SAFETY FACTORS OF TROPICAL VERSUS TEMPERATE LIMPET SHELLS: MULTIPLE SELECTION PRESSURES ON A SINGLE STRUCTURE

A comparison of the safety factors of tropical and temperate limpet shells in the eastern Pacific yielded two results of significance. A safety factor was defined as shell strength/maximum tenacity, where maximum tenacity (force required to detach foot) determines the maximum prying force that a crab or bird predator can exert on the shell. 1) On average, shell strength and foot tenacity for the tropical limpets were twice those for the temperate limpets. In contrast, the average safety factors for the two groups were approximately equal. This comparatively narrow range of safety factors was due to a highly significant association of greater shell strengths with greater foot tenacities. The implication of this result is that selection has acted to closely link the mechanical performances of these two rather independent structures, the shell and the foot. 2) The presence of an additional class of predators which feed on the tropical limpets was reflected in the safety factors of their shells. Whereas the shells of both tropical and temperate limpets are exposed to predator-induced prying forces, the shells of the tropical group are also exposed to lateral crushing forces generated by fish predators. This additional selection pressure was associated with several deviations from a regression of safety factor versus variability in shell strength which had been calculated previously for the temperate limpets. As predicted, the magnitudes of these deviations were correlated with the degree of exposure to this additional selection pressure. Hence, the presence of more than one selection pressure appears to have influenced the precision with which the shells of these species have become adapted to a single selection pressure. The use of safety factor analysis provides a very useful methodology for identifying additional selection pressures or adaptive constraints on biological structures.     

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Discrete movements of foot epithelium during adhesive locomotion of a land snail

During the adhesive locomotion of land snails a series of short dark transverse bands, called pedal or foot waves, is visible ifa moving snail's ventral surface is observed through a sheet of glass. Moreover, the mucus secreted from the pedal glands and some pedal epithelial cells forms a thin layer which acts as a glue augmenting adherence, while also acting as a lubricant under the moving parts of the snail's foot. The relationships between velocity and the frequency of pedal waves as well as changes in the volume of small air bubbles under foot waves were analyzed by means of digital recordings made through a glass sheet on which the snails were moving. On the ventral surface of a moving snail foot, the adhering parts of the foot constituted about 80% of the total area, while several moving parts only about 20%. The single moving region of the foot (the pedal wave) amounted to about 3% of snail length. The epithelium in the region of the pedal wave was arched above the substrate and was also more wrinkled than the stationary epithelium, which enabled the forward motion of each specific point of epithelium during the passage of a pedal wave above it. The actual area of epithelium engaged by a pedal wave was at least 30% greater than the area of the epithelium as recorded through a glass sheet. In the region of the pedal wave, the tiny subepithelial muscles acting on the epithelium move it up in the front part of the wave, and then down at the end of the wave, operating vertically in relation to the substrate. In the middle part of the wave, the epithelium only moves forward. In summary, during the adhesive locomotion of snails, the horizontal movement of the ventral surface epithelium proceeds as temporally separate phases of upward, forward and downward movement.     

The tenacity of the limpet, Patella vulgata L.: An experimental approach

It has been shown that adhesion of the limpet, Patella vulgata L. is influenced by both physical and physiological factors. The tenacity is sensitive to surface properties of the substratum, varying inversely with the contact angle which water makes with a substratum. This can be explained in terms of thermodynamics. Surface roughness also affects tenacity and this is explained in the same manner. Different angles of detachment were tested and it was clearly shown that when a strong peeling component was introduced, a much reduced force was needed to detach a limpet. Contrary to a normal pull, when a shear pull is exerted the force is not proportional to the surface area of the foot. It has also been shown that the speed of separation affects the measured tenacity; there is a speed at which tenacity will be maximum. The effect of water temperature on tenacity has been tested, tenacity increasing with rising temperature (7, 13, 20 °C). At the higher temperatures limpets are able to contract the foot muscles more powerfully, indicating that increased foot rigidity increases tenacity. By measuring the tenacity of limpets left out of water for different periods of time it has been shown that desiccation has no effect on tenacity, but a change from aquatic to aerial respiration increases tenacity. Tenacity has also been measured when the limpets have been subjected to a reduction in metabolic rate. The effect of both anoxia and narcotization shows that reduced muscle tonus, especially in the foot, results in decreased tenacity. These results further demonstrate that foot rigidity is essential for efficient adhesion. Eimpets from different habitats (exposed and sheltered) and vertical distribution (high and low level on shore) exhibited no differences in tenacity. During locomotion limpets leave a mucous trail, most of the mucus being confined to the edge of the trail. Water is incorporated anteriorly under each new locomotory wave and these pockets of water are used to release the mucus from the substratum during locomotion. It is concluded from this study that limpet adhesion can be explained solely by the tackiness of the pedal mucus, tack being due to the stored elastic energy within the mucous layer itself.     

COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUBANTARCTIC LIMPETS NACELLA CONCINNA AND KERGUELENELLA LATERALIS

Two limpet species occur intertidally on subantarctic SouthGeorgia, the patellid Nacella concinna and the siphonarlid Kerguelenellalateralis. N. concinna is confined to the lower shore closeto LWS; K. lateralis occurs in middle shore pools, so theirdistributions do not overlap. N. concinna has a much narrowerthermal niche (–12.9°C to +15.6°C) than K. lateralis(–17.8°C to +31.8°C). Environmental data are presentedto show that the upper lethal temperature of N. concinna islow enough to prevent the limpet living higher on the shore.Both limpet species are slow-moving, but K. lateralis showsincreasing speed with rising temperature, peaking at 15–20°C.In contrast, N. concinna moves actively down to –1.9°C(when sea water freezes), but there is a steady decrease inspeed of locomotion above +2°C. Locomotion ceases at 14°Cin N. concinna (c.f. 30°C in K. lateralis). Both speciesexhibit very low tenacities, but in N. concinna tenacity decreaseswith increasing shell length. In K. lateralis there is no effectof temperature on tenacity. Both species show a positive allometricrelationship between foot area and shell length. N. concinnafeeds upon microbial films and microepiflora, but K. lateraliseats colonial diatoms and Enteromorpha bulbosa. Observationson shell middens of the kelp gull Larus dominicanus showed thatthe gulls did not eat K. lateralis, though they ate great quantitiesof the less accessible N. concinna. Gulls ate N. concinna assmall as 11 mm shell length (within the size range of K. lateralis).Experiments on gulls demonstrated an unwillingness to eat K.lateralis, probably because the siphonariid extrudes a viscidwhite mucus when the foot is touched. (Received 9 May 1996; accepted 8 July 1996)     

Involvement of pedal peptide in locomotion in Aplysia: modulation of foot muscle contractions

Pedal peptide (Pep) is a 15-amino-acid neuropeptide that is localized within the Aplysia central nervous system (CNS) predominantly to a broad band of neurons in each pedal ganglion. Pep-neurons were identified by intracellular staining and immunocytology or by radioimmunoassay (RIA) of extracts from identified neurons. RIA reveals that 97% of all Pep-like immunoreactivity (IR-Pep) in pedal nerves is found in the three nerves that innervate the foot. Nearly every Pep-neuron sends an axon out at least one of these three nerves. Application of Pep to foot muscle causes an increase in the amplitude and relaxation rate of contractions driven by nerve stimulation or intracellular stimulation of pedal motor neurons. The increase in relaxation rate was the predominant effect. Intracellular recording in "split-foot" preparations reveals that Pep-neurons increase their overall firing rates and fire in bursts with each step during locomotion. Recovery of IR-Pep from foot perfusate following pedal nerve stimulation increases in a frequency-dependent fashion. Thus it appears that one function of Pep-neurons is to modulate foot muscle contractility during locomotion in Aplysia.     

