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Tail-flip escape trajectories of the brown shrimp Crangon crangon have been investigated in response to a natural predator, the cod Gadus morhua, and an artificial stimulus. Shrimps escaped by rolling to their left or right during the initial tail-flip of a response, and thereafter swam on their side. As a result of the laterally directed first tail-flip, initial escape angles always lay between 75° and 156° with respect to the (pre-escape) longitudinal axis (anterior=0°) of the shrimp. Symmetrical attacks from either head-on or tail-on produced escapes to the shrimp’s left or right in equal proportions, although a contralateral bias did occur if the shrimp experienced a looming object from one side before a symmetrical attack was applied. Lateral attacks produced a significantly greater proportion of contralateral responses than ipsilateral ones. Empirical and theoretical analyses indicate that the initial escape direction is influenced by an interaction between the range of first tail-flip escape angles that the shrimp is capable of performing and the risk of being intercepted by a predator during the initial stage of an escape. Thus, the unpredictability (‘protean behaviour’) of the response may be affected by the conditions of the interaction. Subsequent tail-flips of an escape usually directed the response away from the stimulus, but sometimes escapes were instead steered to the side of the stimulus and then behind it. The probability of each type of escape occurring changed with attack direction. The elements of protean behaviour that have been identified in both the initial and subsequent stages of the escape may prevent predators from learning a fixed pattern of response, but a trade-off occurs when escape trajectories infringe upon zones of high capture risk.

The burial mechanism ofCrangon crangonwas examined in detail using high-speed video-recording equipment. Four stages of burial were identified which were associated with three modes of burial. As the mode of burial changed, a change in the angle of substratum penetration was also noted.The effect of particle size on the burying ability of three discrete size classes of shrimp was examined using both artificially graded and naturally occurring sediments. Particle size was found to be a major influencing factor on the degree of burial achieved by C.crangon, but no significant effect of shrimp size on burying ability was found over the size range tested.

Surface activity of the deep-burrowing echiuran Maxmuelleria lankesteri (Herdman) was observed in Loch Sween, Argyll (Scotland, UK) using underwater television. Activity, consisting of proboscis emergence and sediment venting, was observed in all seasons. Proboscis emergence was strictly nocturnal, occurring on average on one night in two. Emergence occurred at irregular intervals during the night, with no consistent rhythmicity. Feeding extensions of the proboscis usually lasted about 10 min, but were often curtailed by accidental contact with ophiuroids or other epifauna. The extending proboscis skims off the top layer of sediment, then rapidly pulls the collected material into the burrow. Animals showed a capacity to select material from particular areas within the feeding radius. The feeding strategy may be to accumulate several loads of sediment in the burrow, then selectively ingest material over the following day. Large volumes of sediment were often vented from burrows, either in a fine suspension...

Tail-flip escape swimming by the brown shrimp Crangon crangon has been investigated across a range of body lengths (11–69 mm) using high-speed video analysis. This has revealed several novel aspects of the tail-flip mechanism when compared with that of other decapod crustaceans that have been studied. (i) The pattern of body flexion in C. crangon produces movement of the cephalothorax as well as the abdomen about the centre of mass. (ii) Shrimps form a ‘head-fan’ with their antennal scales, in addition to the tail-fan formed by their uropods, apparently for generating thrust during tail-flips. (iii) Shrimps typically swim on their side rather than in an upright body position. It is suggested that these features may be interlinked and derive from habitat specialisation. The kinematic properties of tail-flips were found to vary with shrimp size. As shrimp body length increased, the rate of body flexion and re-extension decreased whilst the duration of tail-flips increased. Mean (and maximum) velocity estimates ranged between 0.4 m s−1 (0.7 m s−1) and 1.1 m s−1 (1.8 m s−1) for shrimps of different sizes. The combined effects of escape behaviour and size-dependent variability in tail-flip kinematics will have important implications with regard to predation risk.

Burrowing in Cardium echinatum, Laevicardium crassum and Cardium edule involves similar integrated cyclic movements, in which the active stage of downward movement is that of contraction of the pedal retractor muscles. Final position in the sand is reached only after several repetitions of the digging cycle comprised in a digging period. Digging cycles in the three species differ in the timing of the various stages. The species also differ in the number of cycles comprised in a digging period. These differences may be related to shell form. Recovery from deep burial occurs by similar cyclic movements, but modifications in anchorage and in the time and duration of the forces acting on the shell cause upward movement. The stage of protraction and straightening of the foot is the active stage of recovery. Leaping in C. echinatum and in L. crassum involves also cyclic movements, the leaping cycle, and occurs in response to starfish contact. The species differ in the number of repetitions comprising a single response and in the duration of various stages of the cycle. In burrowing, recovery and leaping succeeding cycles in a single period occupy progressively longer time intervals.

SUMMARYThe functional morphology of the veliger, pediveliger, and early post-larva of Venus striatula is described. Settlement takes place at a length of 220- 240 /u, and is marked by the rapid loss of the velum, functioning of the byssus gland, and commencement of secretion of the post-larval shell (dissoconch). With the exception of the loss of the velum, the changes which occur at metamorphosis take place relatively slowly. The significance to phylogeny of the mode of development of certain organs in the post-larva is discussed. It is concluded that the form of many of the organs in development, especially of those organs concerned in producing a respiratory and feeding current, are juvenile adaptations, resulting in the early functional re-adjustment of the free-swimming larval habit to a bottom-living habit, but having little or no phylogenetic significance.

DURING recent studies of the rock and wood boring molluscs of the super-family Adesmacea (Mollusca : Bivalvia) the occurrence of haemocoelic erythrocytes in the blood of Xylophaga dorsalis Turton was noticed while examining serial sections. The erythrocytes are slightly flattened disks of diameter 10–12µ, each containing a prominent nucleus (Fig. 1).