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• W2100074648 abstract "(In this abstract, numbers in parentheses refer to key figures in the text which facilitate understanding.) Selected aspects of notostracan morphology are considered from a functional standpoint and related to habits whenever possible. Scanning electron microscopy and observations on the early stages of development have revealed differences, some of functional significance, between what have been regarded as no more than racially different populations of certain species. These suggest that our taxonomic understanding of the Notostraca is still incomplete. The natural history and general habits of notostracans are briefly described. Many structural features are related to benthic habits. The development of a dorsal, dome-shaped carapace (2 and 3) has probably influenced the evolution of the trunk and other features involved in maintaining hydrodynamic efficiency when swimming. Eggs, which in some species are firmly attached to substrata (4), are probably less easily dispersed than often supposed. Besides its ecological significance, this has a bearing on taxonomy, zoogeography and evolution. The gross morphology of the trunk limbs is re-described as an essential preliminary to interpreting function, and various hitherto undescribed structural features are reported. The anterior series of trunk limbs (5—9) fulfil a multiplicity of roles (in standing, swimming, digging, clambering, food collection and manipulation, egg-carrying, respiration and sensing the environment) and have evolved within the constraints imposed by the overlying carapace, whose presence has, conversely, been exploited. Limbs of the posterior series (10-15) are less constrained by the carapace and have developed large exopodite paddles that set up a respiratory stream, but they are also involved in food handling. Scanning electron microscopy has revealed details of trunk limb armature previously poorly known and often of great complexity. Many of the spines and setae that play an important part in food handling are hinged at the base in a characteristic manner (21, 24, 33, 34 and 40), a feature of great functional significance. Spectacular groups of sensillae are present on the trunk limbs of Lepidurus apus (54—62). The profusion of sensillae on the trunk limbs of the Notostraca stands in marked contrast to the situation in other branchiopods and is probably related to the versatility of food collecting techniques employed, which necessitates an ability to handle a wide range of food items, ranging from detrital particles to large prey organisms. An account of the functional anatomy of the head (63—67) is presented. The mandibles are biting appendages (29, 69-71, 87 and 102) that can abduct widely. In this they contrast strikingly with the rolling, grinding and crushing mandibles of most branchiopods whose ability to abduct is extremely limited. Nevertheless they share many features with such mandibles. Skeletally they have the same hollow, boat-like structure, but their arm ature is very different, consisting of heavily sclerotized, toothed ridges with additional refinements (69-76). The articulating surface is broad (68), not pointed as in most branchiopods. There is a massive transverse mandibular tendon (64, 81, 82, 85, 87, 88 and 90) homologous with that of other branchiopods but vastly thicker. On each side it is suspended and braced by three dorsal ligaments, and braced by an anteroventral ligament and by four dorsal muscles (66, 67, 87 and 88). It is also supported posteriorly by a pair of complex cuticular apodemes and anchored by fibres to simpler anterior apodemes (64-66, 91 and 93). The mandibular muscles share many basic attributes with those of other branchiopods but differ strikingly in ways that are related to the different actions that notostracan mandibles and those of the rolling and grinding type have to perform. The transverse muscles originate essentially from the ends of the transverse mandibular tendon (19, 81 and 102). None does so from the main part of its posterior face as they often do in other branchiopods. There are no 5c muscles, in which respect adult notostracan mandibles appear to be unique among branchiopods. Abduction is achieved by dorsally located abductor muscles (66, 67, 86 and 87) that have no counterpart in other branchiopods. The broad articulating surface can slide as these muscles contract, allowing abduction of the molar regions. The paragnaths, maxillules, maxillae and post-mandibular apodemes make up a functionally integrated complex (64, 66, 67, 94, 97 and 99). The maxillules (100 and 101) are much more complex than hitherto appreciated and far more so than in any other order of the Branchiopoda. Each is two-segmented, the proximal segment having elaborate armature. Intrinsic and extrinsic muscles are involved in maxillulary movements. Among their several roles the post-mandibular apodemes provide firm anchorage for the ventral longitudinal muscles of the trunk (81). In contrast to the fleshy labrum of most branchiopods with its often conspicuous labral glands, that of the Notostraca is a flattened structure that lacks labral glands (63 and 64). Standing, swimming and digging are important elements in the life of notostracans and are related to feeding habits. Aspects of trunk limb structure in relation to the feeding mechanism are discussed. Notostracans feed both on finely particulate material and on large items. These food sources call for different handling techniques, both of which are described. In neither case is filtration involved. In both, food is passed forward along, or adjacent to, the shallow food groove from gnathobase to gnathobase and eventually to the mouthparts. The morphological specializations involved in the process and in the actions of the mouthparts are described. Notostracans hatch as nauplii (119 and 120) (with the possible exception of certain populations of Lepidurus arcticus in which this stage may have been eliminated, though the evidence is ambiguous). These do not feed. Feeding begins at stage 2 in Triops . Food is collected by the m andibular palps and passed to the oesophagus by a masticatory spine on each mandibular gnathobase (105). At this stage the gnathobase has no armature on the prospective masticatory surface, exactly as in the Anostraca. Proximal masticatory spines of the antennae help to pass food forward but the distal masticatory spines do not collect food and, in T. cancriformis at least, seem not to play any part in food handling. In T. cancriformis the mandibular gnathobases develop their first masticatory armature at stage 3 (106). During subsequent ontogeny in Triops the naupliar mechanism is gradually replaced by that of the adult. Development is essentially anamorphic but the trunk limbs develop at much earlier stages than in the Anostraca. Lepidurus arcticus hatches as a non-feeding nauplius that very quickly moults to a stage much more advanced than stage 2 of Triops , there being a virtual metamorphosis at the first moult. Although the second instar swims by an essentially adult (though still rudimentary) mechanism, it does not feed. Its mandibles indeed are incapable of handling food, being in essentially the same state of development as those in stage 2 of T. cancriformis but with minute, non-functional gnathobasic spines (122) At no stage do the antennae possess either proximal or distal masticatory spines (121) and the mandibular palps never collect food. There is in fact no naupliar feeding mechanism. When feeding begins, at stage 3, the mechanism involved is already of an essentially adult type. Although the absence of the 5c muscles of the transverse mandibular series is an outstanding feature of adult notostracans, these muscles are present in the early stages of development (116, 125 and 129). Swimming of the early stages of Triops has been analysed. As in early anostracan larvae the nauplii and early post-naupliar stages inhabit a low Reynolds number environment and have essentially no momentum. Oar-like movements of the antennae propel the larva forwards during the working stroke but, as soon as this impetus ceases, so does forward motion, and during the recovery phase of the cycle of antennal beat the larva moves backwards (130 and 131). Over a series of moults this mechanism is gradually replaced by the adult mechanism as the trunk limbs develop and the antennae atrophy. A comparison between the early stages of the Notostraca and Anostraca from both functional and evolutionary standpoints reveals striking differences, as well as similarities that are probably indicative of remote common ancestry. The Notostraca occupy an isolated position within the Branchiopoda. Although primitive, the Notostraca may be less so than the Anostraca." @default.
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• W2100074648 date "1988-08-17" @default.
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• W2100074648 title "Studies on the functional morphology and biology of the Notostraca (Crustacea: Branchiopoda)" @default.
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