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Newly hatched Artemia larvae use one pair of limbs to locomote. During development they gradually add additional limbs along the elongating trunk. As larvae grow, body length increases from about 0.4 mm to 4 mm, mean swimming speed increases from 1.8 mm s-l to 9.9 mm s-', and frequency of antenna1 beat decreases from 9.5 to 6.7 Hz. As new limbs are added, they become active in the metachronal rhythm of pre-existing limbs. The body velocity oscillates as early larvae swim; later larvae swim without a cyclic acceleration and deceleration of the body. The change in the pattern of swimming is correlated with the addition of propulsors and a transition in the relative importance of viscous and inertial effects that determine the propulsion in subsequent stages. Reynolds number (based on body length) increases from 2 to 37. A theoretical analysis of rowing propulsion at these in- termediate Reynolds numbers shows that initial devel- opment of new limbs in Artemia larvae is unimportant for propulsion. Rowing propulsion at the low Reynolds numbers is drag-based; as Reynolds number increases, inertial effects become more important, and unsteady forces on the body become significant in the balance between limb and body. A glide of the body develops at the end of the powerstroke, and relative limb velocity changes.

Tylos granulatus is an isopod living above the driftline on sandy shores, burrowing to depths of up to a metre or more. Burrowing has been investigated by analyzing video recordings and traces from isotonic and isometric transducers. In crawling, all seven pairs of pereiopods operate in a similar manner and display metachronal rhythm, while in burrowing pereiopods 1 to 3 do the digging, pairs 4 to 6 compact the excavated sand, and pair 7 pushes the bolus behind the animal. Movements of the head also play a significant role in burrowing. Clockwise or anticlockwise partial rotation occurs during burrowing, which takes place in a stepwise progression. With a Burrowing Rate Index of 3, the animal must be considered a powerful burrower, while the energy cost of burrowing is surprisingly low at some 0.3 J m-1.

Dense populations of Urceolaria mitra (Cilophora, Peritrichid) were observed on the surface of the planarian Polycelis tenuis (Platyhelminthes Turbellaria). Although the epizoite was primarily confined to the anterior dorsal surface of the planarian, heavily infested individuals had peritrichs attached to the posterior, lateral and ciliated ventral surfaces. Unique profiles of the skeletal ring (aboral disc) in relation to the host surface showed that some local erosion of planarian epithelium occurs. Outer and inner rows of cilia extended anti-clockwise around the peristomal disc. The cilia ran for one and one-eighth turns around the disc and overlapped in the form of a spiral when the protist was extended and feeding. The cilia appeared to beat in metachronal rhythm and exhibited a wavelength and amplitude of approximately 10μm. Regular striations having a periodicity of 1μm were observed in the periostimal membrane. A posterior or basal circlet of locomotor cilia was also observed and freeze fracture revealed an internal arrangement of microvilli, cilia and reticular membranes.

Nearly all invertebrates that walk are arthropods. These all use a metachronal rhythm in which the legs move in sequence from rear to font. Rhythmic leg movements are produced by networks of neurons called central pattern generators (CPGs). CPGs are coupled together to produce a patterned sequence of leg movements, but can be influenced by sensory feedback from the legs to adapt locomotion to rough or unpredictable terrain. Higher control of walking, starting, stopping, turning, and speed, are controlled by the brain. Walking in arthropods has been used successfully as a model for the design and construction of walking robots.

