The life habit of Neseuretus is reconstructed on the basis of functional analysis and compared with that of other Calymenacea. Neseuretus, like all Calymenidae, was probably a filter feeder, burrowing downward into the sediment, where it rested hidden in a U-shaped attitude. Homalonotidae developed frontal burrowing and Bathycheilidae probably lived on the sediment. These calymenid families have different enrolment and behavioural patterns. Enrolment structures are developed, analysis of which leads to an understanding of phylogeny. Bathycheilidae maintained an unspecialised spiral enrolment. Flexicalymeninae and Calymeninae developed an advanced spiral enrolment, while a pseudosphaeroidal enrolment was achieved along independent lines in Kerfornellinae and Homalonotidae.

Many Ordovician trilobites possess morphological structures on the cephalon, the pygidium and sometimes also on the thoracic segments, which interlock during enrolment. These coaptative devices, also known in living arthropods, are functional, inherited, and seem to remain stable at the species level. Significant evolutionary changes in coaptative devices occur throughout time within the genera Placoparia (Pliomeridae) and Crozonaspis (Dalmanitidae). New data are presented which have relevance for the details of the coaptative structures in Placoparia (Coplacoparia), and for the classification of the dalmanitid species morenensis and dujardini. In some species of Crozonaspis and in Placoparia (Coplacoparia), interlocking structures prevent the possibility of shearing between cephalon and post-cephalic exoskeleton.

The striking morphological similarity that exists between appendages of the extant portunid crabs, such as Callinectes sapidus, and those of the extinet eurypterids has long been noted. The fifth pair of pereiopods in blue crags and other portunids are modified to form the broad, flat, highly mobile ‘swim paddles.’ A nearly identical modification is seen in the sixth pair of prosomal appendages of many eurypterids. The similarities are due to convergence and not to shared descent.

The kinetics of blue crab swimming were studied using high speed films. The animals are capable of slow upwards locomotion (‘hovering’) and rapid sideways swimming. The blue crab paddles apparently act as reciprocating hydrofoils, employing well-understood principles of lift and thrust generation to overcome the animal's weight and drag. Experimental studies indicated that the paddles are capable of producing appreciable amounts of lift. Drag on the body and paddles was also determined. Resxults are similar to those obtained in previous studies of bird and insect flight.

The physical principles employed to study blue crab swimming can be applied to the study of eurypterid locomotion. The eurypterid paddles may have functioned as hydrofoils, producing lift and thrust on forestroke and backstroke. Eurypterids were probably highly agile and manoueverable swimmers, capable of hovering and of high speed swimming. This model predicts observed morphological correlates. Predicted morphological correlates of earlier models (often based on analogies with Limulus) were not found.

The observed convergence between eurypterids and blue crabs may have resulted from similar functional constraints and parallel phylogenetic histories.

Cyrtoctenus wittebergensis sp. nov. is described from a unique holotype from the Witteberg Group of the Cape Supergroup. It is a giant hibbertopteroid eurypterid having combs and specialised movable spines of crytoctenid type (Størmer & Waterston 1968) on the more distal podomeres of the second to fourth prosomal appendages. The function of the combs and their associated movable spines is discussed and it is suggested that together they formed a unique adaptation of eurypterid structures to sweep filter-feeding, the combs forming the filters and the spines the cleaners. The digestive tract is remarkably preserved and shows a spiral valve, posterior to the stomach, which is interpreted as an adaptive feature in this large arthropod to increase the absorptive area of the gut.

The new evidence provided by the South African specimen has required the re-interpretation of the disarticulated Cyrtoctenus specimens previously described from Europe. Disjecta membra recently obtained from the Tournaisian of Foulden, Berwickshire, which may belong to Cyrtoctenus, are described and show characters previously unknown in Scottish material but similar to certain features in the South African specimen.

The taxonomic relationships within the Hibbertopteroidea are discussed in the light of the new combination of characters found in C. wittebergensis. Two families are recognised in the superfamily, the Hibbertopteridae, including Hibbertopterus and Campylocephalus, and the new family Cyrtoctenidae which is here erected to include Cyrtoctenus, Dunsopterus and possibly also Hastimima.

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.