The effects of exposure and predation on the shell of two British winkles

Changes in shell size and shell shape of the two British winkles Littorina nigrolineata and L. rudis were studied in relation to exposure and to crab-size. In both species, shells from exposed shores are smaller and more globose than those from sheltered shores. Also, in rudis of exposed shores the mouth is relatively wider. In shores of equally sheltered conditions, shells are bigger at those localities where crabs are large than at those localities where they are small. The largest shells are found in those localities where it is extremely sheltered, and the crabs are very large.
It is argued that on exposed shores, small shells are favoured because they have more possibilities than large ones to shelter in crevices and in barnacle interspaces, from the impact of winds and waves. A globose shell could accommodate more foot muscle and thus enable a stronger adherence to the rock; and an increased mouth diameter would increase the area of foot adherence to the rock. On sheltered shores, on the other hand, large, narrow-mouthed shells are favoured because they discourage crab predation, large crabs being abundant mainly on sheltered shores.
The possible significance of shell size and shape in relation to zonation is discussed, in view of the different predatory and physical conditions which prevail in different zones of the shore, and the different shell specializations which these conditions would require.     

Functional morphology of the hindlimb of tupaiids (Mammalia,Scandentia) and its phylogenetic implications

In this study, the hindlimb of 12 species of tupaiids was analyzed functionally and compared to that of primates, dermopterans, and chiropterans. Many aspects of the tupaiid hindlimb vary in relation to differential substrate use. These differences include width of the ilium, shape of the acetabulum, size of the anterior inferior iliac spine, size of the greater and third trochanters, depth of the femoral condyles, shape of the patellar groove, and size of the tibial tuberosity. The hindlimb of the arboreal Ptilocercus lowii, the only ptilocercine, is better adapted for arboreal locomotion, whereas that of tupaiines is better adapted for rapid terrestrial (or scansorial) locomotion. The hindlimb of Ptilocercus seems to be habitually flexed and has more joint mobility, a condition necessary for movement on uneven, discontinuous arboreal supports. The tarsus of Ptilocercus facilitates inversion of the foot and its grasping hallux is capable of a great range of abduction. Tupaiines, on the other hand, are characterized by more extended hindlimbs and less mobility in their joints. These restricted joints limit movements more to the parasagittal plane, which increases the efficiency of locomotion on a more even and continuous surface like the ground. The hindlimb of tupaiines is adapted for powerful flexion and extension. Even the most arboreal tupaiines remain similar to terrestrial tupaiines in their hindlimb morphology, which probably reflects the terrestrial ancestry of Tupaiinae (but not Tupaiidae). Many attributes of the tupaiid hindlimb, especially those of the foot, reflect the arboreal ancestry of Tupaiidae and it is proposed that the ancestral tupaiid was arboreal like Ptilocercus. Also, compared to the hindlimb character states of tupaiines, those of Ptilocercus are more similar to those of other archontans, and it is proposed that the hindlimb features of Ptilocercus are primitive for the Tupaiidae. Hence, Ptilocercus should be considered in any phylogenetic analysis that includes Scandentia.     

PALAEOBIOLOGY OF THE CLIMACTICHNITES TRACEMAKER

Abstract:  Trace fossils such as Climactichnites offer rare insights into the palaeobiology of Cambrian soft-bodied animals, especially those that inhabited emergent sand flats and are not known from body fossils. Analysis of field and museum Climactichnites , together with experiments on the preservation of similar modern trails, indicates that the tracemaker was an elongate, bilaterally symmetrical, dorsoventrally flattened, soft-bodied animal with a muscular foot. These characteristics are consistent with the tracemaker being a primitive mollusc or mollusc-like animal. Unlike most Neoproterozoic and Cambrian molluscs, the tracemaker could reach considerable size; at up to c . 69 cm long, it was one of the largest Cambrian animals of its time. When moving on the sediment surface, locomotion resulted from muscular waves generated along the sole of its flexible foot; the foot was extended and then clamped onto the substrate. Contraction of pedal muscles then pulled the body forward. Sedimentary structures associated with Climactichnites  wilsoni , such as polygonal desiccation cracks, raindrop impressions, adhesion structures and gas escape structures demonstrate that the animal inhabited intermittently subaerially exposed environments. The tracemaker's method of locomotion is similar to that employed by modern intertidal gastropods, which make Climactichnites -like trails on exposed sand flats. However, these modern trails are not preserved because of erosion by wind, waves, tides and subsequent bioturbation. Abundant microbial sedimentary structures are associated with C. wilsoni , and together with low levels of vertical bioturbation, intimate that microbial binding may have mediated the preservation of these early mollusc trails.     