SUMMARY Feeding and escape swimming in adult females of the calanoid copepod Temora longicornis Müller were investigated and compared. Swimming velocities were calculated using a 3-D filming setup. Foraging velocities ranged between 2 and 6 mm s-1, while maximum velocities of up to 80 mm s-1 were reached during escape responses. Foraging took place at Reynolds numbers between 2 and 6, indicating that viscous forces are considerable during this swimming mode. Inertial forces are much more important during escape responses, when Reynolds numbers of more than 100 are reached. High-speed film recordings at 500 frames s-1 of the motion pattern of the feeding appendages and the escape movement of the swimming legs revealed that the two swimming modes are essentially very different. While foraging, the first three mouth appendages (antennae, mandibular palps and maxillules) create a backwards motion of water with a metachronal beating pattern. During escape movements the mouth appendages stop moving and the swimming legs beat in a very fast metachronal rhythm, accelerating a jet of water backwards. The large antennules are folded backwards, resulting in a streamlined body shape. Particle image velocimetry analysis of the flow around foraging and escaping copepods revealed that during foraging an asymmetrical vortex system is created on the ventral side of the animal. The feeding motion is steady over a long period of time. The rate of energy dissipation due to viscous friction relates directly to the energetic cost of the feeding current. During escape responses a vortex ring appears behind the animal, which dissipates over time. Several seconds after cessation of swimming leg movements, energy dissipation can still be measured. During escape responses the rate of energy dissipation due to viscous friction increases by up to two orders of magnitude compared to the rate when foraging.

Decapod Crustacea display a slow metachronal rhythm of the third maxillipeds and pereiopod pairs one to four when undisturbed in the natural habitat. The dactyl tips are lifted off the substrate, and the unweighted limbs promote and remote at a slow frequency (range for all species 9–30min− 1). Movement is limited to the T-C joint. This behaviour, called waving, has been observed in many macrurans and an analogous activity, limited to the third maxillipeds, has been seen in several brachyurans. Analysis of EMGs shows: (a) the promotor activity increases in strength during a burst due to facilitation and motoneurone recruitment; (b) the rhythm period is stable throughout long waving sessions; (c) the promotor and remotor strokes are of equal duration and co-vary with period. Ipsilateral coupling is strong, with equal phases between adjacent limbs of 0· 05. The metachronal wave can pass anteriorly or posteriorly along an ipsilateral row. Bilateral coupling is weak. The two sides maintain equal frequency, and antiphasic coordination between the two ipsilateral sets of limbs is favoured. Possible functions for waving include gill grooming and supplementary gill ventilation. Comparisons between waving and other rhythmical motor programmes are discussed. Waving is an alternative programme for the walking legs, and is expressed when proprioceptive feedback is reduced.

Leeches swim by undulating their extended and flattened body in the dorsoventral direction, to form a wave that travels backwards along the animal. The troughs and crests of this body wave are produced by a metachronal rhythm of antiphasic contractions of the dorsal and ventral longitudinal musculature of the body wall of successive segments. Cinematographic records of swimming leeches show that over a range of cycle periods from 390 to 1100 msec the animal maintains one full wave along the length of its body. This constant wave form is achieved by compensating for the increase in the cycle period by an increase in the intersegmental travel time of the wave from 19 to 77 msec per segment. Direct muscle tension measurements of the segmental body wall during the swimming movement of a restrained, brainless and partially denervated leech lead to values for the dynamic parameters of the body wave which are in agreement with those abstracted from cinematographic records. In order to study the neuronal control of the swimming movement, a semi-intact leech preparation was developed. This preparation carries out movements that are clearly vestiges of the swimming rhythm while allowing the taking of electrophysiological records from exposed and immobilized parts of its peripheral and central nervous systems. The records reveal a nerve cell activity rhythm whose period matches that of the swimming rhythm. The swimming rhythm and its body wave must be generated by a system of distributed, phase-locked segmental oscillators that cause an antiphasic activity of the motor neurons innervating the segmental dorsal and ventral longitudinal muscles. The cycle of these oscillators can be inferred to consist of a variable time sector whose changes in length are responsible for changes in the cycle period, and of a constant time sector whose length is independent of the cycle period.

