close to the eyes, forming "eyelashes". The most conspicuous appendages are the pereiopods, and the largest of these is the first pair. These each end in a chela (claw), and are therefore known as the "chelipeds"; they can be more than six times the body length, although some groups show sexual dimorphism, with females having proportionally shorter chelipeds. The following three pairs of pereiopods are somewhat smaller than the chelipeds and are without claws, but are otherwise similar; they are used for walking. The fifth pair of pereiopods are much smaller than the preceding pairs, and are held inconspicuously under the carapace. They each end in a tiny chela, and are generally believed to be used for cleaning the body, especially the gills, which are in a cavity protected by the carapace. The pleon is made up of six somites, each bearing a pair of pleopods, and terminating in a telson. The first somite is narrower than the succeeding somites, and the last pair of pleopods are modified into uropods, which flank the telson. The pleon is usually curled under the thorax, such that only the first three somites are visible from above. The form of the pleopods varies between the sexes. In females, the first one or two pairs are missing, while the remaining pairs are uniramous, and have long setae, to which the eggs can be attached. In males, the first two pairs are formed into gonopods, and are used to transfer the spermatophore to the female during mating; the first pair is often missing. The remaining pleopods can be similar to those of the females, or reduced in size, or entirely absent. In both sexes, the uropods are biramous. Ecology The majority of squat lobsters are benthic, spending their lives on the sea-floor. A few, however, spend part of their lives living in the water column, often forming dense pelagic swarms of juveniles, especially in areas with high densities of plankton; this behaviour is seen in the species Pleuroncodes planipes (off Baja California, Mexico), P. monodon (Peru and Chile) and Munida gregaria (Otago, New Zealand and southernmost South America). Although Munida quadrispina also has pelagic juveniles, they have not been observed to form swarms. As well as these pelagic swarms, many species form dense aggregations on the sea floor. This is particularly prominent around hydrothermal vents, where species such as Shinkaia crosnieri are particularly abundant. Squat lobsters