CAUSE OF BIMODAL DISTRIBUTION IN THE SHAPE OF A TERRESTRIAL GASTROPOD

The distribution of a phenotypic state is often discontinuous and dispersed. An example of such a distribution can be found in the shell shapes of terrestrial gastropods, which exhibit a bimodal distribution whereby species possess either a tall shell or a flat shell. Here we propose a simple model to test the hypothesis that the bimodal distribution relates to the optimum shape for shell balance on the substrates. This model calculates the theoretical shell balance by moment and obtains empirical distribution of shell shape by compiling published data and performing a new analysis. The solution of the model supports one part of the hypothesis, showing that a low-spired shell is the best balanced and is better suited for locomotion on horizontal surface. Additionally, the model shows that both high- and low-spired shells are well balanced and suited on vertical surfaces. The shell with a spire index (shell height divided by diameter) of 1.4 is the least well balanced as a whole. Thus, spire index is expected to show a bimodal distribution with a valley at 1.4. This expectation was supported by empirical distribution of a spire index, suggesting that the bimodality of shell shape in terrestrial gastropods is related to shell balance.     

The burrowing process of Dentalium (Scaphopoda)

Investigations of the burrowing activity of Dentalium , using cine film and electronic recording techniques, have shown it to penetrate the sand in a series of steps, each termed a "digging cycle". Cycles involve first, pedal dilation, second, retraction followed by extension and probing of the foot. The epipodial lobes are elevated during pedal dilation and form a secure pedal anchor so that at retraction the shell is drawn down over the foot.
A comparison of the burrowing process in the Scaphopoda with that of the Bivalvia indicates that essentially the same mechanisms and sequence of activities are involved, for in both digging consists of the integration of pedal protraction and retraction with the application of shell and pedal anchors. The principal differences, such as the absence in Dentalium of water jets to loosen the sand and high pressures in the pedal haemocoele, are related to the form of their shell. The strength of the pedal anchor was determined and, relative to the weight of Dentalium , is comparable to that of bivalves. In contrast the probing force was relatively weak since the shell anchor of Dentalium , which holds the shell still during probing, is largely limited to its own weight, whereas that attained by the Bivalvia is principally due to the valves being pressed against the substrate by the opening moment of the ligament.     

Shell shape and land-snail habitat in a Mediterranean and desert fauna

The bimodal distribution of shell shape (height: diameter), that is found in various geographically widely separate and taxonomically distinct land snail faunas of many different regions of the world, occurs also in a Mediterranean fauna and in a desert fauna that is derived from it. The desert fauna is, however, closer to the bisector than the Mediterranean one. High-spired snails are mainly rock-dwellers, and equidimensional to low-spired snails are bush-dwellers or soil-diggers, with a few rock-dwellers; litter-dwellers are small-sized species that may have either high- or low-spired shells. These results are discussed in adaptive terms. Litter is probably the more primitive of these micro-environments. Many of the small, litter-dwelling snails are ovo-viviparous rather than oviparous, perhaps so as to avoid attacks on the eggs by saprophytic fungi. The shift away from the litter environment is accompanied by a trend to abandon the ovo-viviparous strategy, in favour of oviparity, the snail using its foot to dig into the soil and lay eggs. The conchometric differences between bush-, ground- and rock-dwelling snails may perhaps reflect selective pressure to increase the size of the foot; and constraints of a habitat that consists of narrow interspaces between rocky boulders. Snails that habitually dig into the ground during periods of inactivity, and roam over the ground when active, requires a very large foot and, consequently, a very large-mouthed shell to accommodate it; the result is an equidimensional shell, globose or turbiniform in shape. Snails that climb up vertical vegetation would also require a large foot, and consequently a large-mouthed shell to contain it. A fully globose shell would however be disadvantageous, since it might cause undesired torque. Hence, bush-dwellers tend to be flatter than soil-diggers. Snails that habitually live in rock crevices, and on hard substrata, would not require a very large foot; they would need a narrow shell, both to enable easy manoeuvring through crevices and to reduce torque, the result being a small-mouthed, usually high-spired shell. The classification of land snails into bush-, soil- or rock-dwellers closely follows the taxonomic classification. In those species that depart from the habitat that is typical of their taxonomic group towards another habitat, the shell alters its shape accordingly.     