The gills of some Ephemerid nymphs are always motionless, e. g ., many Bætine forms of English streams. In many others, however, the gills move rapidly in metachronal rhythm, by virtue of which currents are created in the water. These currents are peculiar to the species and probably have an adaptational significance. In many forms already under investigation, e. g ., Cholen dipterum. Siphlurus sp. Ecdyonurus venosus, Ephemerella sp. Leptophlebia marginata and Ephemera vulgata , a common feature is noticeable. This is that in their rhythmical movements both members of each pair of gills beat together, i. e ., and their movements are co-phasedly synchronized. Since, therefore, the effect of the gills on one side of the body is exactly duplicated on the other, whatever may be the precise mechanism for the production of currents, the latter are symmetrical with the longitudinal axis of the body (Eastham, 1932). An intersting exception in the nymph of Caenis horaria . In this animal the currents pass over the body from one side to the other. The gills beat in metachronal rhythm down each side of the body, but though the rhythms are synchronous there is a time phase difference between them. In other words, members of a pair are not co-phasedly synchronized in movement. We have thus in Caenis horaria a bi-laterally symmetrical animal producing movements in the surrounding medium which are not of the nature of an axial flow. It is with this phenomenon that this paper deals.

Termination of peripheral filaments of the axoneme of gill cilia of fresh-water mussels (Elliptio or Anodonta) occurs in characteristic fashion: (a) subfiber b of certain doublets ends leaving a single simplified tubular unit; (b) the wall of the unit becomes thick and may even obliterate the interior; and (c) the filament drops out of the 9 + 2 pattern. The order in which doublets begin simplifying is also characteristic. This may be determined by numbering the filaments, those with the bridge being 5–6, with the direction of numbering determined by the apparent enantiomorphic configuration (I to IV) of the cross-section. Shorter filaments can be identified in simplifying tips with mixed double and single peripheral units. In this material, laterofrontal cirri show a morphological specialization in the region where individual cilia simplify. The cilia studied run frontally from the body of the cirrus and point in the direction of effective stroke. The longest filaments (Nos. 3, 4, 5, 6, 7) appear as the doublets at the bottom of the cross-section, nearest the surface of the cell of origin. Above them, and above the central pair, a dark band (a section of a dense rod) runs through the matrix. The remaining filaments are the single units. Effective-pointing frontal and lateral ciliary tips end in a fashion similar to laterofrontal tips, although no dense band is present. For all effective-pointing tips studied, the order in which the peripheral filaments end appears to be Nos. (9, 1), 8, 2, 7, 6, 3, 4, 5. However, recovery-pointing lateral tips show a different order: Nos. 7, 6, 8, 5, 9, 4, 1 (3, 2), although the longer filaments are still at the bottom of the cross-section. In simple models of ciliary movement involving contraction of the peripheral filaments, filaments at the top of the cross-section should be longer, if any are. Such models are not supported by the evidence here. These results can be interpreted as supporting sliding-filament models of movement where no length change of peripheral filaments occurs.

The swimming and burrowing habits of four species of the genus Bathyporeia have been examined and shown to be alike, and similar to B. robertsoni as described by Schellenberg.Swimming is effected by the pleopods working in a metachronal rhythm, drawing water along a ventral groove formed by the coxal plates of the second peraeopods and the expanded bases of peraeopods 3, 4 and 5.The main water current enters anteriorly between the second gnathopods and is filtered by a series of feathered bristles borne on the basis of this appendage; a subsidiary current enters between the second and third peraeopods.Burrowing is effected mainly by the sweeping action of the second gnathopods and first two pairs of peraeopods, aided by the first and second uropods as soon as they can obtain a purchase on the sand. The adaptation of these three pairs of appendages to burrowing is explained.

1. The gills of the nymph of Ecdyonurus consist each of a gill plate and a proximal gill tuft. The gill plate of the first gill is reduced in size while the seventh gill possesses no gill tuft. 2. By movements of the gills, currents are set up in the water which are symmetrical with the body axis, and which pass from the sides of the body upwards to the mid-dorsal line and backwards. Members of pairs beat synchronously and in phase with each other. 3. Each gill pivots on its pedicel and moving backwards and upwards, increases its angle of incidence with the water through which it passes. This phase is the effective part of an oscillation and is brought about with the posterior gill surface in the leading position. The recovery beat is effected by the gill turning so as to place the anterior border in a leading position. The direction of flow is largely determined by these gill movements. 4. The gills beat in metachronal rhythm from before backwards, the time phase difference between adjacent gills being so small as to make the gills appear to work together and without rhythm. The rhythm, however, is important in setting up suction and compression phases, the compression phase occurring during backward effective movement, the suction during forward recovery movement. Compression expresses itself as a flow from between the gills upwards and backwards, suction as a flow into the intergill space from the sides and below. 5. The principles of gill movement in relation to water flow in Caenis, Leptophlebia and Ecdyonurus are compared. 6. The currents and gill movements in adaptation to environment are briefly discussed. 7. Campaniform sense organs occur on both surfaces of the gill plates and it is suggested that these are concerned with informing the animal of changes in the environmental flow of water.

