become more structured and contractile. Therefore, physiological contractions are possible during and after the conversion of endoplasm to ectoplasm.     

Ultrastructural study of sensory cells of the proboscidial glandular epithelium of Riseriellus occultus (Nemertea,Heteronemertea)

Only one sensory cell type has been observed within the glandular epithelium of the proboscis in the heteronemertine Riseriellus occultus. These bipolar cells are abundant and scattered singly throughout the proboscis length. The apical surface of each dendrite bears a single cilium enclosed by a ring of six to eight prominent microvilli. The cilium has the typical 9×2 + 2 axoneme arrangement and is equipped with a cross-striated vertical rootlet extending from the basal body. No accessory centriole or horizontal rootlet was observed. Large, modified microvilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvillus contains a bundle of actin-like filaments which anchor on the indented inner surface of a dense, apical ring situated beneath the level of the ciliary basal body. The tip of the cilium is expanded and modified to form a bulb-like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerges from the microvillar cone, the membrane of the microvillar apices makes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junctional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors. © 1996 Wiley-Liss, Inc.     

Capture and Ingestion of Paramecium by Didinium nasutum

SYNOPSIS. This study attempts to analyze how a number of specialized organelles are coordinated in a rapid but complex sequence of events during the capture and ingestion of prey by Didinium . Accidental contact between a Paramecium and the proboscis of a hungry Didinium causes instantaneous discharge of 2 kinds of extrusive organelles from the latter. The short pexicysts attach to the surface of Paramecium but do not penerate. The long toxicysts penetrate the victim, altho their proximal end s remain embedded in the proboscis of Didinium . Cytoplasmic streaming pulls the discharged toxicysts inward, and since they are firmly attached to the prey, the latter is drawn steadily into the cytostome. As the prey is being swallowed, it is compressed, and its volume is reduced by elimination of fluid. When the prey is completely inside the predator, the temporary cytostome closes, and the proboscis is reformed.