Locomotion in the pulmonate snail Melampus--II. Recovery after pedal ganglion excision

1. When one pedal ganglion is removed, snails first crawl using the unoperated side of the foot, but in 4-8 weeks the operated side exhibits an anterior-to-posterior gradient of recovery. 2. A ganglion bud bridges the site of the missing ganglion and axons project from intact central ganglia into the foot. 3. Rhythmic activity in right and left pedal nerve pairs is correlated during locomotion in the regenerated snails. 4. The oscillator in the remaining pedal ganglion drives bilaterally coordinated activity. Regenerated projections from the cerebral ganglia through the bud to the remaining pedal ganglion suffice to initiate locomotion.     

Resistance to shell breaking in two intertidal snails

The ability of shells to withstand shell breaking forces has been examined in two intertidal prosobranchs, Nucella lapillus and Littorina littorea , using four methods: measuring shell strength on a compressive testing machine, measuring the shell to body mass ratio, measuring the shell thickness and measuring the ability of crabs to break shells in aquarium experiments. Nucella lapillus consistently showed a relationship between shell vulnerability and environmental variables: the shells were easier to break at sites where rock and boulder movement was the least. Although some between-site differences were found in L. littorea shells, these were less than in N. lapillus and did not relate to environment variables: the shells were easier to break at sites where exposure to wave action was the least. Although some between-site differences were found in L. littorea shells , these were less than in N. lapillus and did not relate to environmental factors. However, both species appear to grow into a size refuge in which they are secure from predation by shore crabs at the sites where these crabs are commonest.     

Kinematics of the Coordination of Pointing during Locomotion

In natural motor behaviour arm movements, such as pointing or reaching, often need to be coordinated with locomotion. The underlying coordination patterns are largely unexplored, and require the integration of both rhythmic and discrete movement primitives. For the systematic and controlled study of such coordination patterns we have developed a paradigm that combines locomotion on a treadmill with time-controlled pointing to targets in the three-dimensional space, exploiting a virtual reality setup. Participants had to walk at a constant velocity on a treadmill. Synchronized with specific foot events, visual target stimuli were presented that appeared at different spatial locations in front of them. Participants were asked to reach these stimuli within a short time interval after a “go” signal. We analysed the variability patterns of the most relevant joint angles, as well as the time coupling between the time of pointing and different critical timing events in the foot movements. In addition, we applied a new technique for the extraction of movement primitives from kinematic data based on anechoic demixing. We found a modification of the walking pattern as consequence of the arm movement, as well as a modulation of the duration of the reaching movement in dependence of specific foot events. The extraction of kinematic movement primitives from the joint angle trajectories exploiting the new algorithm revealed the existence of two distinct main components accounting, respectively, for the rhythmic and discrete components of the coordinated movement pattern. Summarizing, our study shows a reciprocal pattern of influences between the coordination patterns of reaching and walking. This pattern might be explained by the dynamic interactions between central pattern generators that initiate rhythmic and discrete movements of the lower and upper limbs, and biomechanical factors such as the dynamic gait stability.     