1. 1. Two sets of extensor and two sets of flexor muscles at the pereiopod base of Ligia oceanica provide most of the locomotory effort. 2. 2. The segments of the endopodite have only a modifying influence on the precise position of the limb and evidently do not contribute much to the effective locomotory stroke. 3. 3. Stepping pattern is basically a metachronal rhythm. The gait of the animal does not alter at different walking speeds.

SUMMABY 1 The rearing of Balanus nauplii in the laboratory and the stage at which hatching occurs are discussed. 2 The external morphology of the second stage nauplius of Balanus per-foratus is described. The setae on the limbs I divide into two types– swimming setae and feeding setae. 3 The limbs beat in a metachronal rhythm. During the back-stroke particles are swept backwards and inwards, converging from each side just behind the body. From here they are sucked forward along the ventral surface during the fore-stroke of the mandibles and antennae. 4 They are retained in the region behind the labrum by the feeding setae and their setules, by spinules on the ventral surface of the body, and by setules on the tip of the labrum. 5 As the limbs beat back the feeding setae push the particles towards the mouth. A groove under the labrum leads to the mouth itself, into which the particles are pushed by prongs on the antennal jaw processes. 6 Feeding is rapid and not selective. But particles over 6/x in diameter cannot be swallowed, though diatoms 25 fi long are freely taken. It appears that single cells may be torn from filamentous algae and then swallowed. But there is never any true mastication, and larger bodies do not form part of the food. 7 The first stage nauplius does not feed. The mouth is apparently open, but the feeding structures are very rudimentary, and particles can thus reach the mouth only by accident.

1. The six pairs of gills in Ephemera danica causing currents in the water are bilamellate and feather-like. 2. The gills move in metachronal rhythm from before backwards, and set up currents which are symmetrical with the body axis. Members of pairs of gills beat synchronously and in phase with each other. The difference of phase between two adjacent gills of the same side is about one-eighth of a complete oscillation. 3. For a single gill there is little or no difference in the angle presented by the gill to its own path of motion, in the two halves of an oscillation. This factor (angle of incidence) is not of great importance in directing water backwards. Forces of importance in moving water in the direction taken are (i) the resultant of axial and transverse force components for single gills, (ii) the pressure effected on the water over the animal's back by the two members of a pair of gills falling, (iii) the movement backwards of alternating pressure regions over the back, (iv) the undulations of the second screw-like gills which feed the main current from in front and from the sides. 4. The fringing filaments of adjacent gills overlap each other from before backwards (as seen from the middle line of the body), and thus each row of gills forms a "membrane" composed of separate gill units. The latter appear never to lose contact with each other during movement, and since no sideways flow is observed between the gills it is assumed that the overlapping filaments between the gills form a membrane impermeable to flow. 5. The adaptation of Ephemera to its sandy environment is shortly discussed.

1. 1. Lateral cilia on the gill of the fingernail clam, Musculium transversum, normally beats with a metachronal rhythm. 2. 2. The metachronal rhythm is maintained in the presence of 10.0 mM calcium chloride (CaCl2), 15 mM potassium chloride (KCl) or 15 mM NaCl. 3. 3. Lateral ciliary arrest occurred when the gill perfusion contained 10 mM CaCl2 and 10−5 M (small clams) or 10−6 M (large clams) A23187 (a calcium ionophore). 4. 4. Lateral ciliary arrest is not produced in either CaCl2 or ionophore and monovalent cations alone. 5. 5. Restoration of the metachronal rhythm results with superfusion of lateral cilia in KCl, CaCl2 or NaCl alone. 6. 6. These data show the important role that intracellular Ca2 + concentration has on the behavior of lateral cilia. The physiological significance seems to be the momentary or complete shut down of water currents under unfavorable environmental conditions.

