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While molluscs represent the most numerous marine phylum, arthropods as a whole account for over a million species and make up more than 80% of all extant animals (Ødegaard 2000). Of these species, approximately 85,000 are considered to be marine. Most are contained in the subphylum Crustacea, which includes crabs, lobsters, shrimp, krill, barnacles, and small organisms such as isopods, amphipods, and copepods. However, sparse species of the subphylum Chelicerata are represented in the horseshoe crabs and Pycnogonids (sea spiders). Members of the phylum Arthropoda are distinguished by having chitinous, segmented exoskeletons, which they molt as they grow, and segmented appendages. The bodies of most arthropods are made of two or three distinct parts: a cephalum (head), a thorax (chest), and an abdomen. In some cases, the head and chest fuse into a single cephalothorax.
Each body segment can bear a pair of appendages. The number and diversity of appendages in different species may be partly responsible for the group's success, and the study of appendage adaptations using SEM is common. Scanning electron analysis of hooked setae on decorator crabs proved a mechanical means of attachment (Wicksten 1978, Fürböck & Patzner 2005), putting to rest an alternative hypotheses regarding adhesives secreted by either the first maxillipeds (Aurivillus 1889) or the first and second maxillae (Schafer 1954). The dichotomy of predator-prey adaptations is also prevalent in the literature. Imaging of grooming appendages in porcelain crabs found morphological features such as smooth sensilla and hard chela thought to aid in the removal of potential parasites from the carapace and gills (Fleischer et al. 1992). Nevertheless, parasitic copepods have specially-adapted mouthparts which allow them to attach and feed on host species (Kabata 1974). Similarily, non-sensory microstructures such as surface pores and their associated microtrichs (peglike projections) are being considered as analytical tools (Halcrow & Bousfield 1987). Their distinct morphology and ordered distribution suggest that they might be useful characteristics in the taxonomy of the Amphipoda, a group whose classification continues to be a source of much controversy.