Morphology and tenacity of the tube foot disc of three common European sea urchin species: a comparative study

The variation in tenacity of single tube feet from three sea urchin species with contrasted habitats was assessed and correlated with the ultrastructure of their adhesive secretory granules. The tube feet of Arbacia lixula and Sphaerechinus granularis have larger discs and more complex adhesive granules than those of Paracentrotus lividus, but A. lixula attaches to glass with significantly lower tenacity (0.05-0.09 MPa) than the other two species (0.10-0.20 and 0.11 -0.29 MPa, respectively). However, the estimated maximal attachment force one tube foot can produce is similar for all three species investigated. No clear relationship between tube foot size, tenacity, adhesive secretory granule ultrastructure and species habitat can therefore be established. For P. lividus the tenacity of single tube foot discs on four different smooth substrata was also compared, which showed that both the total surface energy and the ratio of polar to non-polar forces at the surface influence tube foot attachment strength. This influence of the surface characteristics of the substratum appears to affect the cohesiveness of the adhesive secretion more than its adhesiveness.     

PEDAL AND OPERCULAR SECRETORY GLANDS OF POMATIAS, BITHYNIA AND LITTORINA

The histochemical properties of the pedal and opercular glandcells of three prosobranchs from different habitats were examined.The suprapedal gland of Pomatias elegans contained 3 gland celltypes producing mucoprotein, protein and sulphated muco-polysaccharide.The ventral surfaces of the foot were devoid of gland cellsexcept in the median furrow in which two cell types producea neutral and a sulphated mucopolysacharide. The dorsal surfaceof the foot possesses 5 cell types which produce a variety ofmucosubstances. The anterior pedal gland of Bithynia tentaculata produces neutraland weakly acidic mucoprotein from one cell type. The ventralsurface of the foot is generally populated by gland cells oftwo types producing acid mucopolysaccharide and protein. A furthercell type producing carboxylated mucopolysaccharide is restrictedto a transverse band mid-way down the foot. The dorsal surfaceof the foot is covered by mucus from three cell types producinga variety of mucosubstances. The anterior pedal gland of Littorina littorea possesses twocell types both of which secrete mucoprotein. The ventral surfaceof the foot secretes sulphated and carboxylated mucopolysaccharideand a mucoprotein from three cell types which form a thick subepi-dermalglandular layer. The dorsal surface is lubricated by a sulphatedmucopolysaccharide and a mucoprotein. The secretory cells of the dorsal surface extend into the operculargroove and disc whose specialised cells generally secrete mucopolysaccharideand at least two differently staining proteins. ^*Present address: Department of Zoology, University of Durham,Durham City, U.K. (Received 19 August 1986;     

Octopamine boosts snail locomotion: behavioural and cellular analysis

We measured the reduction in locomotion of unrestrained pond snails, Lymnaea stagnalis, subsequent to transdermal application of two selective octopamine antagonists, epinastine and phentolamine. After 3 h in fresh standard snail water following treatment with 4 mM epinastine or 3.5 mM phentolamine, the snails’ speed was reduced to 25 and 56% of the controls (P < 0.001 and P = 0.02, respectively). The snails’ speed decreased as the drug concentration increased. In the isolated CNS, 0.5 mM octopamine increased the firing rate of the pedal A cluster motoneurons, which innervate the cilia of the foot. In normal saline the increase was 26% and in a high magnesium/low calcium saline 22% (P < 0.05 and 0.01, respectively). We conclude that octopamine is likely to modulate snail locomotion, partially through effects on pedal motoneurons.     

The mechanism of locomotion of Agriolimax reticulatus (Mollusca: Gastropoda)

The locomotion of Agriolimax reticulatus is described and the musculature directly responsible for producing the pedal waves has been identified. The probable mechanism of locomotion is discussed.     

Conserved ergot-sensitive subunit of the central mechanism of locomotion in mollusca

Alkaloids (ergotamine and ergometrine) were shown to slow down rhythmic locomotion in even distantly related molluscs. The same is true for crawling pulmonate, pond snail Lymnaea stagnalis. The ergotamine activation of a powerful inhibitory input to pedal neurones involved in locomotor rhythmicity, was shown. The data obtained suggest that the conservative target for ergots is located outside rather than within the central pattern generator for locomotion.