SWIMMERETS in the crayfish are the paired ventral abdominal appendages which beat in a metachronal rhythm during behaviours such as swimming and burrow ventilation. Each swimmeret is driven by alternating bursts of impulses in antagonistic power- and return-stroke motoneurones. Although proprioceptive input affects the detailed structure of the rhythm, the basic motor programme can be elicited from the deafferented abdominal central nervous system (CNS) by tonic stimulation of single ‘command fibres’ in the ventral nerve cord1–3. Each swimmeret is controlled by a hemiganglion capable of generating the rhythm in isolation, although normally the most posterior swimmeret ganglion (the 5th) acts as a pacemaker, regulating the activity of the other ganglia through a series of coordinating fibres located in the medial region of the ventral nerve cord4. I report here the preliminary results of an investigation into the structure of the central neural oscillator controlling swimmeret movements in crayfish. The results suggest that motoneurones in this system do not act merely as output elements which passively relay the activity of an oscillator formed from pre-motor neurones within the CNS, but that the motoneurones themselves are an integral part of the pattern generating circuits. There is both excitatory and inhibitory central coupling between motoneurones mediated either by electrical or graded chemical synapses. Furthermore, the activity of some motoneurones can directly influence the intensity, period, and phase of the oscillation.

Lateral cilia of freshwater mussel gills, which normally beat with metachronal rhythm, are arrested pointing frontally by perfusion with 6.25 to 12.5 millimolar calcium and 10(-5) molar A23187, a calcium ionophore. Arrest does not occur in either calcium or ionophore and monovalent cations alone. Activity returns with continued perfusion in potassium chloride or calcium chloride, and more slowly in sodium chloride, after removal of ionophore. These results support the hypothesis that a local rise in internal calcium causes ciliary arrest.

SYNOPSIS. Elaboration of ciliated feeding organelles in the protozoon Stentor coeruleus was reinvestigated for the first time by scanning electron microscopy which gives the most realistic 3-dimensional images. Parallel transmission EM studies of synchronized regenerating stentors gave further ultrastructural details of stomatogenesis, while also confirming the expectation that in the structure of its kineties this now classical experimental object does not differ from other species of Stentor previously studied. Within 2 hr after the stimulus to regeneration, several generations of new kinetosomes for the oral primordium are produced, first in association with kinetosomes of kineties at the restricted primordium site. These kinetosomes rapidly sprout membranellar cilia as well as subpellicular microtubules but are still randomly oriented (anarchic field). The forming membranellar band increases from its center-line to both sides while it grows in length. Young cilia are blunt-ended. Recession of the early anlage occurs without rupture of the pellicle; soon apparent is the clear border stripe of unknown function along the right side of the membranellar band. Instantaneous fixation of beating cilia in early primordia revealed random beating, with coordination and presumably membranellar organization not yet attained. In late anlagen there are 2 types of metachronal rhythm: transversely from cilium to cilium across any given membranelle, as well as the easily observable serial beating of membranelles along the entire band. A single file of cilia leads the subsequent cytostomal invagination. The posterior end of the membranellar band then follows to line the cytopharynx.

THE membranelles (compound cilia) of the protozoan Stentor are arranged in a single row around the oral end of the animal. Each membranelle swings through an angle of beat of about 140° in a plane at right angles to the row, while the metachronal waves pass along the row from one end to the other. It has been established1,2 that these membranelles are normally co-ordinated to beat in a metachronal rhythm by an internal transmission mechanism, so that the conduction of metachronal ‘impulses’ and the beating of the cilia are separable activities of the system, having quite different properties.