Microstructural features of eurypterid cuticles are analysed from a biomechanical viewpoint: some fibrous elements are now considered to resemble the macrofibres of extant arthropod cuticles; possible preferred orientation zones in Mycterops are related to directional stresses; pore canals are not viewed as acting as crack-stoppers but laminae (sensu Dennell 1978) may have served this function. Could some eurypterids have walked on land?—this problem is approached by using extant Limulus as a model. It leads on to the use of scaling exponents to determine the limits that possessing an exoskeleton places on the size of land arthropods: moulting may be the limiting factor. Possible critical factors limiting the size of aquatic arthropods are discussed briefly.

Large scale models in polyester and araldite have been made of Sidneyia, Leanchoilia and Emeraldella from the Middle Cambrian, Burgess Shale. Details are given of how the models have been produced and used in functional morphology studies, particularly with respect to walking in Sidneyia.

The present work deals with the structure of the cephalic part, traces of the digestive system and gills of Ostenocaris cypriformis (Thylacocephala). The structure of the cephalic part has provoked the most substantial controversy since it has been interpreted as being formed by a pair of enormous compound eyes or as a sac structure that we have called the cephalic sac. Particular attention has been devoted to the minute structure of the cephalic sac. Structures that we have previously termed ‘microsclerites’ can be readily observed, but are by no means easy to interpret.

In many specimens, the basal part of the cephalic area contains abundant traces of alimentary residues. The area thus appears to be part of the digestive system. Our observations do not enable us to give a definitive picture of the anatomical organisation of these animals but do lead to substantial corrections of previously proposed interpretations.

Conchyliocarida, from the Jurassic of France, have a conch-like carapace enclosing the whole body. The number of their somites, identified by the visible appendages, excludes them from Malacostraca. Caridean-like characters, including the large lamellar gills, bring them nearer to Malacostraca, end exclude them from other Classes. Their large functional compound eyes and the morphology of their limbs suggest a benthic life and predatory behaviour.

Conchyliocarida are represented by several forms at La Voulte sur Rhône, which are compared and contrasted to related forms elsewhere.

Differences in the preservation of Jurassic thylacocephalans and conchyliocarids have given rise to different interpretations of the form of these fossils, and thus their mode of life. When evidence from these two groups is combined with that derived from Palaeozoic concavicarids, it becomes possible to unify the several interpretations of this one group of organisms, the Thylacocephala. The group ranges from at least the Silurian to the Cretaceous.

A review is given of how these differences of interpretation have arisen, and some resolution is attempted. If the thylacocephalan “anterior structure” is reinterpreted by analogy with hyperiid amphipods as a paired compound eye occupying most of the surface of the head, it explains its bilobed nature and the position of the stomach within the structure, but it raises the difficulty of a post-cephalic origin for the carapace. The simpler solution is preferred of regarding this structure as discrete paired eyes with a smooth cornea and subjacent crystal cones.

The raptorial appendages are post-oral and post-adductor in insertion. They are therefore tentatively identified as the maxillae and maxilliped, but verification of the mandible's position is needed to test this. The postero-ventral battery of “body somites” is reinterpreted as paired protopods of abdominal limbs. A respiratory current is deduced to have entered a branchiostegal chamber ventrally, and left it posterodorsally. It is speculated that the looped linear pattern of intra-cuticular spheres in Paraostenia are photophores. The large eyes with small interommatidial angles were probably used to discern low contrast prey or carrion against a dim background. By analogy with hyperiid amphipods, it is suggested that at least some thylacocephalans were mesopelagic predators. They may have attained neutral buoyancy from their food substrate of shark and coleoids.